title: Late Pleistocene

text: A butchered brown bear patella found in Alice and Gwendoline Cave in County Clare and dated to 10,860 to 10,641 BC indicates the first known human activity in Ireland.


title: Late Pleistocene

text: The very first human habitation in the Japanese archipelago has been traced to prehistoric times between 40,000 BC and 30,000 BC. The earliest fossils are radiocarbon dated to c. 35,000 BC. Japan was once linked to the Asian mainland by land bridges via Hokkaido and Sakhalin Island to the north, but was unconnected at this time when the main islands of Hokkaido, Honshu, Kyushu and Shikoku were all separate entities.


title: Late Pleistocene

text: From about 28 ka, there were migrations across the Bering land bridge from Siberia to Alaska. The people became the Native Americans. It is believed that the original tribes subsequently moved down to Central and South America under pressure from later migrations.


title: Late Pleistocene

text: In the North American land mammal age scale, the Rancholabrean spans the time from c. 240,000 years ago to c. 11,000 years ago. It is named after the Rancho La Brea fossil site in California, characterised by extinct forms of bison in association with other Pleistocene species such as the mammoth.


title: Late Pleistocene

text: "Bison occidentalis" and "Bison antiquus", an extinct subspecies of the smaller present-day bison, survived the Late Pleistocene period, between about 12 and 11 ka ago. Clovis peoples depended on these bison as their major food source. Earlier kills of camels, horses, and muskoxen found at Wally's beach were dated to 13.1–13.3 ka B.P.


title: Late Pleistocene

text: There is evidence of human habitation in mainland Australia, Indonesia, New Guinea and Tasmania from c. 45,000 BC. The finds include rock engravings, stone tools and evidence of cave habitation.


title: Jacky Rosen

text: Jacklyn Sheryl Rosen (née Spektor; born August 2, 1957) is an American politician serving as the junior United States senator from Nevada since 2019. A member of the Democratic Party, she was the U.S. representative for Nevada's 3rd congressional district from 2017 to 2019.


title: Jacky Rosen

text: Rosen was elected to the Senate in 2018, defeating Republican incumbent Dean Heller. She was the only House freshman to win a Senate seat in the 2018 midterm elections and the only challenger to defeat a Republican incumbent senator in 2018.


title: Jacky Rosen

text: Rosen was born on August 2, 1957, in Chicago, Illinois, to Carol, a homemaker, and Leonard Spektor, a car dealership owner who had served in the U.S. Army during the Korean War. Rosen's mother was of Irish, German, and Austrian descent, and her father's family were Jewish emigrants from Russia and Austria.


title: Jacky Rosen

text: Rosen attended the University of Minnesota and graduated with a bachelor's degree in psychology in 1979. While she was in college, her parents moved to Las Vegas, where Rosen moved after graduating. She took a job with Summa Corporation and worked summers as a waitress at Caesars Palace throughout the 1980s. While working for Summa, she attended Clark County Community College (now the College of Southern Nevada) and received an associate degree in computing and information technology in 1985. She began working for Southwest Gas in 1990 before leaving to open her own consulting business three years later.


title: Jacky Rosen

text: A former computer programmer with no political experience at the time, Rosen was asked by U.S. Senate Minority Leader Harry Reid, also from Nevada, to run in the 2016 election for the U.S. House seat being vacated by Republican Joe Heck. On January 26, she declared her candidacy for . Rosen won 60% of the vote in the Democratic primary election and narrowly defeated Republican nominee Danny Tarkanian in the general election. She was sworn into office on January 3, 2017.


title: Jacky Rosen

text: Rosen was elected to the U.S. Senate on November 6, 2018, defeating one-term Republican senator Dean Heller to become the junior senator from Nevada. Her candidacy, announced on July 5, 2017, was endorsed by former President Barack Obama and former vice president Joe Biden. During the campaign, Rosen emphasized her support for the Affordable Care Act (Obamacare) and criticized Heller's vote to repeal it in 2017. At the time, Rosen voted in the U.S. House against Republicans' attempts to repeal Obamacare.


title: Jacky Rosen

text: Rosen defeated Heller, 50.4%–45.4%. Heller carried 15 of Nevada's 17 county-level jurisdictions, but Rosen carried the state's two largest, Clark (home to Las Vegas) and Washoe (home to Reno). She won Clark County by over 92,000 votes, almost double her statewide margin of more than 48,900 votes.


title: Jacky Rosen

text: Rosen was one of only two non-incumbent Democrats, alongside Kyrsten Sinema of Arizona, to win election to the Senate in 2018. She is also the 37th freshman member of the U.S. House to win a Senate seat and the first woman to do so.


title: Jacky Rosen

text: Rosen was on Capitol Hill for the 2021 United States Electoral College vote count when supporters of President Donald Trump stormed the U.S. Capitol. At the time, she was in the Russell Senate Office Building before being evacuated to a secure, undisclosed location. She tweeted during the attack, calling the event "reprehensible" and writing, "It's time for us as a nation to come together and denounce hate and violence."


title: Jacky Rosen

text: Rosen has been described as a liberal Democrat at times and as a moderate at others. As of April 2020, "FiveThirtyEight" found that Rosen's votes aligned with President Trump's legislative positions about 36% of the time. The American Conservative Union gave her a 5% lifetime conservative rating in 2020.


title: Jacky Rosen

text: In April 2019, Rosen was one of 34 senators to sign a letter to Trump encouraging him "to listen to members of your own Administration and reverse a decision that will damage our national security and aggravate conditions inside Central America." The letter asserted that Trump had "consistently expressed a flawed understanding of U.S. foreign assistance" since becoming president and that he was "personally undermining efforts to promote U.S. national security and economic prosperity" through preventing the use of Fiscal Year 2018 national security funding. The senators argued that foreign assistance to Central American countries created less migration to the U.S. by helping to improve conditions in those countries.


title: Jacky Rosen

text: Rosen supports the Affordable Care Act and its provisions that prevent patients from being denied insurance or charged more due to age or a preexisting condition. She supports allowing citizens to buy into Medicaid as an alternative option that would compete with private insurance companies.


title: Jacky Rosen

text: In January 2019, during the 2018-2019 government shutdown, Rosen was one of 34 senators to sign a letter to Commissioner of Food and Drugs Scott Gottlieb recognizing the FDA's efforts to address the effect of the shutdown on the public health and employees while remaining alarmed "that the continued shutdown will result in increasingly harmful effects on the agency's employees and the safety and security of the nation's food and medical products."


title: Jacky Rosen

text: In February 2019, Rosen was one of 11 senators to sign a letter to insulin manufactures Eli Lilly and Company, Novo Nordisk, and Sanofi over increased insulin prices, saying the increases kept patients from receiving "access to the life-saving medications they need."


title: Jacky Rosen

text: In August 2019, Rosen was one of 19 senators to sign a letter to United States Secretary of the Treasury Steve Mnuchin and United States Secretary of Health and Human Services Alex Azar requesting data from the Trump administration in order to help states and Congress understand the potential consequences if the "Texas v. United States" Affordable Care Act lawsuit prevailed in courts. They wrote that an overhaul of the present health care system would form "an enormous hole in the pocketbooks of the people we serve as well as wreck state budgets".


title: Jacky Rosen

text: In April 2019, Rosen was one of 41 senators to sign a bipartisan letter to the housing subcommittee praising the United States Department of Housing and Urban Development's Section 4 Capacity Building program as authorizing "HUD to partner with national nonprofit community development organizations to provide education, training, and financial support to local community development corporations (CDCs) across the country" and expressing disappointment that Trump's budget "has slated this program for elimination after decades of successful economic and community development." The senators wrote of their hope that the subcommittee would support continued funding for Section 4 in Fiscal Year 2020.


title: Jacky Rosen

text: Rosen supports "comprehensive immigration reform" but does not believe the Immigration and Customs Enforcement agency should be abolished.


title: Jacky Rosen

text: In 2018, Rosen was one of three U.S. House Democrats to break with their party and vote to make individual tax cuts permanent. She supports a $15 hourly minimum wage.


title: Jacky Rosen

text: Rosen resides in Henderson, Nevada, with her husband, Larry, a radiologist. They have a daughter. Before entering politics, she served as the president of the Congregation Ner Tamid synagogue, a Reform Jewish synagogue in Henderson. She has cited the philosophy of "tikkun olam" as a key part of her decision to enter politics.


title: North York

text: North York is one of the six administrative districts of Toronto, Ontario, Canada. It is located directly north of York, Old Toronto and East York, between Etobicoke to the west and Scarborough to the east. As of the 2016 Census, it had a population of 869,401.


title: North York

text: North York was created as a township in 1922 out of the northern part of the former township of York, a municipality that was located along the western border of Old Toronto. Following its inclusion in Metropolitan Toronto in 1953, it was one of the fastest-growing parts of the region due to its proximity to Old Toronto. It was declared a borough in 1967, and later became a city in 1979, attracting high-density residences, rapid transit, and a number of corporate headquarters in North York City Centre, its central business district. In 1998, North York was amalgamated with the rest of Metropolitan Toronto to form the new city of Toronto and has since been a secondary economic hub of the city outside Downtown Toronto.


title: North York

text: The was formed on June 13, 1922 out of the rural part of the Township of York. In the previous decade, the southern part of York, bordering the Old City of Toronto had become increasingly urbanized while the northern portion remained rural farmland. The northern residents increasingly resented that they made up 20% of York's tax base while receiving few services and little representation in return, particularly after 1920 when their sole member on York's council, which was elected on an at-large basis, was defeated. Dairy farmer Robert Franklin Hicks organized with other farmers to petition the Ontario legislature to carve out what was then the portion of York Township north of Eglinton Avenue to create the separate township of North York. With the support of the pro-farmer United Farmers of Ontario government, a plebiscite was organized and held and the 6,000 residents voted in favour of separating from York by margin of 393 votes.


title: North York

text: The township remained largely rural and agrarian until World War II. After the war, in the late 1940s and 1950s, a housing shortage led to the township becoming increasingly developed as a suburb of Toronto and a population boom. In 1953, the province federated 11 townships and villages with the Old City of Toronto, to become Metropolitan Toronto.


title: North York

text: As North York became more populous, it became the in 1967, and then on February 14, 1979, the . To commemorate receiving its city charter on Valentine's Day, the city's corporate slogan was "The City with Heart". It now forms the largest part of the area served by the "North York Community Council", a committee of Toronto City Council.


title: North York

text: North York used to be known as a regional agricultural hub composed of scattered villages. The area boomed following World War II, and by the 1950s and 1960s, it resembled many other sprawling North American suburbs.


title: North York

text: On August 10, 2008, the Toronto propane explosion occurred at the Sunrise Propane Industrial Gases propane facility just southwest of the Downsview Airport. This destroyed the depot and damaged several homes nearby. About 13,000 residents were evacuated for several days before being allowed back home. One employee at the company was killed in the blast and one firefighter died while attending to the scene of the accident. A follow-up investigation to the incident made several recommendations concerning propane supply depots. It asked for a review of setback distances between depots and nearby residential areas but didn't call for restrictions on where they can be located.


title: North York

text: Canada's deadliest pedestrian attack occurred in the North York City Centre district on April 23, 2018 when a van collided with numerous pedestrians killing 10 and injuring 16 others on Yonge Street between Finch and Sheppard Avenues.


title: North York

text: The district's central business district is known as North York Centre, which was the location of the former city's government and major corporate headquarters. North York Centre continues to be one of Toronto's major corporate areas with many office buildings and businesses. The former city hall of North York, the North York Civic Centre, is located within North York City Centre.


title: North York

text: Downsview Airport, near Sheppard and Allen Road, employs 1,800 workers. Downsview Airport will be the location of the Centennial College Aerospace campus, a $60 million investment from the Government of Ontario and Government of Canada. Private partners include Bombardier, Honeywell, MDA Corporation, Pratt & Whitney Canada, Ryerson University, Sumitomo Precision Products Canada Aircraft, Inc. and UTC Aerospace Systems.


title: North York

text: Flemingdon Park, located near Eglinton and Don Mills, is an economic hub located near the busy Don Valley Parkway and busy Toronto Transit Commission (TTC) routes. McDonald's Canada and Celestica are located in this area, and Foresters Insurance has a major office tower and Bell Canada has a data centre. The Concorde Corporate Centre has of leasable area and is 85% occupied with tenants such as Home Depot Canada, Sport Alliance of Ontario, Toronto-Dominion Bank, Esri Canada and Deloitte. Home Depot's Canadian head office is located in Flemingdon Park.


title: North York

text: North York houses two of Toronto's five major shopping malls: the Yorkdale Shopping Centre and Fairview Mall. Other neighbourhood malls locations include Centerpoint Mall, Bayview Village, Sheridan Mall, Yorkgate Mall, Shops at Don Mills, Steeles West Market Mall, Jane Finch Mall and Sheppard Centre.


title: North York

text: Health care is another major industry in North York, with the district housing several major hospitals, including the North York General Hospital, Humber River Hospital and the Sunnybrook Health Sciences Centre.


title: North York

text: Four public school boards operate primary and secondary institutions in North York, Conseil scolaire catholique MonAvenir (CSCM), Conseil scolaire Viamonde (CSV), the Toronto Catholic District School Board (TCDSB), and the Toronto District School Board (TDSB). CSV and TDSB operate as secular public school boards, the former operating French first language institution, whereas the latter operated English first language institutions. The other two school boards, CSCM and TCDSB, operate as public separate school boards, the former operating French first language separate schools, the latter operating English first language separate schools. All four Toronto-based public school boards are headquartered within North York. Prior to 1998, the North York Board of Education and Conseil des écoles françaises de la communauté urbaine de Toronto operated English and French public secular schools, while the Metropolitan Separate School Board operated English and French public separate schools for North York pupils.


title: North York

text: In addition to primary and secondary schools, several post-secondary institutions were established in North York. York University is a university that was established in 1959. The university operates two campuses in North York, the Keele campus located in the north, and Glendon College, a bilingual campus operated by the university. There are also two colleges that operate campuses in North York. Seneca College was established in North York in 1967, and presently operates several campuses throughout North York, and Greater Toronto. One of Centennial College's campuses are also located in North York, known as the Downsview Park Aerospace Campus.


title: North York

text: North York is a district of the City of Toronto, and is represented by councillors elected to the Toronto City Council, members elected to the Legislative Assembly of Ontario, as well as members elected to the Parliament of Canada. North York Civic Centre is presently used by North York's community council and other city departments servicing North York.


title: North York

text: Prior to North York's amalgamation with Toronto in 1998, North York operated as a lower-tier municipality within the regional municipality of Metropolitan Toronto. The municipality operated its own municipal council, the North York City Council, and met at the North York Civic Centre prior to the municipality's dissolution. The following is a list of reeves (1922–1966) and mayors (1967–1997) of North York.


title: North York

text: North York had a Board of Control from 1964 until it was abolished with the 1988 election and replaced by directly elected Metro Councillors. The Board of Control consisted of four Controllers elected at large and the mayor and served as the executive committee of North York Council. Controllers concurrently sat on Metropolitan Toronto Council


title: North York

text: North York is home to several museums including the (now closed) Canadian Air and Space Museum (formerly the Toronto Aerospace Museum) in Downsview Park. The closed museum was relocated to Edenvale, Ontario in 2019 (northwest of Barrie) and opened and renamed as the "Canadian Air and Space Conservancy". North York is also home to a number of interactive museums, including Black Creek Pioneer Village, an authentic nineteenth-century village and a living museum, the Ontario Science Centre is an interactive science museum, and the Aga Khan Museum, which includes a collection of Islamic art from the Middle-East and Northern Africa.


title: North York

text: An aircraft manufacturing facility and a former military base are located in the Downsview neighbourhood. With the end of the Cold War, much of the land was transformed into a large park now called Downsview Park. Located within the park is the Downsview Park Sports Centre, a multi-purpose facility built by Maple Leaf Sports & Entertainment (MLSE), owners of Toronto FC, of Major League Soccer. MLSE invested $26 million to build the Kia Training Ground, the state-of-the-art practice facility for Toronto FC. Volleyball Canada made Downsview Park its headquarters and training facility.


title: North York

text: There are a multitude of sports clubs based in North York including the North York Storm, a girls' hockey league, Gwendolen Tennis Club, and the North York Aquatic Club, which was founded in 1958 as the North York Lions Swim Club. The Granite Club, located at Bayview and Lawrence, is an invitation-only athletic club. In 2012, the club made a major expansion in North York for their members.


title: North York

text: The North York Ski Centre at Earl Bales Park is one of the only urban ski centres of its kind in Canada. After several incidents involving failures of the club's two-person chairlift incited talks of closing the ski centre, the city revitalized the facilities with a new four-person chairlift. Sports clubs based in North York include:


title: North York

text: Several major controlled-access highways pass through North York, including Highway 400, Highway 401, Highway 404, Allen Road, and the Don Valley Parkway. The former three controlled access highways are operated by the province as 400-series highways, whereas the latter two roadways are managed by the City of Toronto. The section of Highway 401 which traverses North York is the busiest section of freeway in North America, exceeding 400,000 vehicles per day, and one of the widest.


title: North York

text: Public transportation in North York is primarily provided by the Toronto Transit Commission's (TTC) bus or subway system. Two lines of the Toronto subway have stations in North York, the Line 1 Yonge–University, and Line 4 Sheppard. Finch station, the terminus of the Yonge Street branch of the Yonge–University line, is the busiest TTC bus station and the sixth-busiest subway station, serving around 97,460 people per day. The Line 4 Sheppard subway which runs from its intersection with the Yonge-University line at Sheppard Avenue easterly to Fairview Mall at Don Mills Road, is entirely in North York, averaging around 55,000 riders per day. Line 5 Eglinton is a light rail line that is under construction and will traverse through the southeast portion of North York. Line 6 Finch West is another line under construction and will traverse through the northwestern portion of North York. The Ontario Line is expected to have two stops in North York, Science Centre and Flemingdon Park. The intersection of York Mills and Yonge, located next to York Mills station is home to an office and a TTC commuter parking lot, which was sold for $25 million. A $300-million project is expected to create about 300 jobs and bring a new hotel, perhaps a four star Marriott, to the intersection.


title: North York

text: In addition to the TTC, other public transit services that may be accessed from North York include GO Transit, and York Region Transit. GO Transit provides access to commuter rail and bus services to communities throughout Greater Toronto. Both services may be accessed at GO or TTC stations located in North York.


title: Elbow

text: The elbow is the region between the arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and the medial epicondyles of the humerus. The elbow joint is a hinge joint between the arm and the forearm; more specifically between the humerus in the upper arm and the radius and ulna in the forearm which allows the forearm and hand to be moved towards and away from the body.


title: Elbow

text: The term "elbow" is specifically used for humans and other primates, and in other vertebrates forelimb plus joint is used.


title: Elbow

text: The name for the elbow in Latin is "cubitus", and so the word cubital is used in some elbow-related terms, as in "cubital nodes" for example.


title: Elbow

text: The elbow joint has three different portions surrounded by a common joint capsule. These are joints between the three bones of the elbow, the humerus of the upper arm, and the radius and the ulna of the forearm.


title: Elbow

text: When in anatomical position there are four main bony landmarks of the elbow. At the lower part of the humerus are the medial and lateral epicondyles, on the side closest to the body (medial) and on the side away from the body (lateral) surfaces. The third landmark is the olecranon found at the head of the ulna. These lie on a horizontal line called the "Hueter line". When the elbow is flexed, they form a triangle called the "Hueter triangle", which resembles an equilateral triangle.


title: Elbow

text: At the surface of the humerus where it faces the joint is the trochlea. In most people, the groove running across the trochlea is vertical on the anterior side but it spirals off on the posterior side. This results in the forearm being aligned to the upper arm during flexion, but forming an angle to the upper arm during extension — an angle known as the carrying angle.


title: Elbow

text: The superior radioulnar joint shares the joint capsule with the elbow joint but plays no functional role at the elbow.


title: Elbow

text: The elbow joint and the superior radioulnar joint are enclosed by a single fibrous capsule. The capsule is strengthened by ligaments at the sides but is relatively weak in front and behind.


title: Elbow

text: On the anterior side, the capsule consists mainly of longitudinal fibres. However, some bundles among these fibers run obliquely or transversely, thickening and strengthening the capsule. These bundles are referred to as the capsular ligament. Deep fibres of the brachialis muscle insert anteriorly into the capsule and act to pull it and the underlying membrane during flexion in order to prevent them from being pinched.


title: Elbow

text: On the posterior side, the capsule is thin and mainly composed of transverse fibres. A few of these fibres stretch across the olecranon fossa without attaching to it and form a transverse band with a free upper border. On the ulnar side, the capsule reaches down to the posterior part of the annular ligament. The posterior capsule is attached to the triceps tendon which prevents the capsule from being pinched during extension.


title: Elbow

text: The synovial membrane of the elbow joint is very extensive. On the humerus, it extends up from the articular margins and covers the coronoid and radial fossae anteriorly and the olecranon fossa posteriorly. Distally, it is prolonged down to the neck of the radius and the superior radioulnar joint. It is supported by the quadrate ligament below the annular ligament where it also forms a fold which gives the head of the radius freedom of movement.


title: Elbow

text: These folds or plicae are remnants of normal embryonic development and can be categorized as either anterior (anterior humeral recess) or posterior (olecranon recess).


title: Elbow

text: A crescent-shaped fold is commonly present between the head of the radius and the capitulum of the humerus.


title: Elbow

text: On the humerus there are extrasynovial fat pads adjacent to the three articular fossae. These pads fill the radial and coronoid fossa anteriorly during extension, and the olecranon fossa posteriorly during flexion. They are displaced when the fossae are occupied by the bony projections of the ulna and radius.


title: Elbow

text: The elbow, like other joints, has ligaments on either side. These are triangular bands which blend with the joint capsule. They are positioned so that they always lie across the transverse joint axis and are, therefore, always relatively tense and impose strict limitations on abduction, adduction, and axial rotation at the elbow.


title: Elbow

text: The ulnar collateral ligament has its apex on the medial epicondyle. Its anterior band stretches from the anterior side of the medial epicondyle to the medial edge of the coronoid process, while the posterior band stretches from posterior side of the medial epicondyle to the medial side of the olecranon. These two bands are separated by a thinner intermediate part and their distal attachments are united by a transverse band below which the synovial membrane protrudes during joint movements. The anterior band is closely associated with the tendon of the superficial flexor muscles of the forearm, even being the origin of flexor digitorum superficialis. The ulnar nerve crosses the intermediate part as it enters the forearm.


title: Elbow

text: The radial collateral ligament is attached to the lateral epicondyle below the common extensor tendon. Less distinct than the ulnar collateral ligament, this ligament blends with the annular ligament of the radius and its margins are attached near the radial notch of the ulna.


title: Elbow

text: Brachialis is the main muscle used when the elbow is flexed slowly. During rapid and forceful flexion all three muscles are brought into action assisted by the superficial forearm flexors originating at the medial side of the elbow.


title: Elbow

text: The efficiency of the flexor muscles increases dramatically as the elbow is brought into midflexion (flexed 90°) — biceps reaches its angle of maximum efficiency at 80–90° and brachialis at 100–110°.


title: Elbow

text: Active flexion is limited to 145° by the contact between the anterior muscles of the upper arm and forearm, more so because they are hardened by contraction during flexion. Passive flexion (forearm is pushed against the upper arm with flexors relaxed) is limited to 160° by the bony projections on the radius and ulna as they reach to shallow depressions on the humerus; i.e. the head of radius being pressed against the radial fossa and the coronoid process being pressed against the coronoid fossa. Passive flexion is further limited by tension in the posterior capsular ligament and in triceps brachii.


title: Elbow

text: A small accessory muscle, so called epitrochleoanconeus muscle, may be found on the medial aspect of the elbow running from the medial epicondyle to the olecranon.


title: Elbow

text: Elbow extension is simply bringing the forearm back to anatomical position. This action is performed by triceps brachii with a negligible assistance from anconeus. Triceps originates with two heads posteriorly on the humerus and with its long head on the scapula just below the shoulder joint. It is inserted posteriorly on the olecranon.


title: Elbow

text: Triceps is maximally efficient with the elbow flexed 20–30°. As the angle of flexion increases, the position of the olecranon approaches the main axis of the humerus which decreases muscle efficiency. In full flexion, however, the triceps tendon is "rolled up" on the olecranon as on a pulley which compensates for the loss of efficiency. Because triceps' long head is biarticular (acts on two joints), its efficiency is also dependent on the position of the shoulder.


title: Elbow

text: Extension is limited by the olecranon reaching the olecranon fossa, tension in the anterior ligament, and resistance in flexor muscles. Forced extension results in a rupture in one of the limiting structures: olecranon fracture, torn capsule and ligaments, and, though the muscles are normally left unaffected, a bruised brachial artery.


title: Elbow

text: The arteries supplying the joint are derived from an extensive circulatory anastomosis between the brachial artery and its terminal branches. The superior and inferior ulnar collateral branches of the brachial artery and the radial and middle collateral branches of the profunda brachii artery descend from above to reconnect on the joint capsule, where they also connect with the anterior and posterior ulnar recurrent branches of the ulnar artery; the radial recurrent branch of the radial artery; and the interosseous recurrent branch of the common interosseous artery.


title: Elbow

text: There are two sets of lymphatic nodes at the elbow, normally located above the medial epicondyle — the deep and superficial cubital nodes (also called epitrochlear nodes). The lymphatic drainage at the elbow is through the deep nodes at the bifurcation of the brachial artery, the superficial nodes drain the forearm and the ulnar side of the hand. The efferent lymph vessels from the elbow proceed to the lateral group of axillary lymph nodes.


title: Elbow

text: The elbow is innervated anteriorly by branches from the musculocutaneous, median, and radial nerve, and posteriorly from the ulnar nerve and the branch of the radial nerve to anconeus.


title: Elbow

text: The elbow undergoes dynamic development of ossification centers through infancy and adolescence, with the order of both the appearance and fusion of the apophyseal growth centers being crucial in assessment of the pediatric elbow on radiograph, in order to distinguish a traumatic fracture or apophyseal separation from normal development. The order of appearance can be understood by the mnemonic CRITOE, referring to the capitellum, radial head, internal epicondyle, trochlea, olecranon, and external epicondyle at ages 1, 3, 5, 7, 9 and 11 years. These apophyseal centers then fuse during adolescence, with the internal epicondyle and olecranon fusing last. The ages of fusion are more variable than ossification, but normally occur at 13, 15, 17, 13, 16 and 13 years, respectively. In addition, the presence of a joint effusion can be inferenced by the presence of the fat pad sign, a structure that is normally physiologically present, but pathologic when elevated by fluid, and always pathologic when posterior.


title: Elbow

text: The function of the elbow joint is to extend and flex the arm grasp and reach for objects. The range of movement in the elbow is from 0 degrees of elbow extension to 150 degrees of elbow flexion. Muscles contributing to function are all flexion (biceps brachii, brachialis, and brachioradialis) and extension muscles (triceps and anconeus).


title: Elbow

text: In humans, the main task of the elbow is to properly place the hand in space by shortening and lengthening the upper limb. While the superior radioulnar joint shares joint capsule with the elbow joint, it plays no functional role at the elbow.


title: Elbow

text: With the elbow extended, the long axis of the humerus and that of the ulna coincide. At the same time, the articular surfaces on both bones are located in front of those axes and deviate from them at an angle of 45°. Additionally, the forearm muscles that originate at the elbow are grouped at the sides of the joint in order not to interfere with its movement. The wide angle of flexion at the elbow made possible by this arrangement — almost 180° — allows the bones to be brought almost in parallel to each other.


title: Elbow

text: When the arm is extended, with the palm facing forward or up, the bones of the upper arm (humerus) and forearm (radius and ulna) are not perfectly aligned. The deviation from a straight line occurs in the direction of the thumb, and is referred to as the "carrying angle".


title: Elbow

text: The carrying angle permits the arm to be swung without contacting the hips. Women on average have smaller shoulders and wider hips than men, which tends to produce a larger carrying angle (i.e., larger deviation from a straight line than that in men). There is, however, extensive overlap in the carrying angle between individual men and women, and a sex-bias has not been consistently observed in scientific studies.


title: Elbow

text: The angle is greater in the dominant limb than the non-dominant limb of both sexes, suggesting that natural forces acting on the elbow modify the carrying angle. Developmental, aging and possibly racial influences add further to the variability of this parameter.


title: Elbow

text: Two of the most common injuries at the elbow are overuse injuries: tennis elbow and golfer's elbow. Golfer's elbow involves the tendon of the common flexor origin which originates at the medial epicondyle of the humerus (the "inside" of the elbow). Tennis elbow is the equivalent injury, but at the common extensor origin (the lateral epicondyle of the humerus).


title: Elbow

text: There are three bones at the elbow joint, and any combination of these bones may be involved in a fracture of the elbow. Patients who are able to fully extend their arm at the elbow are unlikely to have a fracture (98% certainty) and an X-ray is not required as long as an olecranon fracture is ruled out. Acute fractures may not be easily visible on X-ray.


title: Elbow

text: Elbow dislocations constitute 10% to 25% of all injuries to the elbow. The elbow is one of the most commonly dislocated joints in the body, with an average annual incidence of acute dislocation of 6 per 100,000 persons. Among injuries to the upper extremity, dislocation of the elbow is second only to a dislocated shoulder.


title: Elbow

text: A full dislocation of the elbow will require expert medical attention to re-align, and recovery can take approximately 8–14 weeks.


title: Elbow

text: Infection of the elbow joint (septic arthritis) is uncommon. It may occur spontaneously, but may also occur in relation to surgery or infection elsewhere in the body (for example, endocarditis).


title: Elbow

text: Elbow arthritis is usually seen in individuals with rheumatoid arthritis or after fractures that involve the joint itself. When the damage to the joint is severe, fascial arthroplasty or elbow joint replacement may be considered.


title: Elbow

text: Olecranon bursitis, tenderness, warmth, swelling, pain in both flexion and extension-in chronic case great flexion-is extremely painful.


title: Elbow

text: Elbow pain occurs when the tenderness of the tissues in the elbow become inflamed. Frequent exercise of the inflamed elbow will assist with healing.


title: Elbow

text: Elbow pain can occur for a multitude of reasons, including injury, disease, and other conditions. Common conditions include tennis elbow, golfer's elbow, distal radioulnar joint rheumatoid arthritis, and cubital tunnel syndrome.


title: Elbow

text: Tennis elbow is a very common type of overuse injury. It can occur both from chronic repetitive motions of the hand and forearm, and from trauma to the same areas. These repetitions can injure the tendons that connect the extensor supinator muscles (which rotate and extend the forearm) to the olecranon process (also known as “the elbow”). Pain occurs, often radiating from the lateral forearm. Weakness, numbness, and stiffness are also very common, along with tenderness upon touch.


title: Elbow

text: A non-invasive treatment for pain management is rest. If achieving rest is an issue, a wrist brace can also be worn. This keeps the wrist in flexion, thereby relieving the extensor muscles and allowing rest. Ice, heat, ultrasound, steroid injections, and compression can also help alleviate pain. After the pain has been reduced, exercise therapy is important to prevent injury in the future. Exercises should be low velocity, and weight should increase progressively. Stretching the flexors and extensors is helpful, as are strengthening exercises. Massage can also be useful, focusing on the extensor trigger points.


title: Elbow

text: Golfer's elbow is very similar to tennis elbow, but less common. It is caused by overuse and repetitive motions like a golf swing. It can also be caused by trauma. Wrist flexion and pronation (rotating of the forearm) causes irritation to the tendons near the medial epicondyle of the elbow. It can cause pain, stiffness, loss of sensation, and weakness radiating from the inside of the elbow to the fingers.


title: Elbow

text: Rest is the primary intervention for this injury. Ice, pain medication, steroid injections, strengthening exercises, and avoiding any aggravating activities can also help. Surgery is a last resort, and rarely used. Exercises should focus on strengthening and stretching the forearm, and utilizing proper form when performing movements.


title: Elbow

text: Rheumatoid arthritis is a chronic disease that affects joints. It is very common in the wrist, and is most common at the radioulnar joint. It results in pain, stiffness, and deformities.


title: Elbow

text: There are many different treatments for rheumatoid arthritis, and there is no one consensus for which methods are best. Most common treatments include wrist splints, surgery, physical and occupational therapy, and antirheumatic medication.


title: Elbow

text: Cubital tunnel syndrome, more commonly known as ulnar neuropathy, occurs when the ulnar nerve is irritated and becomes inflamed. This can often happen where the ulnar nerve is most superficial, at the elbow. The ulnar nerve passes over the elbow, at the area known as the “funny bone”. Irritation can occur due to constant, repeated stress and pressure at this area, or from a trauma. It can also occur due to bone deformities, and oftentimes from sports. Symptoms include tingling, numbness, and weakness, along with pain.


title: Elbow

text: First line pain management techniques include the use of nonsteroidal anti-inflammatory oral medicines. These help to reduce inflammation, pressure, and irritation of the nerve and around the nerve. Other simple fixes include learning more ergonomically friendly habits that can help prevent nerve impingement and irritation in the future. Protective equipment can also be very helpful. Examples of this include a protective elbow pad, and an arm splint. More serious cases often involve surgery, in which the nerve or the surrounding tissue is moved to relieve the pressure. Recovery from surgery can take awhile, but the prognosis is often a good one. Recovery often includes movement restrictions, and range of motion activities, and can last a few months (cubital and radial tunnel syndrome, 2).


title: Elbow

text: The now obsolete length unit ell relates closely to the elbow. This becomes especially visible when considering the Germanic origins of both words, "Elle" (ell, defined as the length of a male forearm from elbow to fingertips) and "Ellbogen" (elbow). It is unknown when or why the second "l" was dropped from English usage of the word. The "ell" as in the English measure could also be taken to come from the letter L, being bent at right angles, as an elbow. The ell as a measure was taken as six handbreadths; three to the elbow and three from the elbow to the shoulder. Another measure was the "cubit" (from "cubital"). This was taken to be the length of a man's arm from the elbow to the end of the middle finger.


title: Elbow

text: Though the elbow is similarly adapted for stability through a wide range of pronation-supination and flexion-extension in all apes, there are some minor difference. In arboreal apes such as orangutans, the large forearm muscles originating on the epicondyles of the humerus generate significant transverse forces on the elbow joint. The structure to resist these forces is a pronounced keel on the trochlear notch on the ulna, which is more flattened in, for example, humans and gorillas. In knuckle-walkers, on the other hand, the elbow has to deal with large vertical loads passing through extended forearms and the joint is therefore more expanded to provide larger articular surfaces perpendicular to those forces.


title: Elbow

text: Derived traits in catarrhini (apes and Old World monkeys) elbows include the loss of the entepicondylar foramen (a hole in the distal humerus), a non-translatory (rotation-only) humeroulnar joint, and a more robust ulna with a shortened trochlear notch.


title: Elbow

text: The proximal radioulnar joint is similarly derived in higher primates in the location and shape of the radial notch on the ulna; the primitive form being represented by New World monkeys, such as the howler monkey, and by fossil catarrhines, such as "Aegyptopithecus". In these taxa, the oval head of the radius lies in front of the ulnar shaft so that the former overlaps the latter by half its width. With this forearm configuration, the ulna supports the radius and maximum stability is achieved when the forearm is fully pronated.


title: Evolutionary history of plants

text: The evolution of plants has resulted in a wide range of complexity, from the earliest algal mats, through multicellular marine and freshwater green algae, terrestrial bryophytes, lycopods and ferns, to the complex gymnosperms and angiosperms (flowering plants) of today. While many of the earliest groups continue to thrive, as exemplified by red and green algae in marine environments, more recently derived groups have displaced previously ecologically dominant ones; for example, the ascendance of flowering plants over gymnosperms in terrestrial environments.


title: Evolutionary history of plants

text: There is evidence that cyanobacteria and multicellular photosynthetic eukaryotes lived in freshwater communities on land as early as 1 billion years ago, and that communities of complex, multicellular photosynthesizing organisms existed on land in the late Precambrian, around .


title: Evolutionary history of plants

text: Evidence of the emergence of embryophyte land plants first occurs in the mid-Ordovician (~), and by the middle of the Devonian (~), many of the features recognised in land plants today were present, including roots and leaves. By Late Devonian (~) some free-sporing plants such as "Archaeopteris" had secondary vascular tissue that produced wood and had formed forests of tall trees. Also by late Devonian, "Elkinsia", an early seed fern, had evolved seeds.


title: Evolutionary history of plants

text: Evolutionary innovation continued throughout the rest of the Phanerozoic eon and still continues today. Most plant groups were relatively unscathed by the Permo-Triassic extinction event, although the structures of communities changed. This may have set the scene for the appearance of the flowering plants in the Triassic (~), and their later diversification in the Cretaceous and Paleogene. The latest major group of plants to evolve were the grasses, which became important in the mid-Paleogene, from around . The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low and warm, dry conditions of the tropics over the last .


title: Evolutionary history of plants

text: Land plants evolved from a group of green algae, perhaps as early as 850 mya, but algae-like plants might have evolved as early as 1 billion years ago. The closest living relatives of land plants are the charophytes, specifically Charales; assuming that the habit of the Charales has changed little since the divergence of lineages, this means that the land plants evolved from a branched, filamentous alga dwelling in shallow fresh water, perhaps at the edge of seasonally desiccating pools. However, some recent evidence suggests that land plants might have originated from unicellular terrestrial charophytes similar to extant Klebsormidiophyceae. The alga would have had a haplontic life cycle. It would only very briefly have had paired chromosomes (the diploid condition) when the egg and sperm first fused to form a zygote that would have immediately divided by meiosis to produce cells with half the number of unpaired chromosomes (the haploid condition). Co-operative interactions with fungi may have helped early plants adapt to the stresses of the terrestrial realm.


title: Evolutionary history of plants

text: Plants were not the first photosynthesisers on land. Weathering rates suggest that organisms capable of photosynthesis were already living on the land , and microbial fossils have been found in freshwater lake deposits from , but the carbon isotope record suggests that they were too scarce to impact the atmospheric composition until around . These organisms, although phylogenetically diverse, were probably small and simple, forming little more than an algal scum.


title: Evolutionary history of plants

text: Evidence of the earliest land plants occurs much later at about 470Ma, in lower middle Ordovician rocks from Saudi Arabia and Gondwana in the form of spores with decay-resistant walls.


title: Evolutionary history of plants

text: These spores, known as cryptospores, were produced either singly (monads), in pairs (dyads) or groups of four (tetrads), and their microstructure resembles that of modern liverwort spores, suggesting they share an equivalent grade of organisation. Their walls contain sporopollenin – further evidence of an embryophytic affinity. It could be that atmospheric 'poisoning' prevented eukaryotes from colonising the land prior to this, or it could simply have taken a great time for the necessary complexity to evolve.


title: Evolutionary history of plants

text: Trilete spores similar to those of vascular plants appear soon afterwards, in Upper Ordovician rocks about 455 million years ago. Depending exactly when the tetrad splits, each of the four spores may bear a "trilete mark", a Y-shape, reflecting the points at which each cell squashed up against its neighbours. However, this requires that the spore walls be sturdy and resistant at an early stage. This resistance is closely associated with having a desiccation-resistant outer wall—a trait only of use when spores must survive out of water. Indeed, even those embryophytes that have returned to the water lack a resistant wall, thus don't bear trilete marks. A close examination of algal spores shows that none have trilete spores, either because their walls are not resistant enough, or, in those rare cases where they are, because the spores disperse before they are compressed enough to develop the mark or do not fit into a tetrahedral tetrad.


title: Evolutionary history of plants

text: The earliest megafossils of land plants were thalloid organisms, which dwelt in fluvial wetlands and are found to have covered most of an early Silurian flood plain. They could only survive when the land was waterlogged. There were also microbial mats.


title: Evolutionary history of plants

text: Once plants had reached the land, there were two approaches to dealing with desiccation. Modern bryophytes either avoid it or give in to it, restricting their ranges to moist settings or drying out and putting their metabolism "on hold" until more water arrives, as in the liverwort genus "Targionia". Tracheophytes resist desiccation by controlling the rate of water loss. They all bear a waterproof outer cuticle layer wherever they are exposed to air (as do some bryophytes), to reduce water loss, but since a total covering would cut them off from in the atmosphere tracheophytes use variable openings, the stomata, to regulate the rate of gas exchange. Tracheophytes also developed vascular tissue to aid in the movement of water within the organisms (see below), and moved away from a gametophyte dominated life cycle (see below). Vascular tissue ultimately also facilitated upright growth without the support of water and paved the way for the evolution of larger plants on land.


title: Evolutionary history of plants

text: A snowball earth, from around 720-635 mya in the Cryogenian period, is believed to have been caused by early photosynthetic organisms, which reduced the concentration of carbon dioxide and increased the amount of oxygen in the atmosphere. Based on molecular clock studies of the previous decade or so, a 2022 study observed that the estimated time for the origin of the "multicellular" streptophytes (all except the unicellular basal clade Mesostigmatophyceae) fell in the cool Cryogenian while that of the subsequent separation of streptophytes fell in the warm Ediacaran, which they interpreted as an indication of selective pressure by the glacial period to the photosynthesizing organisms, a group of which succeeded in surviving in relatively warmer edaphic refugia, subsequently flourishing in the later Ediacaran and Phanerozoic on land as embryophytes. The study also theorized that the unicellular morphology and other unique features of the Zygnematophyceae may reflect further adaptations to a cryophilic life. The establishment of a land-based flora increased the rate of accumulation of oxygen in the atmosphere, as the land plants produced oxygen as a waste product. When this concentration rose above 13%, around 0.45 billion years ago, wildfires became possible, evident from charcoal in the fossil record. Apart from a controversial gap in the Late Devonian, charcoal is present ever since.


title: Evolutionary history of plants

text: Charcoalification is an important taphonomic mode. Wildfire or burial in hot volcanic ash drives off the volatile compounds, leaving only a residue of pure carbon. This is not a viable food source for fungi, herbivores or detritovores, so it is prone to preservation. It is also robust and can withstand pressure, displaying exquisite, sometimes sub-cellular, detail in remains.


title: Evolutionary history of plants

text: All multicellular plants have a life cycle comprising two generations or phases. The gametophyte phase has a single set of chromosomes (denoted 1"n) and produces gametes (sperm and eggs). The sporophyte phase has paired chromosomes (denoted 2"n) and produces spores. The gametophyte and sporophyte phases may be homomorphic, appearing identical in some algae, such as "Ulva lactuca", but are very different in all modern land plants, a condition known as heteromorphy.


title: Evolutionary history of plants

text: The pattern in plant evolution has been a shift from homomorphy to heteromorphy. The algal ancestors of land plants were almost certainly haplobiontic, being haploid for all their life cycles, with a unicellular zygote providing the 2N stage. All land plants (i.e. embryophytes) are diplobiontic – that is, both the haploid and diploid stages are multicellular. Two trends are apparent: bryophytes (liverworts, mosses and hornworts) have developed the gametophyte as the dominant phase of the life cycle, with the sporophyte becoming almost entirely dependent on it; vascular plants have developed the sporophyte as the dominant phase, with the gametophytes being particularly reduced in the seed plants.


title: Evolutionary history of plants

text: It has been proposed as the basis for the emergence of the diploid phase of the life cycle as the dominant phase that diploidy allows masking of the expression of deleterious mutations through genetic complementation. Thus if one of the parental genomes in the diploid cells contains mutations leading to defects in one or more gene products, these deficiencies could be compensated for by the other parental genome (which nevertheless may have its own defects in other genes). As the diploid phase was becoming predominant, the masking effect likely allowed genome size, and hence information content, to increase without the constraint of having to improve accuracy of replication. The opportunity to increase information content at low cost is advantageous because it permits new adaptations to be encoded. This view has been challenged, with evidence showing that selection is no more effective in the haploid than in the diploid phases of the lifecycle of mosses and angiosperms.


title: Evolutionary history of plants

text: The interpolation theory (also known as the antithetic or intercalary theory) holds that the interpolation of a multicellular sporophyte phase between two successive gametophyte generations was an innovation caused by preceding meiosis in a freshly germinated zygote with one or more rounds of mitotic division, thereby producing some diploid multicellular tissue before finally meiosis produced spores. This theory implies that the first sporophytes bore a very different and simpler morphology to the gametophyte they depended on. This seems to fit well with what is known of the bryophytes, in which a vegetative thalloid gametophyte nurtures a simple sporophyte, which consists of little more than an unbranched sporangium on a stalk. Increasing complexity of the ancestrally simple sporophyte, including the eventual acquisition of photosynthetic cells, would free it from its dependence on a gametophyte, as seen in some hornworts ("Anthoceros"), and eventually result in the sporophyte developing organs and vascular tissue, and becoming the dominant phase, as in the tracheophytes (vascular plants). This theory may be supported by observations that smaller "Cooksonia" individuals must have been supported by a gametophyte generation. The observed appearance of larger axial sizes, with room for photosynthetic tissue and thus self-sustainability, provides a possible route for the development of a self-sufficient sporophyte phase.


title: Evolutionary history of plants

text: The alternative hypothesis, called the transformation theory (or homologous theory), posits that the sporophyte might have appeared suddenly by delaying the occurrence of meiosis until a fully developed multicellular sporophyte had formed. Since the same genetic material would be employed by both the haploid and diploid phases, they would look the same. This explains the behaviour of some algae, such as "Ulva lactuca", which produce alternating phases of identical sporophytes and gametophytes. Subsequent adaption to the desiccating land environment, which makes sexual reproduction difficult, might have resulted in the simplification of the sexually active gametophyte, and elaboration of the sporophyte phase to better disperse the waterproof spores. The tissue of sporophytes and gametophytes of vascular plants such as "Rhynia" preserved in the Rhynie chert is of similar complexity, which is taken to support this hypothesis. By contrast, modern vascular plants, with the exception of "Psilotum", have heteromorphic sporophytes and gametophytes in which the gametophytes rarely have any vascular tissue.


title: Evolutionary history of plants

text: There is no evidence that early land plants of the Silurian and early Devonian had roots, although fossil evidence of rhizoids occurs for several species, such as "Horneophyton". The earliest land plants did not have vascular systems for transport of water and nutrients either. "Aglaophyton", a rootless vascular plant known from Devonian fossils in the Rhynie chert was the first land plant discovered to have had a symbiotic relationship with fungi which formed arbuscular mycorrhizas, literally "tree-like fungal roots", in a well-defined cylinder of cells (ring in cross section) in the cortex of its stems. The fungi fed on the plant's sugars, in exchange for nutrients generated or extracted from the soil (especially phosphate), to which the plant would otherwise have had no access. Like other rootless land plants of the Silurian and early Devonian "Aglaophyton" may have relied on arbuscular mycorrhizal fungi for acquisition of water and nutrients from the soil.


title: Evolutionary history of plants

text: The fungi were of the phylum Glomeromycota, a group that probably first appeared 1 billion years ago and still forms arbuscular mycorrhizal associations today with all major land plant groups from bryophytes to pteridophytes, gymnosperms and angiosperms and with more than 80% of vascular plants.


title: Evolutionary history of plants

text: Evidence from DNA sequence analysis indicates that the arbuscular mycorrhizal mutualism arose in the common ancestor of these land plant groups during their transition to land and it may even have been the critical step that enabled them to colonise the land. Appearing as they did before these plants had evolved roots, mycorrhizal fungi would have assisted plants in the acquisition of water and mineral nutrients such as phosphorus, in exchange for organic compounds which they could not synthesize themselves. Such fungi increase the productivity even of simple plants such as liverworts.


title: Evolutionary history of plants

text: To photosynthesise, plants must absorb from the atmosphere. However, making the tissues available for to enter allows water to evaporate, so this comes at a price. Water is lost much faster than is absorbed, so plants need to replace it. Early land plants transported water apoplastically, within the porous walls of their cells. Later, they evolved three anatomical features that provided the ability to control the inevitable water loss that accompanied  acquisition.  First, a waterproof outer covering or cuticle evolved that reduced water loss. Secondly, variable apertures, the  stomata that could open and close to regulate the amount of water lost by evaporation during uptake and thirdly intercellular space between photosynthetic parenchyma cells that allowed improved internal distribution of the to the chloroplasts. This three-part system provided improved homoiohydry, the regulation of water content of the tissues, providing a particular advantage when water supply is not constant. The high concentrations of the Silurian and early Devonian, when plants were first colonising land, meant that they used water relatively efficiently. As was withdrawn from the atmosphere by plants, more water was lost in its capture, and more elegant water acquisition and transport mechanisms evolved. Plants growing upwards into the air needed a system for transporting water from the soil to all the different parts of the above-soil plant, especially to photosynthesising parts. By the end of the Carboniferous, when concentrations had been reduced to something approaching that of today, around 17 times more water was lost per unit of uptake. However, even in the "easy" early days, water was always at a premium, and had to be transported to parts of the plant from the wet soil to avoid desiccation.


title: Evolutionary history of plants

text: Water can be wicked by capillary action along a fabric with small spaces. In narrow columns of water, such as those within the plant cell walls or in tracheids, when molecules evaporate from one end, they pull the molecules behind them along the channels. Therefore, evaporation alone provides the driving force for water transport in plants.  However, without specialized transport vessels, this cohesion-tension mechanism can cause negative pressures sufficient to collapse water conducting cells, limiting the transport water to no more than a few cm, and therefore limiting the size of the earliest plants.


title: Evolutionary history of plants

text: To be free from the constraints of small size and constant moisture that the parenchymatic transport system inflicted, plants needed a more efficient water transport system.  As plants grew upwards, specialised water transport vascular tissues evolved, first in the form of simple hydroids of the type found in the setae of moss sporophytes. These simple elongated cells were dead and water-filled at maturity, providing a channel for water transport, but their thin, unreinforced walls would collapse under modest water tension, limiting the plant height.  Xylem tracheids, wider cells with lignin-reinforced cell walls that were more resistant to collapse under the tension caused by water stress, occur  in more than one plant group by mid-Silurian, and may have a single evolutionary origin, possibly within the hornworts, uniting all tracheophytes.  Alternatively,  they may have evolved more than once.  Much later, in the Cretaceous, tracheids were followed by vessels in flowering plants. As water transport mechanisms and waterproof cuticles evolved, plants could survive without being continually covered by a film of water. This transition from poikilohydry to homoiohydry opened up new potential for colonisation.


title: Evolutionary history of plants

text: The early Devonian pretracheophytes "Aglaophyton" and "Horneophyton" have unreinforced water transport tubes with wall structures very similar to moss hydroids, but they grew alongside several species of tracheophytes, such as "Rhynia gwynne-vaughanii" that had xylem tracheids that were well reinforced by bands of lignin. The earliest macrofossils known to have xylem tracheids are small, mid-Silurian plants of the genus "Cooksonia". However, thickened bands on the walls of isolated tube fragments are apparent from the early Silurian onwards.


title: Evolutionary history of plants

text: Plants continued to innovate ways of reducing the resistance to flow within their cells, progressively  increasing the efficiency of their water transport and to increase the resistance of the tracheids to collapse under tension. During the early Devonian, maximum tracheid diameter increased with time, but may have plateaued in the zosterophylls by mid-Devonian.  Overall transport rate also depends on the overall cross-sectional area of the xylem bundle itself, and some mid-Devonian plants, such as the Trimerophytes, had much larger steles than their early ancestors. While wider tracheids provided higher rates of water transport, they increased the risk of cavitation, the formation of air bubbles resulting from the breakage of the water column under tension. Small pits in tracheid walls allow water to by-pass a defective tracheid while preventing air bubbles from passing through but at the cost of restricted flow rates. By the Carboniferous, Gymnosperms had developed bordered pits, valve-like structures that allow high-conductivity  pits to seal when one side of a tracheid is depressurized.


title: Evolutionary history of plants

text: Tracheids have perforated end walls, which impose a great deal of resistance on water flow, but may have had the advantage of isolating air embolisms caused by cavitation or freezing. Vessels first evolved during the dry, low periods of the Late Permian, in the horsetails, ferns and Selaginellales independently, and later appeared in the mid Cretaceous in gnetophytes and angiosperms.  Vessel members are open  tubes with no end walls, and are arranged end to end  to operate as if they were one continuous vessel. Vessels allowed the same cross-sectional area of wood to transport much more water than tracheids. This allowed plants to fill more of their stems with structural fibres and also opened a new niche to vines, which could transport water without being as thick as the tree they grew on. Despite these advantages, tracheid-based wood is a lot lighter, thus cheaper to make, as vessels need to be much more reinforced to avoid cavitation. Once plants had evolved this level of control over water evaporation and water transport, they were truly homoiohydric, able to extract water from their environment through root-like organs rather than relying on a film of surface moisture, enabling them to grow to much greater size but as a result of their increased independence from their surroundings, most vascular plants lost their ability to survive desiccation - a costly trait to lose. In early land plants, support was mainly provided by turgor pressure, particularly of the outer layer of cells known as the sterome tracheids, and not by the xylem, which was too small, too weak and in too central a position to provide much structural support.  Plants with secondary xylem that  had appeared by mid-Devonian, such as the Trimerophytes and Progymnosperms had much larger vascular cross sections producing strong woody tissue.


title: Evolutionary history of plants

text: An endodermis may have evolved in the earliest plant roots during the Devonian, but the first fossil evidence for such a structure is Carboniferous. The endodermis in the roots surrounds the water transport tissue and regulates ion exchange between the groundwater and the tissues and prevents unwanted pathogens etc. from entering the water transport system. The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver.


title: Evolutionary history of plants

text: Leaves are the primary photosynthetic organs of a modern plant. The origin of leaves was almost certainly triggered by falling concentrations of atmospheric during the Devonian period, increasing the efficiency with which carbon dioxide could be captured for photosynthesis.


title: Evolutionary history of plants

text: Leaves certainly evolved more than once. Based on their structure, they are classified into two types: microphylls, which lack complex venation and may have originated as spiny outgrowths known as enations, and megaphylls, which are large and have complex venation that may have arisen from the modification of groups of branches. It has been proposed that these structures arose independently. Megaphylls, according to Walter Zimmerman's telome theory, have evolved from plants that showed a three-dimensional branching architecture, through three transformations—overtopping, which led to the lateral position typical of leaves, planation, which involved formation of a planar architecture, webbing or fusion, which united the planar branches, thus leading to the formation of a proper leaf lamina. All three steps happened multiple times in the evolution of today's leaves.


title: Evolutionary history of plants

text: It is widely believed that the telome theory is well supported by fossil evidence. However, Wolfgang Hagemann questioned it for morphological and ecological reasons and proposed an alternative theory. Whereas according to the telome theory the most primitive land plants have a three-dimensional branching system of radially symmetrical axes (telomes), according to Hagemann's alternative the opposite is proposed: the most primitive land plants that gave rise to vascular plants were flat, thalloid, leaf-like, without axes, somewhat like a liverwort or fern prothallus. Axes such as stems and roots evolved later as new organs. Rolf Sattler proposed an overarching process-oriented view that leaves some limited room for both the telome theory and Hagemann's alternative and in addition takes into consideration the whole continuum between dorsiventral (flat) and radial (cylindrical) structures that can be found in fossil and living land plants. This view is supported by research in molecular genetics. Thus, James (2009) concluded that "it is now widely accepted that... radiality [characteristic of axes such as stems] and dorsiventrality [characteristic of leaves] are but extremes of a continuous spectrum. In fact, it is simply the timing of the KNOX gene expression!"


title: Evolutionary history of plants

text: Before the evolution of leaves, plants had the photosynthetic apparatus on the stems, which they retain albeit leaves have largely assumed that job. Today's megaphyll leaves probably became commonplace some 360mya, about 40my after the simple leafless plants had colonized the land in the Early Devonian. This spread has been linked to the fall in the atmospheric carbon dioxide concentrations in the Late Paleozoic era associated with a rise in density of stomata on leaf surface. This would have resulted in greater transpiration rates and gas exchange, but especially at high concentrations, large leaves with fewer stomata would have heated to lethal temperatures in full sunlight. Increasing the stomatal density allowed for a better-cooled leaf, thus making its spread feasible, but increased uptake at the expense of decreased water use efficiency.


title: Evolutionary history of plants

text: The rhyniophytes of the Rhynie chert consisted of nothing more than slender, unornamented axes. The early to middle Devonian trimerophytes may be considered leafy. This group of vascular plants are recognisable by their masses of terminal sporangia, which adorn the ends of axes which may bifurcate or trifurcate. Some organisms, such as "Psilophyton", bore enations. These are small, spiny outgrowths of the stem, lacking their own vascular supply.


title: Evolutionary history of plants

text: The zosterophylls were already important in the late Silurian, much earlier than any rhyniophytes of comparable complexity. This group, recognisable by their kidney-shaped sporangia which grew on short lateral branches close to the main axes, sometimes branched in a distinctive H-shape. Many zosterophylls bore pronounced spines on their axes but none of these had a vascular trace. The first evidence of vascularised enations occurs in a fossil clubmoss known as "Baragwanathia" that had already appeared in the fossil record in the Late Silurian. In this organism, these leaf traces continue into the leaf to form their mid-vein. One theory, the "enation theory", holds that the microphyllous leaves of clubmosses developed by outgrowths of the protostele connecting with existing enations The leaves of the Rhynie genus "Asteroxylon", which was preserved in the Rhynie chert almost 20 Million years later than "Baragwanathia" had a primitive vascular supply – in the form of leaf traces departing from the central protostele towards each individual "leaf". "Asteroxylon" and "Baragwanathia" are widely regarded as primitive lycopods, a group still extant today, represented by the quillworts, the spikemosses and the club mosses. Lycopods bear distinctive microphylls, defined as leaves with a single vascular trace. Microphylls could grow to some size, those of Lepidodendrales reaching over a meter in length, but almost all just bear the one vascular bundle. An exception is the rare branching in some "Selaginella" species.


title: Evolutionary history of plants

text: The more familiar leaves, megaphylls, are thought to have originated four times independently, in the ferns, horsetails, progymnosperms and seed plants. They appear to have originated by modifying dichotomising branches, which first overlapped (or "overtopped") one another, became flattened or planated and eventually developed "webbing" and evolved gradually into more leaf-like structures. Megaphylls, by Zimmerman's telome theory, are composed of a group of webbed branches and hence the "leaf gap" left where the leaf's vascular bundle leaves that of the main branch resembles two axes splitting. In each of the four groups to evolve megaphylls, their leaves first evolved during the Late Devonian to Early Carboniferous, diversifying rapidly until the designs settled down in the mid Carboniferous.


title: Evolutionary history of plants

text: The cessation of further diversification can be attributed to developmental constraints, but why did it take so long for leaves to evolve in the first place? Plants had been on the land for at least 50 million years before megaphylls became significant. However, small, rare mesophylls are known from the early Devonian genus "Eophyllophyton" – so development could not have been a barrier to their appearance. The best explanation so far incorporates observations that atmospheric was declining rapidly during this time – falling by around 90% during the Devonian. This required an increase in stomatal density by 100 times to maintain rates of photosynthesis. When stomata open to allow water to evaporate from leaves it has a cooling effect, resulting from the loss of latent heat of evaporation. It appears that the low stomatal density in the early Devonian meant that evaporation and evaporative cooling were limited, and that leaves would have overheated if they grew to any size. The stomatal density could not increase, as the primitive steles and limited root systems would not be able to supply water quickly enough to match the rate of transpiration. Clearly, leaves are not always beneficial, as illustrated by the frequent occurrence of secondary loss of leaves, famously exemplified by cacti and the "whisk fern" "Psilotum".


title: Evolutionary history of plants

text: Secondary evolution can also disguise the true evolutionary origin of some leaves. Some genera of ferns display complex leaves which are attached to the pseudostele by an outgrowth of the vascular bundle, leaving no leaf gap. Further, horsetail ("Equisetum") leaves bear only a single vein, and appear to be microphyllous; however, both the fossil record and molecular evidence indicate that their forebears bore leaves with complex venation, and the current state is a result of secondary simplification.


title: Evolutionary history of plants

text: Deciduous trees deal with another disadvantage to having leaves. The popular belief that plants shed their leaves when the days get too short is misguided; evergreens prospered in the Arctic circle during the most recent greenhouse earth. The generally accepted reason for shedding leaves during winter is to cope with the weather – the force of wind and weight of snow are much more comfortably weathered without leaves to increase surface area. Seasonal leaf loss has evolved independently several times and is exhibited in the ginkgoales, some pinophyta and certain angiosperms. Leaf loss may also have arisen as a response to pressure from insects; it may have been less costly to lose leaves entirely during the winter or dry season than to continue investing resources in their repair.


title: Evolutionary history of plants

text: Various physical and physiological factors such as light intensity, humidity, temperature, wind speeds etc. have influenced evolution of leaf shape and size. High trees rarely have large leaves, because they are damaged by high winds. Similarly, trees that grow in temperate or taiga regions have pointed leaves presumably to prevent nucleation of ice onto the leaf surface and reduce water loss due to transpiration. Herbivory, by mammals and insects, has been a driving force in leaf evolution. An example is that plants of the New Zealand genus "Aciphylla" have spines on their laminas, which probably functioned to discourage the extinct Moas from feeding on them. Other members of "Aciphylla", which did not co-exist with the moas, do not have these spines.


title: Evolutionary history of plants

text: At the genetic level, developmental studies have shown that repression of KNOX genes is required for initiation of the leaf primordium. This is brought about by "ARP" genes, which encode transcription factors. Repression of KNOX genes in leaf primordia seems to be quite conserved, while expression of KNOX genes in leaves produces complex leaves. The "ARP" function appears to have arisen early in vascular plant evolution, because members of the primitive group Lycophytes also have a functionally similar gene. Other players that have a conserved role in defining leaf primordia are the phytohormones auxin, gibberelin and cytokinin.


title: Evolutionary history of plants

text: The arrangement of leaves or phyllotaxy on the plant body can maximally harvest light and might be expected to be genetically robust. However, in maize, a mutation in only one gene called "ABPHYL" "(ABnormal PHYLlotaxy)" is enough to change the phyllotaxy of the leaves, implying that mutational adjustment of a single locus on the genome is enough to generate diversity.


title: Evolutionary history of plants

text: Once the leaf primordial cells are established from the SAM cells, the new axes for leaf growth are defined, among them being the abaxial-adaxial (lower-upper surface) axes. The genes involved in defining this, and the other axes seem to be more or less conserved among higher plants. Proteins of the "HD-ZIPIII" family have been implicated in defining the adaxial identity. These proteins deviate some cells in the leaf primordium from the default abaxial state, and make them adaxial. In early plants with leaves, the leaves probably just had one type of surface — the abaxial one, the underside of today's leaves. The definition of the adaxial identity occurred some 200 million years after the abaxial identity was established.


title: Evolutionary history of plants

text: How the wide variety of observed plant leaf morphology is generated is a subject of intense research. Some common themes have emerged. One of the most significant is the involvement of KNOX genes in generating compound leaves, as in the tomato "(see above)". But, this is not universal. For example, the pea uses a different mechanism for doing the same thing. Mutations in genes affecting leaf curvature can also change leaf form, by changing the leaf from flat, to a crinkly shape, like the shape of cabbage leaves. There also exist different morphogen gradients in a developing leaf which define the leaf's axis and may also affect the leaf form. Another class of regulators of leaf development are the microRNAs.


title: Evolutionary history of plants

text: Roots are important to plants for two main reasons: Firstly, they provide anchorage to the substrate; more importantly, they provide a source of water and nutrients from the soil. Roots allowed plants to grow taller and faster.


title: Evolutionary history of plants

text: The evolution of roots had consequences on a global scale. By disturbing the soil and promoting its acidification (by taking up nutrients such as nitrate and phosphate), they enabled it to weather more deeply, injecting carbon compounds deeper into soils with huge implications for climate. These effects may have been so profound they led to a mass extinction.


title: Evolutionary history of plants

text: While there are traces of root-like impressions in fossil soils in the Late Silurian, body fossils show the earliest plants to be devoid of roots. Many had prostrate branches that sprawled along the ground, with upright axes or thalli dotted here and there, and some even had non-photosynthetic subterranean branches which lacked stomata. The distinction between root and specialised branch is developmental. differing in their branching pattern and in possession of a root cap. So while Siluro-Devonian plants such as "Rhynia" and "Horneophyton" possessed the physiological equivalent of roots, roots – defined as organs differentiated from stems – did not arrive until later. Unfortunately, roots are rarely preserved in the fossil record, and our understanding of their evolutionary origin is sparse.


title: Evolutionary history of plants

text: Rhizoids – small structures performing the same role as roots, usually a cell in diameter – probably evolved very early, perhaps even before plants colonised the land; they are recognised in the Characeae, an algal sister group to land plants. That said, rhizoids probably evolved more than once; the rhizines of lichens, for example, perform a similar role. Even some animals ("Lamellibrachia") have root-like structures. Rhizoids are clearly visible in the Rhynie chert fossils, and were present in most of the earliest vascular plants, and on this basis seem to have presaged true plant roots.


title: Evolutionary history of plants

text: More advanced structures are common in the Rhynie chert, and many other fossils of comparable early Devonian age bear structures that look like, and acted like, roots. The rhyniophytes bore fine rhizoids, and the trimerophytes and herbaceous lycopods of the chert bore root-like structure penetrating a few centimetres into the soil. However, none of these fossils display all the features borne by modern roots, with the exception of "Asteroxylon", which has recently been recognized as bearing roots that evolved independently from those of extant vascular plants. Roots and root-like structures became increasingly common and deeper penetrating during the Devonian, with lycopod trees forming roots around 20 cm long during the Eifelian and Givetian. These were joined by progymnosperms, which rooted up to about a metre deep, during the ensuing Frasnian stage. True gymnosperms and zygopterid ferns also formed shallow rooting systems during the Famennian.


title: Evolutionary history of plants

text: The rhizophores of the lycopods provide a slightly different approach to rooting. They were equivalent to stems, with organs equivalent to leaves performing the role of rootlets. A similar construction is observed in the extant lycopod "Isoetes", and this appears to be evidence that roots evolved independently at least twice, in the lycophytes and other plants, a proposition supported by studies showing that roots are initiated and their growth promoted by different mechanisms in lycophytes and euphyllophytes.


title: Evolutionary history of plants

text: A vascular system is indispensable to rooted plants, as non-photosynthesising roots need a supply of sugars, and a vascular system is required to transport water and nutrients from the roots to the rest of the plant. Rooted plants are little more advanced than their Silurian forebears, without a dedicated root system; however, the flat-lying axes can be clearly seen to have growths similar to the rhizoids of bryophytes today.


title: Evolutionary history of plants

text: By the Middle to Late Devonian, most groups of plants had independently developed a rooting system of some nature. As roots became larger, they could support larger trees, and the soil was weathered to a greater depth. This deeper weathering had effects not only on the aforementioned drawdown of , but also opened up new habitats for colonisation by fungi and animals.


title: Evolutionary history of plants

text: Roots today have developed to the physical limits. They penetrate as much as 60 metres of soil to tap the water table. The narrowest roots are a mere 40 μm in diameter, and could not physically transport water if they were any narrower.


title: Evolutionary history of plants

text: The earliest fossil roots recovered, by contrast, narrowed from 3 mm to under 700 μm in diameter; of course, taphonomy is the ultimate control of what thickness can be seen.


title: Evolutionary history of plants

text: The early Devonian landscape was devoid of vegetation taller than waist height. Greater height provided a competitive advantage in the harvesting of sunlight for photosynthesis, overshadowing of competitors and in spore distribution, as spores (and later, seeds) could be blown for greater distances if they started higher. An effective vascular system was required in order to achieve greater heights. To attain arborescence, plants had to develop woody tissue that provided both support and water transport, and thus needed to evolve the capacity for secondary growth. The stele of plants undergoing secondary growth is surrounded by a vascular cambium, a ring of meristematic cells which produces more xylem on the inside and phloem on the outside. Since xylem cells comprise dead, lignified tissue, subsequent rings of xylem are added to those already present, forming wood. Fossils of plants from the early Devonian show that a simple form of wood first appeared at least 400 million years ago, at a time when all land plants were small and herbaceous. Because wood evolved long before shrubs and trees, it is likely that its original purpose was for water transport, and that it was only used for mechanical support later.


title: Evolutionary history of plants

text: The first plants to develop secondary growth and a woody habit, were apparently the ferns, and as early as the Middle Devonian one species, "Wattieza", had already reached heights of 8 m and a tree-like habit.


title: Evolutionary history of plants

text: Other clades did not take long to develop a tree-like stature. The Late Devonian "Archaeopteris", a precursor to gymnosperms which evolved from the trimerophytes, reached 30 m in height. The progymnosperms were the first plants to develop true wood, grown from a bifacial cambium. The first appearance of one of them, "Rellimia", was in the Middle Devonian. True wood is only thought to have evolved once, giving rise to the concept of a "lignophyte" clade.


title: Evolutionary history of plants

text: "Archaeopteris" forests were soon supplemented by arborescent lycopods, in the form of Lepidodendrales, which exceeded 50m in height and 2m across at the base. These arborescent lycopods rose to dominate Late Devonian and Carboniferous forests that gave rise to coal deposits. Lepidodendrales differ from modern trees in exhibiting determinate growth: after building up a reserve of nutrients at a lower height, the plants would "bolt" as a single trunk to a genetically determined height, branch at that level, spread their spores and die. They consisted of "cheap" wood to allow their rapid growth, with at least half of their stems comprising a pith-filled cavity. Their wood was also generated by a unifacial vascular cambium – it did not produce new phloem, meaning that the trunks could not grow wider over time.


title: Evolutionary history of plants

text: The horsetail "Calamites" appeared in the Carboniferous. Unlike the modern horsetail "Equisetum", "Calamites" had a unifacial vascular cambium, allowing them to develop wood and grow to heights in excess of 10 m and to branch repeatedly.


title: Evolutionary history of plants

text: While the form of early trees was similar to that of today's, the Spermatophytes or seed plants, the group that contain all modern trees, had yet to evolve. The dominant tree groups today are all seed plants, the gymnosperms, which include the coniferous trees, and the angiosperms, which contain all fruiting and flowering trees. No free-sporing trees like "Archaeopteris" exist in the extant flora. It was long thought that the angiosperms arose from within the gymnosperms, but recent molecular evidence suggests that their living representatives form two distinct groups. The molecular data has yet to be fully reconciled with morphological data, but it is becoming accepted that the morphological support for paraphyly is not especially strong.


title: Evolutionary history of plants

text: This would lead to the conclusion that both groups arose from within the pteridosperms, probably as early as the Permian.


title: Evolutionary history of plants

text: The angiosperms and their ancestors played a very small role until they diversified during the Cretaceous. They started out as small, damp-loving organisms in the understorey, and have been diversifying ever since the Cretaceous, to become the dominant member of non-boreal forests today.


title: Evolutionary history of plants

text: Early land plants reproduced in the fashion of ferns: spores germinated into small gametophytes, which produced eggs and/or sperm. These sperm would swim across moist soils to find the female organs (archegonia) on the same or another gametophyte, where they would fuse with an egg to produce an embryo, which would germinate into a sporophyte.


title: Evolutionary history of plants

text: Heterosporic plants, as their name suggests, bear spores of two sizes – microspores and megaspores. These would germinate to form microgametophytes and megagametophytes, respectively. This system paved the way for ovules and seeds: taken to the extreme, the megasporangia could bear only a single megaspore tetrad, and to complete the transition to true ovules, three of the megaspores in the original tetrad could be aborted, leaving one megaspore per megasporangium.


title: Evolutionary history of plants

text: The transition to ovules continued with this megaspore being "boxed in" to its sporangium while it germinated. Then, the megagametophyte was contained within a waterproof integument, which enclosed the seed. The pollen grain, which contained a microgametophyte germinated from a microspore , was employed for dispersal of the male gamete, only releasing its desiccation-prone flagellate sperm when it reached a receptive megagametophyte.


title: Evolutionary history of plants

text: Lycopods and sphenopsids got a fair way down the path to the seed habit without ever crossing the threshold. Fossil lycopod megaspores reaching 1 cm in diameter, and surrounded by vegetative tissue, are known (Lepidocarpon, Achlamydocarpon);– these even germinated into a megagametophyte "in situ". However, they fell short of being ovules, since the nucellus, an inner spore-covering layer, does not completely enclose the spore. A very small slit (micropyle) remains, meaning that the megasporangium is still exposed to the atmosphere. This has two consequences – firstly, it means it is not fully resistant to desiccation, and secondly, sperm do not have to "burrow" to access the archegonia of the megaspore.


title: Evolutionary history of plants

text: A Middle Devonian precursor to seed plants from Belgium has been identified predating the earliest seed plants by about 20 million years. "Runcaria", small and radially symmetrical, is an integumented megasporangium surrounded by a cupule. The megasporangium bears an unopened distal extension protruding above the multilobed integument. It is suspected that the extension was involved in anemophilous pollination. "Runcaria" sheds new light on the sequence of character acquisition leading to the seed. "Runcaria" has all of the qualities of seed plants except for a solid seed coat and a system to guide the pollen to the ovule.


title: Evolutionary history of plants

text: The first spermatophytes (literally: "seed plants") – that is, the first plants to bear true seeds – are called pteridosperms: literally, "seed ferns", so called because their foliage consisted of fern-like fronds, although they were not closely related to ferns. The oldest fossil evidence of seed plants is of Late Devonian age, and they appear to have evolved out of an earlier group known as the progymnosperms. These early seed plants ranged from trees to small, rambling shrubs; like most early progymnosperms, they were woody plants with fern-like foliage. They all bore ovules, but no cones, fruit or similar. While it is difficult to track the early evolution of seeds, the lineage of the seed ferns may be traced from the simple trimerophytes through homosporous Aneurophytes.


title: Evolutionary history of plants

text: This seed model is shared by basically all gymnosperms (literally: "naked seeds"), most of which encase their seeds in a woody cone or fleshy aril (the yew, for example), but none of which fully enclose their seeds. The angiosperms ("vessel seeds") are the only group to fully enclose the seed, in a carpel.


title: Evolutionary history of plants

text: Fully enclosed seeds opened up a new pathway for plants to follow: that of seed dormancy. The embryo, completely isolated from the external atmosphere and hence protected from desiccation, could survive some years of drought before germinating.


title: Evolutionary history of plants

text: Gymnosperm seeds from the Late Carboniferous have been found to contain embryos, suggesting a lengthy gap between fertilisation and germination. This period is associated with the entry into a greenhouse earth period, with an associated increase in aridity. This suggests that dormancy arose as a response to drier climatic conditions, where it became advantageous to wait for a moist period before germinating. This evolutionary breakthrough appears to have opened a floodgate: previously inhospitable areas, such as dry mountain slopes, could now be tolerated, and were soon covered by trees.


title: Evolutionary history of plants

text: Seeds offered further advantages to their bearers: they increased the success rate of fertilised gametophytes, and because a nutrient store could be "packaged" in with the embryo, the seeds could germinate rapidly in inhospitable environments, reaching a size where it could fend for itself more quickly. For example, without an endosperm, seedlings growing in arid environments would not have the reserves to grow roots deep enough to reach the water table before they expired from dehydration. Likewise, seeds germinating in a gloomy understory require an additional reserve of energy to quickly grow high enough to capture sufficient light for self-sustenance.


title: Evolutionary history of plants

text: A combination of these advantages gave seed plants the ecological edge over the previously dominant genus "Archaeopteris", thus increasing the biodiversity of early forests.


title: Evolutionary history of plants

text: Despite these advantages, it is common for fertilized ovules to fail to mature as seeds. Also during seed dormancy (often associated with unpredictable and stressful conditions) DNA damage accumulates. Thus DNA damage appears to be a basic problem for survival of seed plants, just as DNA damage is a A major problem for life in general.


title: Evolutionary history of plants

text: Flowers are modified leaves possessed only by the angiosperms, which are relatively late to appear in the fossil record. The group originated and diversified during the Early Cretaceous and became ecologically significant thereafter. Flower-like structures first appear in the fossil records some ~130 mya, in the Cretaceous. However, in 2018, scientists reported the finding of a fossil flower from about 180 million years ago, 50 million years earlier than thought earlier. The interpretation has been however highly disputed.


title: Evolutionary history of plants

text: Colorful and/or pungent structures surround the cones of plants such as cycads and Gnetales, making a strict definition of the term "flower" elusive.


title: Evolutionary history of plants

text: The main function of a flower is reproduction, which, before the evolution of the flower and angiosperms, was the job of microsporophylls and megasporophylls. A flower can be considered a powerful evolutionary innovation, because its presence allowed the plant world to access new means and mechanisms for reproduction.


title: Evolutionary history of plants

text: The flowering plants have long been assumed to have evolved from within the gymnosperms; according to the traditional morphological view, they are closely allied to the Gnetales. However, as noted above, recent molecular evidence is at odds with this hypothesis, and further suggests that Gnetales are more closely related to some gymnosperm groups than angiosperms, and that extant gymnosperms form a distinct clade to the angiosperms, the two clades diverging some .


title: Evolutionary history of plants

text: The relationship of stem groups to the angiosperms is important in determining the evolution of flowers. Stem groups provide an insight into the state of earlier "forks" on the path to the current state. Convergence increases the risk of misidentifying stem groups. Since the protection of the megagametophyte is evolutionarily desirable, probably many separate groups evolved protective encasements independently. In flowers, this protection takes the form of a carpel, evolved from a leaf and recruited into a protective role, shielding the ovules. These ovules are further protected by a double-walled integument.


title: Evolutionary history of plants

text: Penetration of these protective layers needs something more than a free-floating microgametophyte. Angiosperms have pollen grains comprising just three cells. One cell is responsible for drilling down through the integuments, and creating a conduit for the two sperm cells to flow down. The megagametophyte has just seven cells; of these, one fuses with a sperm cell, forming the nucleus of the egg itself, and another joins with the other sperm, and dedicates itself to forming a nutrient-rich endosperm. The other cells take auxiliary roles. This process of "double fertilisation" is unique and common to all angiosperms.


title: Evolutionary history of plants

text: In the fossil record, there are three intriguing groups which bore flower-like structures. The first is the Permian pteridosperm "Glossopteris", which already bore recurved leaves resembling carpels. The Mesozoic "Caytonia" is more flower-like still, with enclosed ovules – but only a single integument. Further, details of their pollen and stamens set them apart from true flowering plants.