AHS Centaur

Australian Hospital Ship (AHS) Centaur was a hospital ship which was attacked and sunk by a Japanese submarine off the coast of Queensland, Australia, on 14 May 1943. Of the 332 medical personnel and civilian crew aboard, 268 died, including 63 of the 65 army personnel.

The Scottish-built vessel was launched in 1924 as a combination passenger liner and refrigerated cargo ship and operated a trade route between Western Australia and Singapore via the Dutch East Indies (now Indonesia), carrying passengers, cargo, and livestock. At the start of World War II, Centaur (like all British Merchant Navy vessels) was placed under British Admiralty control, but after being fitted with defensive equipment, was allowed to continue normal operations. In November 1941, the ship rescued German survivors of the engagement between Kormoran and HMAS Sydney. Centaur was relocated to Australia's east coast in October 1942, and used to transport materiel to New Guinea.

In January 1943, Centaur was handed over to the Australian military for conversion to a hospital ship, as her small size made her suitable for operating in Maritime Southeast Asia. The refit (including installation of medical facilities and repainting with Red Cross markings) was completed in March, and the ship undertook a trial voyage: transporting wounded from Townsville to Brisbane, then from Port Moresby to Brisbane. After replenishing in Sydney, Centaur embarked the 2/12th Field Ambulance for transport to New Guinea, and sailed on 12 May. Before dawn on 14 May 1943, during her second voyage, Centaur was torpedoed and sunk by a Japanese submarine off Moreton Island, Queensland. The majority of the 332 aboard died in the attack; the 64 survivors were discovered 36 hours later. The incident resulted in public outrage as attacking a hospital ship is considered a war crime under the 1907 Hague Convention. Protests were made by the Australian and British governments to Japan and efforts were made to discover the people responsible so they could be tried at a war crimes tribunal. In the 1970s the probable identity of the attacking submarine, I-177, became public.

The reason for the attack is unknown; there are theories that Centaur was in breach of the international conventions that should have protected her, that I-177's commander was unaware that Centaur was a hospital ship, or that the submarine commander, Hajime Nakagawa, knowingly attacked a protected vessel. The wreck of Centaur was found on 20 December 2009; a claimed discovery in 1995 has been proven to be a different shipwreck.

## Design and construction

### Original design

In early 1923, the Ocean Steamship Company (a subsidiary of Alfred Holt's Blue Funnel Line) decided that a new vessel would be required to replace the ageing Charon on the Western Australia to Singapore trade route. The vessel had to be capable of simultaneously transporting passengers, cargo, and livestock. She also had to be capable of resting on mud flats out of the water as the tidal variance in ports at the northern end of Western Australia was as great as 8 metres (26 ft).

Scotts Shipbuilding and Engineering Company in Greenock was chosen to build Centaur. The keel was laid on 16 November 1923, and the ship was ready for collection by 29 August 1924. Constructed at a cost of £146,750 sterling, Centaur was designed to carry 72 passengers and 450 cattle. Cargo was carried in four holds; the two decks within the hull were primarily for livestock, and could also be used as extra cargo space. The hull of the ship was a 'turret deck' design; decks below the waterline were wider than those above water, and a flat, reinforced hull allowed the ship to rest on the bottom. Centaur was among the first civilian vessels to be equipped with a diesel engine. One of the most visible characteristics was the 35-foot (11 m) smokestack, the extreme size was more a concession to tradition than of practical advantage on a diesel-powered vessel. Her engine was 6-cylinder 4-stroke, single cycle single action diesel engine. It had cylinders of 2415⁄16 inches (64 cm) diameter by 513⁄16 inches (135 cm) stroke. The engine was built by Burmeister & Wain, Copenhagen, Denmark. One of her holds was fitted with refrigeration equipment. The refrigerant was brine and the insulation was cork. The refrigerated hold had a capacity of 3,000 cubic feet (85 m3).

In December 1939, Centaur underwent a minor refit in Hong Kong, with a supercharger and a new propeller fitted to the engine. The supercharger broke down in April 1942, and could not be repaired because of equipment shortages and restricted dockyard access caused by World War II.

### Hospital ship refit

At the beginning of 1943, Centaur was placed at the disposal of the Australian Department of Defence for conversion to a hospital ship. The conversion was performed by United Ship Services in Melbourne, Australia, and was initially estimated to cost AU£20,000.

The cost increased to almost AU£55,000, for a variety of reasons. It was originally intended for the ship to travel between ports in New Guinea and Townsville, Queensland, Australia. Increasing casualty numbers in the New Guinea campaign meant that the hospitals in Queensland would quickly become unable to deal with the quantity of the casualties and the nature of their injuries, so a longer voyage to Sydney was required. The Army demanded that more facilities and conversions be added to the original plans such as expanded bathing and washing facilities, hot water made available to all parts of the ship through installation of a calorifier, the rerouting of all steam pipes away from patient areas, and ventilation arrangements suitable for tropical conditions. The unions representing the ship's crew requested improved living and dining conditions, including new sinks in the food preparation areas and the replacement of flooring in the quarters and mess rooms.

When AHS Centaur was relaunched on 12 March 1943, she was equipped with an operating theatre, dispensary, two wards (located on the former cattle decks), and a dental surgery, along with quarters for seventy five crew and sixty five permanent Army medical staff. To maintain the ship's mean draught of 6.1 metres (20 ft), 900 tons of ironstone were distributed through the cargo holds as ballast. AHS Centaur was capable of voyages of 18 days before resupply and could carry just over 250 bedridden patients.

## Operational history

### 1924 to 1938

Centaur was allocated the United Kingdom Official Number 147275 and the Code Letters KHHC. Her port of registry was Liverpool. When Centaur entered service at the end of 1924, the Fremantle–Java– Singapore trade route was being serviced by two other Blue Funnel Line vessels; Gorgon (which remained in service until 1928) and Charon (which Centaur was replacing). Centaur's route ran from Fremantle up the Western Australian coast calling at Geraldton, Carnarvon, Onslow, Point Samson, Port Hedland, Broome, and Derby then to the Bali Strait, Surabaya, Semarang, Batavia, and Singapore. Centaur operated as a cross between a tramp steamer and a freight liner; she travelled a set route, but stops at ports along the route varied between journeys. From 1928 until sometime in the 1930s, Centaur remained alone on her route, but the increase in trade along this route prompted Blue Funnel Line to reassign Gorgon and assign the new Charon to work alongside Centaur.

Following the change in Code Letters in 1934, Centaur was allocated the Code Letters GMQP. A highlight of Centaur'''s pre-war career was the rescue of the 385 ton Japanese whale-chaser Kyo Maru II in November 1938. Kyo Maru II had developed boiler problems while returning from the Antarctic and was drifting towards the Houtman Abrolhos Archipelago, where she was in danger of being wrecked by the reefs in the area. Centaur responded to the distress signal and towed Kyo Maru II to Geraldton.

### 1939 to 1942

As a vessel of the British Merchant Navy, Centaur was affected by the British Parliament's 1939 outline of how the Merchant Navy would respond to the declaration of war, primarily submission to the Admiralty in all matters excluding the crewing and management of vessels. Following the outbreak of World War II on 3 September 1939, Centaur was equipped with a stern-mounted 4-inch (100 mm) Mark IX naval gun and two .303 Vickers machine guns located on the bridge wings for protection against Axis warships and aircraft. She was also fitted with port and starboard paravanes and degaussing equipment for protection against naval mines. The weapons were removed during the hospital ship refit, although the anti-mine countermeasures remained. Centaur initially remained in service on her original trade route.

On 26 November 1941, a damaged lifeboat carrying 62 Kriegsmarine (German navy) sailors and officers was spotted by an aircraft looking for the missing Australian cruiser ; the aircraft directed Centaur to the lifeboat. Upon encountering the lifeboat, food was lowered to its occupants, and one person was allowed on board to explain the situation. Initially posing as a Norwegian merchant navy officer, the man quickly revealed that he was the first officer of the German auxiliary cruiser Kormoran and that the lifeboat contained German survivors from Kormoran's battle with HMAS Sydney seven days earlier, including Captain Theodor Detmers.

Unwilling to leave the shipwrecked men at sea, but afraid of having his ship captured by the Germans, Centaur's master decided to take the lifeboat in tow, after allowing nine wounded men aboard. During the tow towards Carnarvon, Western Australia, the lifeboat was swamped and partially sunk by rough seas, so two of Centaur's lifeboats were lowered to carry the Germans. On arrival in Carnarvon, the Germans were relocated to the number one cargo hold, where they were joined by another hundred Kormoran survivors collected by other ships, plus forty Australian Army guards, which were then transported by Centaur to Fremantle.

Following the Japanese attack on Pearl Harbor and the beginning of the Malayan Campaign on 7 December 1941, Centaur's run was curtailed to Broome, Western Australia. On 6 October 1942, Centaur was ordered to sail to Queensland, where she began runs between the east coast of Australia and New Guinea, carrying materiel.

### 1943

With the commencement of hostilities between Japan and the British Empire, it became clear that the three hospital ships currently serving Australia—Manunda, Wanganella, and Oranje—would not be able to operate in the shallow waters typical of Maritime Southeast Asia, so a new hospital ship was required. Of the Australian Merchant Navy vessels able to operate in this region, none were suitable for conversion to a hospital ship, and a request to the British Ministry of Shipping placed Centaur at the disposal of the Australian military on 4 January 1943. The conversion work began on 9 January and Centaur was commissioned as an Australian Hospital Ship on 1 March. During her conversion, Centaur was painted with the markings of a hospital ship as detailed in Article 5 of the tenth Hague Convention of 1907 ("Adaptation to Maritime War of the Principles of the Geneva Convention"); white hull with a green band interspersed by three red crosses on each flank of the hull, white superstructure, multiple large red crosses positioned so that the ship's status would be visible from both sea and air, and an identification number (for Centaur, 47) on her bows. At night, the markings were illuminated by a combination of internal and external lights. Data on the ship's markings and the layout of identifying structural features was provided to the International Committee of the Red Cross during the first week of February 1943, who passed this on to the Japanese on 5 February. This information was also circulated and promoted by the press and media.

Centaur entered operation as a hospital ship on 12 March 1943. The early stages of Centaur's first voyage as a hospital ship were test and transport runs; the initial run from Melbourne to Sydney resulted in the Master, Chief Engineer, and Chief Medical Officer composing a long list of defects requiring attention. Following repairs, she conducted a test run, transporting wounded servicemen from Townsville to Brisbane to ensure that she was capable of fulfilling the role of a medical vessel. Centaur was then tasked with delivering medical personnel to Port Moresby, New Guinea, and returning to Brisbane with Australian and American wounded along with a few wounded Japanese prisoners of war.

Arriving in Sydney on 8 May 1943, Centaur was re-provisioned at Darling Harbour, before departing for Cairns, Queensland on 12 May 1943. From there, her destination was again New Guinea. On board at the time were 74 civilian crew, 53 Australian Army Medical Corps personnel (including 8 officers), 12 female nurses from the Australian Army Nursing Service, 192 soldiers from the 2/12th Field Ambulance, and one Torres Strait ship pilot. Most of the female nurses had transferred from the hospital ship Oranje, and the male Army personnel assigned to the ship aboard were all medical staff. During the loading process, there was an incident when the ambulance drivers attached to the 2/12th attempted to bring their rifles and personal supplies of ammunition aboard. This was met with disapproval from Centaur's Master and Chief Medical Officer, and raised concerns amongst the crew and wharf labourers that Centaur would be transporting military supplies or commandos to New Guinea: the rifles were not allowed on board until Centaur's Master received official reassurance that the ambulance drivers were allowed to carry weapons under the 10th Hague Convention (specifically Article 8), as they were used "for the maintenance of order and the defence of the wounded." The remaining cargo was searched by the crew and labourers for other weapons and munitions.

## Sinking

At approximately 4:10 am on 14 May 1943, while on her second run from Sydney to Port Moresby, Centaur was torpedoed by an unsighted submarine. The torpedo struck the portside oil fuel tank approximately 2 metres (6 ft 7 in) below the waterline, creating a hole 8 to 10 metres (26 to 33 ft) across, igniting the fuel, and setting the ship on fire from the bridge aft. Many of those on board were immediately killed by concussion or perished in the inferno. Centaur quickly took on water through the impact site, rolled to port, then sank bow-first, submerging completely in less than three minutes. The rapid sinking prevented the deployment of lifeboats, although two broke off from Centaur as she sank, along with several damaged liferafts.

According to the position extrapolated by Second Officer Gordon Rippon from the 4:00 am dead reckoning position, Centaur was attacked approximately 24 nautical miles (44 km; 28 mi) east-northeast of Point Lookout, North Stradbroke Island, Queensland. Doubts were initially cast on the accuracy of both the calculated point of sinking and the dead reckoning position, but the 2009 discovery of the wreck found both to be correct, Centaur located within 1 nautical mile (1.9 km; 1.2 mi) of Rippon's coordinates.

### Survivors

Of the 332 people on board, 64 were rescued. Most of the crew and passengers were asleep at the time of attack and had little chance to escape. It was estimated that up to 200 people may have been alive at the time Centaur submerged. Several who made it off the ship later died from shrapnel wounds or burns; others were unable to find support and drowned.

The survivors spent 36 hours in the water, using barrels, wreckage, and the two damaged lifeboats for flotation. During this time, they drifted approximately 19.6 nautical miles (36.3 km; 22.6 mi) north east of Centaur's calculated point of sinking and spread out over an area of 2 nautical miles (3.7 km; 2.3 mi). The survivors saw at least four ships and several aircraft, but could not attract their attention.

At the time of rescue, the survivors were in two large and three small groups, with several more floating alone. Amongst those rescued were Sister Ellen Savage, the only surviving nurse from 12 aboard; Leslie Outridge, the only surviving doctor from 18 aboard; Gordon Rippon, second officer and most senior surviving crew member; and Richard Salt, the Torres Strait ship pilot. In 1944, Ellen Savage was presented with the George Medal for providing medical care, boosting morale, and displaying courage during the wait for rescue.

### Rescue

On the morning of 15 May 1943, the American destroyer USS Mugford departed Brisbane to escort the 11,063 ton New Zealand freighter Sussex on the first stage of the latter's trans-Tasman voyage. At 2:00 pm, a lookout aboard Mugford reported an object on the horizon. Around the same time, a Royal Australian Air Force Avro Anson of No. 71 Squadron, flying ahead on anti-submarine watch, dived towards the object. The aircraft returned to the two ships and signalled that there were shipwreck survivors in the water requiring rescue. Mugford's commanding officer ordered Sussex to continue alone as Mugford collected the survivors. Marksmen were positioned around the ship to shoot sharks, and sailors stood ready to dive in and assist the wounded. Mugford's medics inspected each person as they came aboard and provided necessary medical care. The American crew learned from the first group of survivors that they were from the hospital ship Centaur.

At 2:14 pm, Mugford made contact with the Naval Officer-in-Charge in Brisbane, and announced that the ship was recovering survivors from Centaur at , the first that anyone in Australia had knowledge of the attack on the hospital ship. The rescue of the 64 survivors took an hour and twenty minutes, although Mugford remained in the area until dark, searching an area of approximately 7 by 14 nautical miles (13 by 26 km; 8 by 16 mi) for more survivors. After darkness fell, Mugford returned to Brisbane, arriving shortly before midnight. Further searches of the waters off North Stradbroke Island were made by USS Helm during the night of 15 May until 6:00 pm on 16 May, and by and four motor torpedo boats from 16 to 21 May, neither search finding more survivors.

### Identifying attacker

At the time of the attack, none aboard Centaur witnessed what had attacked the ship. Due to the ship's position, the distance from shore, and the depth, it was concluded that she was torpedoed by one of the Japanese submarines known to be operating off the Australian east coast. Several survivors later claimed to have heard the attacking submarine moving on the surface while they were adrift, and the submarine was seen by the ship's cook, Francis Martin, who was floating alone on a hatch cover, out of sight from the main cluster of survivors. Martin described the submarine to Naval Intelligence following the survivors' return to land; his description matched the profile of a KD7 type Kaidai-class submarine of the Imperial Japanese Navy.

At the time of the attack, three KD7 Kaidai were operating off Australia's east coast: I-177 under the command of Hajime Nakagawa, I-178 under Hidejiro Utsuki, and I-180 under Toshio Kusaka. None of these submarines survived the war; I-177 was sunk by USS Samuel S. Miles on 3 October 1944, I-178 by USS Patterson on 25 August 1943, and I-180 by USS Gilmore on 26 April 1944. Kusaka and Nakagawa were transferred to other submarines before the loss of I-180 and I-177 respectively, but Utsuki and I-178 were sunk while returning from the patrol off the coast of Australia.

In December 1943, following official protests, the Japanese government issued a statement formally denying responsibility for the sinking of Centaur. Records provided by the Japanese following the war also did not acknowledge responsibility. Although Centaur's sinking was a war crime, no one was tried for sinking the hospital ship. Investigations into the attack were conducted between 1944 and 1948, and included the interrogation of the commanders of the submarines operating in Australian waters at the time, their superiors, plus junior officers and crewmen from the submarines who had survived the war. Several of the investigators suspected that Nakagawa and I-177 were most likely responsible, but they were unable to establish this beyond reasonable doubt, and the Centaur case file was closed on 14 December 1948 without any charges laid.

Historians were divided on which submarine was responsible. In Royal Australian Navy, 1942–1945, published in 1968 as part of the series detailing the Australian official history of World War II, George Hermon Gill concluded that either I-178 or I-180 was responsible; the former was more likely as she had served in Australian waters the longest of any Japanese submarine at the time, but had claimed no kills in the three-month period surrounding Centaur's sinking. In 1972, German military historian Jürgen Rohwer claimed in Chronology of the war at sea that I-177 torpedoed Centaur, based on a Japanese report stating that I-177 had attacked a ship on 14 May 1943 in the area the hospital ship had sunk. Japanese Rear Admiral Kaneyoshi Sakamoto, who had shown Rohwer the report, stated that Nakagawa and I-177 were responsible for the attack on Centaur in his 1979 book History of Submarine Warfare.

As an official history of the Japanese Navy, Sakamoto's work was considered to be official admission of the attacking submarine's identity. Subsequently, most sources assumed as fact Nakagawa's and I-177's role in the loss of Centaur. Nakagawa, who died in 1991, refused to speak about the attack on Centaur following the war crimes investigation at the end of World War II or even to defend himself or deny the claims made by Rohwer and Sakamoto.

## Reaction

### Public reaction

The media were notified of Centaur's sinking on 17 May 1943, but were ordered not to release the news until it had been announced in the South West Pacific Area's General Headquarters dispatch at midday on 18 May, and in Parliament by Prime Minister John Curtin that afternoon. News of the attack made front pages throughout the world, including The Times of London, The New York Times, and the Montreal Gazette. In some newspapers, the news took precedence over the 'Dambuster' raids performed in Europe by No. 617 Squadron RAF.

The initial public reaction to the attack on Centaur was one of outrage, significantly different from that displayed following the loss of Australian warships or merchant vessels. As a hospital ship, the attack was a breach of the tenth Hague Convention of 1907, and as such was a war crime. The sinking of Centaur drew strong reactions from both Prime Minister Curtin and General Douglas MacArthur. Curtin stated that the sinking was "an entirely inexcusable act, undertaken in violation of the convention to which Japan is a party and of all the principles of common humanity". MacArthur reflected the common Australian view when he stated that the sinking was an example of Japanese "limitless savagery". Politicians urged the public to use their rage to fuel the war effort, and Centaur became a symbol of Australia's determination to defeat what appeared to be a brutal and uncompromising enemy. The Australian Government produced posters depicting the sinking, which called for Australians to "Avenge the Nurses" by working to produce materiel, purchasing war bonds, or enlisting in the armed forces.

People also expressed their sympathy towards the crew, and there were several efforts to fund a new hospital ship. The councillors of Caulfield, Victoria, organised a fund to replace the lost medical equipment, opening with a donation of AU£2,000. Those who worked on Centaur's conversion contributed money towards a replacement, and employees of Ansett Airways pledged to donate an hour's pay towards the fitting out of such a replacement.

With some people unable to believe that the Japanese would be so ruthless, rumours began to spread almost immediately after news of the attack was made public. The most common rumour was that Centaur had been carrying munitions or commandos at the time of her sinking, the Japanese being made aware of this before her departure. This stemmed from an incident involving the ambulance drivers' weapons during loading in Sydney.

### Military reaction

The attack was universally condemned by Australian servicemen, who commonly believed that the attack on Centaur had been carried out deliberately and in full knowledge of her status. Similar reactions were expressed by other Allied personnel; United States Army Air Forces General George Kenney recalled having to talk a sergeant bombardier out of organising a retaliatory bombing run on a Japanese hospital ship known to be in their area.

Six days after the attack on Centaur, a request was made by the Australian Department of Defence that the identification markings and lights be removed from Australian hospital ship Manunda, weapons be installed, and that she begin to sail blacked out and under escort. The conversion was performed, although efforts by the Department of the Navy, the Admiralty, and authorities in New Zealand and the United States of America caused the completed conversion to be undone. The cost of the roundabout work came to £12,500, and kept Manunda out of service for three months. On 9 June 1943, communications between the Combined Chiefs of Staff on the subject of hospital ships contained a section referring to the Manunda incident as a response to the attack on Centaur, with the conclusion that the attack was the work of an irresponsible Japanese commander, and that it would be better to wait until further attacks had been made before considering the removal of hospital ship markings.

When the consideration was made that the ambulance drivers' weapons incident just before Centaur's voyage may have been partially responsible for the attack, it led to the tightening of rules regarding who was allowed to travel on a hospital ship. Quasi-medical staff, like repatriation teams, were no longer permitted on hospital ships. Ambulance drivers had to transfer from the regular Army to the Australian Army Medical Corps before they were allowed aboard, although they were still permitted to carry their unloaded weapons and ammunition.

### Official protests

After consultation with the Australian armed forces, General MacArthur, the Admiralty, and the Australian Government, an official protest was sent. This was received by the Japanese Government on 29 May 1943. At around the same time, the International Committee of the Red Cross sent a protest on behalf of the major Allied Red Cross organisations to the Japanese Red Cross.

On 26 December 1943, a response to the Australian protest arrived. It stated that the Japanese Government had no information justifying the allegation made, and therefore took no responsibility for what happened. The reply counter-protested that nine Japanese hospital ships had been attacked by the Allies, although these claims were directed against the United States, not Australia. Although several later exchanges were made, the lack of progress saw the British Government inform the Australian Prime Minister on 14 November 1944 that no further communications would be made on the loss of Centaur.

## Theories for attack

Torpedo attacks in Australian waters were common at this time, with 27 Japanese submarines operating in Australian waters between June 1942 and December 1944. These submarines attacked almost 50 merchant vessels, 20 ships confirmed to be sunk as result of a Japanese attack, plus 9 more unconfirmed. This was part of a concentrated effort to disrupt supply convoys from Australia to New Guinea.

Several actions on Centaur's part may have contributed to her demise. Centaur was under orders to sail well out to sea until reaching the Great Barrier Reef; her course keeping her between 50 and 150 nautical miles (90 and 280 km; 60 and 170 mi) from shore. Centaur's Master, believing he had been given a route intended for a merchant vessel, set a course closer to land, but on the seaward side of 2,000 metres (6,600 ft) in depth. Also, Centaur was sailing completely illuminated, with the exception of the two bow floodlights, which had been switched off as they interfered with visibility from the bridge.

There are three main theories as to why Centaur was attacked:

### Legitimate target

This theory stems from the rumours spreading after Centaur's sinking. If Centaur had been in breach of the Hague Convention of 1907, and someone had informed the Japanese of this, I-177 may have been under valid orders to attack. When Centaur left Sydney, her decks were packed with green-uniformed men, and as Field Ambulance uniforms were only distinguishable from other Army uniforms by badge insignia and the colouration of the cloth band ringing the hat, a distant observer could have concluded that the hospital ship was transporting soldiers. Those witnessing the loading in Sydney would have seen the ambulance drivers bring their weapons aboard, and could have come to a similar conclusion. If a spy or informant had passed this information to the Japanese, I-177 could have been lying in wait. The main flaw in this theory is the question of how Nakagawa and his crew were able to predict that Centaur was taking an alternative route and how they were able to determine the new route selected.

Similar but later rumours included that during her first voyage, Centaur had transported soldiers to New Guinea, or Japanese prisoners of war back to Australia for interrogation, and consequently had been marked as a legitimate target by the Japanese. Centaur had carried 10 prisoners of war on her return voyage from New Guinea, but as they were all wounded personnel, transporting them on a hospital ship was legal.

### Mistaken target

This theory states that Nakagawa was unaware that the vessel he was attacking was a hospital ship, and that the sinking was an unfortunate accident. This view was supported by several Japanese officers, both before and after the revelation that Nakagawa was responsible. Amongst them was Lieutenant Commander Zenji Orita, who took command of I-177 after Nakagawa. Orita did not hear anything from the crew about having sunk a hospital ship, not even rumours, and believed that if I-177 had knowingly attacked Centaur, he would have learned this from the crew's gossip.

When compared to the other contemporary Australian hospital ships, Centaur was the smallest, approximately a third of the size of Manunda or Wanganella. Centaur was also slightly shorter than I-177. The observation of Centaur was made through a periscope, and submarine officers attest that at 1,500 metres (4,900 ft), the optimum range of attack for World War II–era Japanese submarines, some officers would not be able to clearly identify a target ship's profile or hull markings. With Centaur's bow floodlights out, and with the observation of the target made through the periscope, there is a possibility Nakagawa would not have seen the hospital ship's markings if he had been in the wrong position. Apart from the two bow floodlights, Centaur was lit up brilliantly. To attack, I-177 would have had to approach from abeam of Centaur, which was illuminated by both its own lights and a full moon.

### Intentional target

This theory states that Nakagawa was fully aware that his target was a hospital ship and decided to sink her regardless, either on his own initiative or on a poor interpretation of his orders. Researchers speculate that as Nakagawa was approaching the end of his tour in Australian waters, and had only sunk a single enemy vessel, the 8,742 ton freighter Limerick, he did not want to return with the disgrace of a single kill. Other claims include that Nakagawa may have been acting in vengeance for casualties inflicted by the Allies during the Battle of the Bismarck Sea, or may have expected praise for the sinking of an enemy naval vessel.

In February 1944, while in command of I-37, Nakagawa ordered the machine-gunning of survivors from three British merchant vessels torpedoed by his submarine (British Chivalry, on 22 February; , on 24 February; and , on 29 February). His defence, that he was acting under orders from Vice Admiral Shiro Takasu, was not accepted, and he was sentenced to four years imprisonment at Sugamo Prison as a Class B war criminal. These incidents showed that Nakagawa was willing to ignore the laws of war.

## Shipwreck

Following World War II, several searches of the waters around North Stradbroke and Moreton Islands failed to reveal Centaur's location. It was believed that she had sunk off the edge of the continental shelf, to a depth at which the Royal Australian Navy did not have the capability to search for a vessel of Centaur's size. Some parties also believed that Rippon's calculated point of sinking was inaccurate, either intentionally or through error.

Several points were incorrectly identified as the location where Centaur sank. The first was in the War Diary Situation Report entry for the hospital ship's sinking, which gives , 7 nautical miles (13 km; 8.1 mi) east of Rippon's position. According to Milligan and Foley, this likely occurred because an estimated 50-nautical-mile (93 km; 58 mi) distance from Brisbane, included as a frame of reference, was interpreted literally. In 1974, two divers claimed to have found the ship approximately 40 nautical miles (74 km; 46 mi) east of Brisbane, in 60 metres (200 ft) of water, but did not disclose its exact location. Attempts to relocate the site between 1974 and 1992 were unsuccessful, an associate of the divers claiming that the Navy destroyed the wreck shortly after its discovery.

### Dennis's claim

In 1995, it was announced that the shipwreck of Centaur had been located in waters 9 nautical miles (17 km; 10 mi) from the lighthouse on Moreton Island, a significant distance from her believed last position. The finding was reported on A Current Affair, during which footage of the shipwreck, 170 metres (560 ft) underwater, was shown. Discoverer Donald Dennis claimed the identity of the shipwreck had been confirmed by the Navy, the Queensland Maritime Museum, and the Australian War Memorial. A cursory search by the Navy confirmed the presence of a shipwreck at the given location, which was gazetted as a war grave and added to navigation charts by the Australian Hydrographic Office.

Over the next eight years, there was growing doubt about the position of Dennis' wreck, due to the distance from both Second Officer Rippon's calculation of the point of sinking and where USS Mugford found the survivors. During this time, Dennis had been convicted on two counts of deception and one of theft through scams. Two wreck divers, Trevor Jackson and Simon Mitchell, used the location for a four-hour world record dive on 14 May 2002, during which they examined the wreck and took measurements, claiming that the ship was too small to be Centaur. Jackson had been studying Centaur for some time, and believed that the wreck was actually another, much smaller ship, the 55-metre-long (180 ft) MV Kyogle, a lime freighter purchased by the Royal Australian Air Force and sunk during bombing practice on 12 May 1951. The facts gathered on the dive were inconclusive, but the divers remained adamant it was not Centaur, and passed this information onto Nick Greenaway, producer of the newsmagazine show 60 Minutes.

On the 60th anniversary of the sinking, 60 Minutes ran a story demonstrating that the wreck was not Centaur. It was revealed that nobody at the Queensland Maritime Museum had yet seen Dennis' footage, and when it was shown to Museum president Rod McLeod and maritime historian John Foley, they stated that the shipwreck could not be Centaur due to physical inconsistencies, such as an incorrect rudder. Following this story, and others published around the same time in newspapers, the Navy sent three ships to inspect the site over a two-month period; HMA Ships , , and , before concluding that the shipwreck was incorrectly identified as Centaur. An amendment was made to the gazettal, and the Hydrographic Office began to remove the mark from charts.

### Discovery

In April 2008, following the successful discovery of HMAS Sydney, several parties began calling for a dedicated search for Centaur. By the end of 2008, the Australian Federal and Queensland State governments had formed a joint committee and contributed A\$2 million each towards a search, and tenders to supply equipment (including the search vessel, side-scan sonar systems, and a remotely operated inspection submersible) were opened in February 2009, and awarded during the year. The search, conducted from the Defence Maritime Services vessel Seahorse Spirit and overseen by shipwreck hunter David Mearns, commenced during the weekend of 12–13 December 2009. The initial search area off Cape Moreton covered 1,365 square kilometres (527 sq mi), the search team being given 35 days to locate and film the wreck before funding was exhausted.

Six sonar targets with similar dimensions to Centaur were located between 15 and 18 December: as none of the contacts corresponded completely to the hospital ship, the search team opted to take advantage of favourable weather conditions and continue investigating the area before returning to each site and making a detailed inspection with a higher-resolution sonar. On the afternoon of 18 December, the sonar towfish separated from the cable, and was lost in 1,800 metres (5,900 ft) of water, forcing the use of the high-resolution sonar to complete the area search. After inspecting the potential targets, Mearns and the search team announced on 20 December that they had found Centaur that morning.

The wreck was found at (30 nautical miles (56 km; 35 mi) east of Moreton Island, and less than 1 nautical mile (1.9 km; 1.2 mi) from Rippon's coordinates), resting 2,059 metres (6,755 ft) below sea level in a steep-walled gully, 150 metres (490 ft) wide and 90 metres (300 ft) deep. After returning to shore for Christmas and to install a remotely operated vehicle (ROV) aboard Seahorse Spirit, the search team commenced efforts to document the wreck, the first photographs being taken by the ROV in the early morning of 10 January 2010 confirming that the wreck is Centaur. Conditions for documenting the hospital ship were not optimal on the first ROV dive, and three more dives were made during 11 and 12 January. During the four dives, over 24 hours of footage were collected, along with several photographs: features identified during the operation include the Red Cross identification number, the hospital ship markings, and the ship's bell. The Centaur wreck site has been marked as a war grave and protected with a navigational exclusion zone under the Historic Shipwrecks Act 1976.

## Memorials

In 1948, Queensland nurses established the "Centaur Memorial Fund for Nurses" which used the money raised to purchase an establishment and name it "Centaur House"; a facility supporting nurses by holding convivial meetings and providing inexpensive accommodation for out-of-town nurses. The original Centaur House was sold in 1971, a new building being purchased and renamed. The second Centaur House was sold in 1979 and although the fund still exists, it no longer owns a physical facility. On 15 September 1968, a cairn was unveiled at Caloundra, Queensland, erected by the local Rotary International Club. In 1990, a stained glass memorial window depicting Centaur, along with a plaque listing the names of those lost in the attack, was installed at Concord Repatriation General Hospital, at a cost of A\$16,000. A display about Centaur was placed at the Australian War Memorial. The centrepiece of the display was a scale model of Centaur presented to the Memorial by Blue Funnel Line, and the display included items that were donated by the survivors, such as a lifejacket, a signal flare, and a medical kit. It was removed in 1992 to make way for a display related to the Vietnam War.

A memorial to Centaur was unveiled at Point Danger, Coolangatta, Queensland on 14 May 1993, the 50th anniversary of the sinking. It consists of a monumental stone topped with a cairn, surrounded by a tiled moat with memorial plaques explaining the commemoration. The memorial is surrounded by a park with a boardwalk, overlooking the sea, with plaques for other Merchant Navy and Royal Australian Navy vessels lost during World War II. The unveiling of the memorial was performed by Minister for Veterans' Affairs, Senator John Faulkner.

A memorial plaque was laid on the foredeck of Centaur on 12 January 2010, during the fourth and final ROV dive on the hospital ship. This would normally be a breach of the Historic Shipwrecks Act, but a special dispensation permitted the manoeuvre, as placing the plaque on the seabed next to the ship would have seen it sink into the sediment. Following the ship's discovery, a national memorial service at St John's Cathedral, Brisbane on 2 March 2010 was attended by over 600 people, including Prime Minister Kevin Rudd. A second ceremony for 300 relatives of the hospital ship's personnel was held aboard on 24 September. During the service, which occurred over the wreck site, wreaths were laid and the ashes of three survivors were scattered.

## See also

- USS Hope (AH-7)
- USS Relief
- USS Comfort (AH-6)
- SS Op ten Noort''
- Japanese war crimes

Hospital ship shipwreck in Queensland, Australia

Australian Shipwrecks with protected zone

World War II shipwrecks in the Coral Sea


Botany

Botany, also called plant science (or plant sciences), plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek word βοτάνη (botanē) meaning "pasture", "herbs" "grass", or "fodder"; βοτάνη is in turn derived from βόσκειν (boskein), "to feed" or "to graze". Traditionally, botany has also included the study of fungi and algae by mycologists and phycologists respectively, with the study of these three groups of organisms remaining within the sphere of interest of the International Botanical Congress. Nowadays, botanists (in the strict sense) study approximately 410,000 species of land plants of which some 391,000 species are vascular plants (including approximately 369,000 species of flowering plants), and approximately 20,000 are bryophytes.

Botany originated in prehistory as herbalism with the efforts of early humans to identify – and later cultivate – plants that were edible, poisonous, and possibly medicinal, making it one of the first endeavours of human investigation. Medieval physic gardens, often attached to monasteries, contained plants possibly having medicinal benefit. They were forerunners of the first botanical gardens attached to universities, founded from the 1540s onwards. One of the earliest was the Padua botanical garden. These gardens facilitated the academic study of plants. Efforts to catalogue and describe their collections were the beginnings of plant taxonomy, and led in 1753 to the binomial system of nomenclature of Carl Linnaeus that remains in use to this day for the naming of all biological species.

In the 19th and 20th centuries, new techniques were developed for the study of plants, including methods of optical microscopy and live cell imaging, electron microscopy, analysis of chromosome number, plant chemistry and the structure and function of enzymes and other proteins. In the last two decades of the 20th century, botanists exploited the techniques of molecular genetic analysis, including genomics and proteomics and DNA sequences to classify plants more accurately.

Modern botany is a broad, multidisciplinary subject with contributions and insights from most other areas of science and technology. Research topics include the study of plant structure, growth and differentiation, reproduction, biochemistry and primary metabolism, chemical products, development, diseases, evolutionary relationships, systematics, and plant taxonomy. Dominant themes in 21st century plant science are molecular genetics and epigenetics, which study the mechanisms and control of gene expression during differentiation of plant cells and tissues. Botanical research has diverse applications in providing staple foods, materials such as timber, oil, rubber, fibre and drugs, in modern horticulture, agriculture and forestry, plant propagation, breeding and genetic modification, in the synthesis of chemicals and raw materials for construction and energy production, in environmental management, and the maintenance of biodiversity.

## History

### Early botany

Botany originated as herbalism, the study and use of plants for their possible medicinal properties. The early recorded history of botany includes many ancient writings and plant classifications. Examples of early botanical works have been found in ancient texts from India dating back to before 1100 BCE, Ancient Egypt, in archaic Ancient Iranic Avestan writings, and in works from China purportedly from before 221 BCE.

Modern botany traces its roots back to Ancient Greece specifically to Theophrastus (c. 371–287 BCE), a student of Aristotle who invented and described many of its principles and is widely regarded in the scientific community as the "Father of Botany". His major works, Enquiry into Plants and On the Causes of Plants, constitute the most important contributions to botanical science until the Middle Ages, almost seventeen centuries later.

Another work from Ancient Greece that made an early impact on botany is De materia medica, a five-volume encyclopedia about preliminary herbal medicine written in the middle of the first century by Greek physician and pharmacologist Pedanius Dioscorides. De materia medica was widely read for more than 1,500 years. Important contributions from the medieval Muslim world include Ibn Wahshiyya's Nabatean Agriculture, Abū Ḥanīfa Dīnawarī's (828–896) the Book of Plants, and Ibn Bassal's The Classification of Soils. In the early 13th century, Abu al-Abbas al-Nabati, and Ibn al-Baitar (d. 1248) wrote on botany in a systematic and scientific manner.

In the mid-16th century, botanical gardens were founded in a number of Italian universities. The Padua botanical garden in 1545 is usually considered to be the first which is still in its original location. These gardens continued the practical value of earlier "physic gardens", often associated with monasteries, in which plants were cultivated for suspected medicinal uses. They supported the growth of botany as an academic subject. Lectures were given about the plants grown in the gardens. Botanical gardens came much later to northern Europe; the first in England was the University of Oxford Botanic Garden in 1621.

German physician Leonhart Fuchs (1501–1566) was one of "the three German fathers of botany", along with theologian Otto Brunfels (1489–1534) and physician Hieronymus Bock (1498–1554) (also called Hieronymus Tragus). Fuchs and Brunfels broke away from the tradition of copying earlier works to make original observations of their own. Bock created his own system of plant classification.

Physician Valerius Cordus (1515–1544) authored a botanically and pharmacologically important herbal Historia Plantarum in 1544 and a pharmacopoeia of lasting importance, the Dispensatorium in 1546. Naturalist Conrad von Gesner (1516–1565) and herbalist John Gerard (1545–c. 1611) published herbals covering the supposed medicinal uses of plants. Naturalist Ulisse Aldrovandi (1522–1605) was considered the father of natural history, which included the study of plants. In 1665, using an early microscope, Polymath Robert Hooke discovered cells, a term he coined, in cork, and a short time later in living plant tissue.

### Early modern botany

During the 18th century, systems of plant identification were developed comparable to dichotomous keys, where unidentified plants are placed into taxonomic groups (e.g. family, genus and species) by making a series of choices between pairs of characters. The choice and sequence of the characters may be artificial in keys designed purely for identification (diagnostic keys) or more closely related to the natural or phyletic order of the taxa in synoptic keys. By the 18th century, new plants for study were arriving in Europe in increasing numbers from newly discovered countries and the European colonies worldwide. In 1753, Carl Linnaeus published his Species Plantarum, a hierarchical classification of plant species that remains the reference point for modern botanical nomenclature. This established a standardised binomial or two-part naming scheme where the first name represented the genus and the second identified the species within the genus. For the purposes of identification, Linnaeus's Systema Sexuale classified plants into 24 groups according to the number of their male sexual organs. The 24th group, Cryptogamia, included all plants with concealed reproductive parts, mosses, liverworts, ferns, algae and fungi.

This clinical categorization of plants was soon followed by the creation of the categories of race and sexuality; the classification of plants necessitated classification of all other living things, including humans. As a result, taxonomy and botany played an influential role in the development of scientific racism. One example of this progression is in the works of Carl Linnaeus, the previously mentioned 18th century botanist. As Linnaeus moved on from classifying plants to classifying all organisms, he published Systema Naturae, a major classificatory piece that he would continue to edit and grow over time. In his 10th edition he expands from four "varieties" of man - Europeans Albus, Americanus Rubescens, Asiaticus Fuscus, and Africanus Niger, based on the four known continents - he also attributes certain skin color, medical temperament, body posture, physical traits, behavior, manner of clothing, and form of government to each variety of people. In these descriptions he labels Asian people as stern, taught, and greedy; black people as sly, sluggish, and neglectful; white people as light, wise, and inventors. Linnaeus is only one of many botanists who influenced scientific racism through the categorization of organisms.

Increasing knowledge of plant anatomy, morphology and life cycles led to the realisation that there were more natural affinities between plants than the artificial sexual system of Linnaeus. Adanson (1763), de Jussieu (1789), and Candolle (1819) all proposed various alternative natural systems of classification that grouped plants using a wider range of shared characters and were widely followed. The Candollean system reflected his ideas of the progression of morphological complexity and the later Bentham & Hooker system, which was influential until the mid-19th century, was influenced by Candolle's approach. Darwin's publication of the Origin of Species in 1859 and his concept of common descent required modifications to the Candollean system to reflect evolutionary relationships as distinct from mere morphological similarity.

Botany was greatly stimulated by the appearance of the first "modern" textbook, Matthias Schleiden's Grundzüge der Wissenschaftlichen Botanik, published in English in 1849 as Principles of Scientific Botany. Schleiden was a microscopist and an early plant anatomist who co-founded the cell theory with Theodor Schwann and Rudolf Virchow and was among the first to grasp the significance of the cell nucleus that had been described by Robert Brown in 1831. In 1855, Adolf Fick formulated Fick's laws that enabled the calculation of the rates of molecular diffusion in biological systems.

The system in which early modern botany was practiced was very extensive. Modern botany emerged following the surge in exploration of other continents by European colonizers. Plant collectors would travel to different countries in search of new specimens for botanists to classify. Plants usable for cultivataion would then be hybridized. The history of botany has been connected to imbalanced power structures in the past. Slave labor was widespread not only in plantations but also in the running of botanical gardens; for example, on St. Vincent Island, plantation slavery was vital for the economic success of the sugar colonies and for the maintenance of the breadfruit cultivation project in the St. Vincent botanical gardens.

### Late modern botany

Building upon the gene-chromosome theory of heredity that originated with Gregor Mendel (1822–1884), August Weismann (1834–1914) proved that inheritance only takes place through gametes. No other cells can pass on inherited characters. The work of Katherine Esau (1898–1997) on plant anatomy is still a major foundation of modern botany. Her books Plant Anatomy and Anatomy of Seed Plants have been key plant structural biology texts for more than half a century.

The discipline of plant ecology was pioneered in the late 19th century by botanists such as Eugenius Warming, who produced the hypothesis that plants form communities, and his mentor and successor Christen C. Raunkiær whose system for describing plant life forms is still in use today. The concept that the composition of plant communities such as temperate broadleaf forest changes by a process of ecological succession was developed by Henry Chandler Cowles, Arthur Tansley and Frederic Clements. Clements is credited with the idea of climax vegetation as the most complex vegetation that an environment can support and Tansley introduced the concept of ecosystems to biology. Building on the extensive earlier work of Alphonse de Candolle, Nikolai Vavilov (1887–1943) produced accounts of the biogeography, centres of origin, and evolutionary history of economic plants.

Particularly since the mid-1960s there have been advances in understanding of the physics of plant physiological processes such as transpiration (the transport of water within plant tissues), the temperature dependence of rates of water evaporation from the leaf surface and the molecular diffusion of water vapour and carbon dioxide through stomatal apertures. These developments, coupled with new methods for measuring the size of stomatal apertures, and the rate of photosynthesis have enabled precise description of the rates of gas exchange between plants and the atmosphere. Innovations in statistical analysis by Ronald Fisher, Frank Yates and others at Rothamsted Experimental Station facilitated rational experimental design and data analysis in botanical research. The discovery and identification of the auxin plant hormones by Kenneth V. Thimann in 1948 enabled regulation of plant growth by externally applied chemicals. Frederick Campion Steward pioneered techniques of micropropagation and plant tissue culture controlled by plant hormones. The synthetic auxin 2,4-dichlorophenoxyacetic acid or 2,4-D was one of the first commercial synthetic herbicides.

20th century developments in plant biochemistry have been driven by modern techniques of organic chemical analysis, such as spectroscopy, chromatography and electrophoresis. With the rise of the related molecular-scale biological approaches of molecular biology, genomics, proteomics and metabolomics, the relationship between the plant genome and most aspects of the biochemistry, physiology, morphology and behaviour of plants can be subjected to detailed experimental analysis. The concept originally stated by Gottlieb Haberlandt in 1902 that all plant cells are totipotent and can be grown in vitro ultimately enabled the use of genetic engineering experimentally to knock out a gene or genes responsible for a specific trait, or to add genes such as GFP that report when a gene of interest is being expressed. These technologies enable the biotechnological use of whole plants or plant cell cultures grown in bioreactors to synthesise pesticides, antibiotics or other pharmaceuticals, as well as the practical application of genetically modified crops designed for traits such as improved yield.

Modern morphology recognises a continuum between the major morphological categories of root, stem (caulome), leaf (phyllome) and trichome. Furthermore, it emphasises structural dynamics. Modern systematics aims to reflect and discover phylogenetic relationships between plants. Modern Molecular phylogenetics largely ignores morphological characters, relying on DNA sequences as data. Molecular analysis of DNA sequences from most families of flowering plants enabled the Angiosperm Phylogeny Group to publish in 1998 a phylogeny of flowering plants, answering many of the questions about relationships among angiosperm families and species. The theoretical possibility of a practical method for identification of plant species and commercial varieties by DNA barcoding is the subject of active current research.

## Scope and importance

The study of plants is vital because they underpin almost all animal life on Earth by generating a large proportion of the oxygen and food that provide humans and other organisms with aerobic respiration with the chemical energy they need to exist. Plants, algae and cyanobacteria are the major groups of organisms that carry out photosynthesis, a process that uses the energy of sunlight to convert water and carbon dioxide into sugars that can be used both as a source of chemical energy and of organic molecules that are used in the structural components of cells. As a by-product of photosynthesis, plants release oxygen into the atmosphere, a gas that is required by nearly all living things to carry out cellular respiration. In addition, they are influential in the global carbon and water cycles and plant roots bind and stabilise soils, preventing soil erosion. Plants are crucial to the future of human society as they provide food, oxygen, biochemicals, and products for people, as well as creating and preserving soil.

Historically, all living things were classified as either animals or plants and botany covered the study of all organisms not considered animals. Botanists examine both the internal functions and processes within plant organelles, cells, tissues, whole plants, plant populations and plant communities. At each of these levels, a botanist may be concerned with the classification (taxonomy), phylogeny and evolution, structure (anatomy and morphology), or function (physiology) of plant life.

The strictest definition of "plant" includes only the "land plants" or embryophytes, which include seed plants (gymnosperms, including the pines, and flowering plants) and the free-sporing cryptogams including ferns, clubmosses, liverworts, hornworts and mosses. Embryophytes are multicellular eukaryotes descended from an ancestor that obtained its energy from sunlight by photosynthesis. They have life cycles with alternating haploid and diploid phases. The sexual haploid phase of embryophytes, known as the gametophyte, nurtures the developing diploid embryo sporophyte within its tissues for at least part of its life, even in the seed plants, where the gametophyte itself is nurtured by its parent sporophyte. Other groups of organisms that were previously studied by botanists include bacteria (now studied in bacteriology), fungi (mycology) – including lichen-forming fungi (lichenology), non-chlorophyte algae (phycology), and viruses (virology). However, attention is still given to these groups by botanists, and fungi (including lichens) and photosynthetic protists are usually covered in introductory botany courses.

Palaeobotanists study ancient plants in the fossil record to provide information about the evolutionary history of plants. Cyanobacteria, the first oxygen-releasing photosynthetic organisms on Earth, are thought to have given rise to the ancestor of plants by entering into an endosymbiotic relationship with an early eukaryote, ultimately becoming the chloroplasts in plant cells. The new photosynthetic plants (along with their algal relatives) accelerated the rise in atmospheric oxygen started by the cyanobacteria, changing the ancient oxygen-free, reducing, atmosphere to one in which free oxygen has been abundant for more than 2 billion years.

Among the important botanical questions of the 21st century are the role of plants as primary producers in the global cycling of life's basic ingredients: energy, carbon, oxygen, nitrogen and water, and ways that our plant stewardship can help address the global environmental issues of resource management, conservation, human food security, biologically invasive organisms, carbon sequestration, climate change, and sustainability.

### Human nutrition

Virtually all staple foods come either directly from primary production by plants, or indirectly from animals that eat them. Plants and other photosynthetic organisms are at the base of most food chains because they use the energy from the sun and nutrients from the soil and atmosphere, converting them into a form that can be used by animals. This is what ecologists call the first trophic level. The modern forms of the major staple foods, such as hemp, teff, maize, rice, wheat and other cereal grasses, pulses, bananas and plantains, as well as hemp, flax and cotton grown for their fibres, are the outcome of prehistoric selection over thousands of years from among wild ancestral plants with the most desirable characteristics.

Botanists study how plants produce food and how to increase yields, for example through plant breeding, making their work important to humanity's ability to feed the world and provide food security for future generations. Botanists also study weeds, which are a considerable problem in agriculture, and the biology and control of plant pathogens in agriculture and natural ecosystems. Ethnobotany is the study of the relationships between plants and people. When applied to the investigation of historical plant–people relationships ethnobotany may be referred to as archaeobotany or palaeoethnobotany. Some of the earliest plant-people relationships arose between the indigenous people of Canada in identifying edible plants from inedible plants. This relationship the indigenous people had with plants was recorded by ethnobotanists.

## Plant biochemistry

Plant biochemistry is the study of the chemical processes used by plants. Some of these processes are used in their primary metabolism like the photosynthetic Calvin cycle and crassulacean acid metabolism. Others make specialised materials like the cellulose and lignin used to build their bodies, and secondary products like resins and aroma compounds.

Plants and various other groups of photosynthetic eukaryotes collectively known as "algae" have unique organelles known as chloroplasts. Chloroplasts are thought to be descended from cyanobacteria that formed endosymbiotic relationships with ancient plant and algal ancestors. Chloroplasts and cyanobacteria contain the blue-green pigment chlorophyll a. Chlorophyll a (as well as its plant and green algal-specific cousin chlorophyll b) absorbs light in the blue-violet and orange/red parts of the spectrum while reflecting and transmitting the green light that we see as the characteristic colour of these organisms. The energy in the red and blue light that these pigments absorb is used by chloroplasts to make energy-rich carbon compounds from carbon dioxide and water by oxygenic photosynthesis, a process that generates molecular oxygen (O<sub>2</sub>) as a by-product.

The light energy captured by chlorophyll a is initially in the form of electrons (and later a proton gradient) that's used to make molecules of ATP and NADPH which temporarily store and transport energy. Their energy is used in the light-independent reactions of the Calvin cycle by the enzyme rubisco to produce molecules of the 3-carbon sugar glyceraldehyde 3-phosphate (G3P). Glyceraldehyde 3-phosphate is the first product of photosynthesis and the raw material from which glucose and almost all other organic molecules of biological origin are synthesised. Some of the glucose is converted to starch which is stored in the chloroplast. Starch is the characteristic energy store of most land plants and algae, while inulin, a polymer of fructose is used for the same purpose in the sunflower family Asteraceae. Some of the glucose is converted to sucrose (common table sugar) for export to the rest of the plant.

Unlike in animals (which lack chloroplasts), plants and their eukaryote relatives have delegated many biochemical roles to their chloroplasts, including synthesising all their fatty acids, and most amino acids. The fatty acids that chloroplasts make are used for many things, such as providing material to build cell membranes out of and making the polymer cutin which is found in the plant cuticle that protects land plants from drying out.

Plants synthesise a number of unique polymers like the polysaccharide molecules cellulose, pectin and xyloglucan from which the land plant cell wall is constructed. Vascular land plants make lignin, a polymer used to strengthen the secondary cell walls of xylem tracheids and vessels to keep them from collapsing when a plant sucks water through them under water stress. Lignin is also used in other cell types like sclerenchyma fibres that provide structural support for a plant and is a major constituent of wood. Sporopollenin is a chemically resistant polymer found in the outer cell walls of spores and pollen of land plants responsible for the survival of early land plant spores and the pollen of seed plants in the fossil record. It is widely regarded as a marker for the start of land plant evolution during the Ordovician period. The concentration of carbon dioxide in the atmosphere today is much lower than it was when plants emerged onto land during the Ordovician and Silurian periods. Many monocots like maize and the pineapple and some dicots like the Asteraceae have since independently evolved pathways like Crassulacean acid metabolism and the carbon fixation pathway for photosynthesis which avoid the losses resulting from photorespiration in the more common carbon fixation pathway. These biochemical strategies are unique to land plants.

### Medicine and materials

Phytochemistry is a branch of plant biochemistry primarily concerned with the chemical substances produced by plants during secondary metabolism. Some of these compounds are toxins such as the alkaloid coniine from hemlock. Others, such as the essential oils peppermint oil and lemon oil are useful for their aroma, as flavourings and spices (e.g., capsaicin), and in medicine as pharmaceuticals as in opium from opium poppies. Many medicinal and recreational drugs, such as tetrahydrocannabinol (active ingredient in cannabis), caffeine, morphine and nicotine come directly from plants. Others are simple derivatives of botanical natural products. For example, the pain killer aspirin is the acetyl ester of salicylic acid, originally isolated from the bark of willow trees, and a wide range of opiate painkillers like heroin are obtained by chemical modification of morphine obtained from the opium poppy. Popular stimulants come from plants, such as caffeine from coffee, tea and chocolate, and nicotine from tobacco. Most alcoholic beverages come from fermentation of carbohydrate-rich plant products such as barley (beer), rice (sake) and grapes (wine). Native Americans have used various plants as ways of treating illness or disease for thousands of years. This knowledge Native Americans have on plants has been recorded by enthnobotanists and then in turn has been used by pharmaceutical companies as a way of drug discovery.

Plants can synthesise coloured dyes and pigments such as the anthocyanins responsible for the red colour of red wine, yellow weld and blue woad used together to produce Lincoln green, indoxyl, source of the blue dye indigo traditionally used to dye denim and the artist's pigments gamboge and rose madder.

Sugar, starch, cotton, linen, hemp, some types of rope, wood and particle boards, papyrus and paper, vegetable oils, wax, and natural rubber are examples of commercially important materials made from plant tissues or their secondary products. Charcoal, a pure form of carbon made by pyrolysis of wood, has a long history as a metal-smelting fuel, as a filter material and adsorbent and as an artist's material and is one of the three ingredients of gunpowder. Cellulose, the world's most abundant organic polymer, can be converted into energy, fuels, materials and chemical feedstock. Products made from cellulose include rayon and cellophane, wallpaper paste, biobutanol and gun cotton. Sugarcane, rapeseed and soy are some of the plants with a highly fermentable sugar or oil content that are used as sources of biofuels, important alternatives to fossil fuels, such as biodiesel. Sweetgrass was used by Native Americans to ward off bugs like mosquitoes. These bug repelling properties of sweetgrass were later found by the American Chemical Society in the molecules phytol and coumarin.

## Plant ecology

Plant ecology is the science of the functional relationships between plants and their habitats – the environments where they complete their life cycles. Plant ecologists study the composition of local and regional floras, their biodiversity, genetic diversity and fitness, the adaptation of plants to their environment, and their competitive or mutualistic interactions with other species. Some ecologists even rely on empirical data from indigenous people that is gathered by ethnobotanists. This information can relay a great deal of information on how the land once was thousands of years ago and how it has changed over that time. The goals of plant ecology are to understand the causes of their distribution patterns, productivity, environmental impact, evolution, and responses to environmental change.

Plants depend on certain edaphic (soil) and climatic factors in their environment but can modify these factors too. For example, they can change their environment's albedo, increase runoff interception, stabilise mineral soils and develop their organic content, and affect local temperature. Plants compete with other organisms in their ecosystem for resources. They interact with their neighbours at a variety of spatial scales in groups, populations and communities that collectively constitute vegetation. Regions with characteristic vegetation types and dominant plants as well as similar abiotic and biotic factors, climate, and geography make up biomes like tundra or tropical rainforest.

Herbivores eat plants, but plants can defend themselves and some species are parasitic or even carnivorous. Other organisms form mutually beneficial relationships with plants. For example, mycorrhizal fungi and rhizobia provide plants with nutrients in exchange for food, ants are recruited by ant plants to provide protection, honey bees, bats and other animals pollinate flowers and humans and other animals act as dispersal vectors to spread spores and seeds.

### Plants, climate and environmental change

Plant responses to climate and other environmental changes can inform our understanding of how these changes affect ecosystem function and productivity. For example, plant phenology can be a useful proxy for temperature in historical climatology, and the biological impact of climate change and global warming. Palynology, the analysis of fossil pollen deposits in sediments from thousands or millions of years ago allows the reconstruction of past climates. Estimates of atmospheric concentrations since the Palaeozoic have been obtained from stomatal densities and the leaf shapes and sizes of ancient land plants. Ozone depletion can expose plants to higher levels of ultraviolet radiation-B (UV-B), resulting in lower growth rates. Moreover, information from studies of community ecology, plant systematics, and taxonomy is essential to understanding vegetation change, habitat destruction and species extinction.

## Genetics

Inheritance in plants follows the same fundamental principles of genetics as in other multicellular organisms. Gregor Mendel discovered the genetic laws of inheritance by studying inherited traits such as shape in Pisum sativum (peas). What Mendel learned from studying plants has had far-reaching benefits outside of botany. Similarly, "jumping genes" were discovered by Barbara McClintock while she was studying maize. Nevertheless, there are some distinctive genetic differences between plants and other organisms.

Species boundaries in plants may be weaker than in animals, and cross species hybrids are often possible. A familiar example is peppermint, Mentha × piperita, a sterile hybrid between Mentha aquatica and spearmint, Mentha spicata. The many cultivated varieties of wheat are the result of multiple inter- and intra-specific crosses between wild species and their hybrids. Angiosperms with monoecious flowers often have self-incompatibility mechanisms that operate between the pollen and stigma so that the pollen either fails to reach the stigma or fails to germinate and produce male gametes. This is one of several methods used by plants to promote outcrossing. In many land plants the male and female gametes are produced by separate individuals. These species are said to be dioecious when referring to vascular plant sporophytes and dioicous when referring to bryophyte gametophytes.

Unlike in higher animals, where parthenogenesis is rare, asexual reproduction may occur in plants by several different mechanisms. The formation of stem tubers in potato is one example. Particularly in arctic or alpine habitats, where opportunities for fertilisation of flowers by animals are rare, plantlets or bulbs, may develop instead of flowers, replacing sexual reproduction with asexual reproduction and giving rise to clonal populations genetically identical to the parent. This is one of several types of apomixis that occur in plants. Apomixis can also happen in a seed, producing a seed that contains an embryo genetically identical to the parent.

Most sexually reproducing organisms are diploid, with paired chromosomes, but doubling of their chromosome number may occur due to errors in cytokinesis. This can occur early in development to produce an autopolyploid or partly autopolyploid organism, or during normal processes of cellular differentiation to produce some cell types that are polyploid (endopolyploidy), or during gamete formation. An allopolyploid plant may result from a hybridisation event between two different species. Both autopolyploid and allopolyploid plants can often reproduce normally, but may be unable to cross-breed successfully with the parent population because there is a mismatch in chromosome numbers. These plants that are reproductively isolated from the parent species but live within the same geographical area, may be sufficiently successful to form a new species. Some otherwise sterile plant polyploids can still reproduce vegetatively or by seed apomixis, forming clonal populations of identical individuals. Durum wheat is a fertile tetraploid allopolyploid, while bread wheat is a fertile hexaploid. The commercial banana is an example of a sterile, seedless triploid hybrid. Common dandelion is a triploid that produces viable seeds by apomictic seed.

As in other eukaryotes, the inheritance of endosymbiotic organelles like mitochondria and chloroplasts in plants is non-Mendelian. Chloroplasts are inherited through the male parent in gymnosperms but often through the female parent in flowering plants.

### Molecular genetics

A considerable amount of new knowledge about plant function comes from studies of the molecular genetics of model plants such as the Thale cress, Arabidopsis thaliana, a weedy species in the mustard family (Brassicaceae). The genome or hereditary information contained in the genes of this species is encoded by about 135 million base pairs of DNA, forming one of the smallest genomes among flowering plants. Arabidopsis was the first plant to have its genome sequenced, in 2000. The sequencing of some other relatively small genomes, of rice (Oryza sativa) and Brachypodium distachyon, has made them important model species for understanding the genetics, cellular and molecular biology of cereals, grasses and monocots generally.

Model plants such as Arabidopsis thaliana are used for studying the molecular biology of plant cells and the chloroplast. Ideally, these organisms have small genomes that are well known or completely sequenced, small stature and short generation times. Corn has been used to study mechanisms of photosynthesis and phloem loading of sugar in plants. The single celled green alga Chlamydomonas reinhardtii, while not an embryophyte itself, contains a green-pigmented chloroplast related to that of land plants, making it useful for study. A red alga Cyanidioschyzon merolae has also been used to study some basic chloroplast functions. Spinach, peas, soybeans and a moss Physcomitrella patens are commonly used to study plant cell biology.

Agrobacterium tumefaciens, a soil rhizosphere bacterium, can attach to plant cells and infect them with a callus-inducing Ti plasmid by horizontal gene transfer, causing a callus infection called crown gall disease. Schell and Van Montagu (1977) hypothesised that the Ti plasmid could be a natural vector for introducing the Nif gene responsible for nitrogen fixation in the root nodules of legumes and other plant species. Today, genetic modification of the Ti plasmid is one of the main techniques for introduction of transgenes to plants and the creation of genetically modified crops.

### Epigenetics

Epigenetics is the study of heritable changes in gene function that cannot be explained by changes in the underlying DNA sequence but cause the organism's genes to behave (or "express themselves") differently. One example of epigenetic change is the marking of the genes by DNA methylation which determines whether they will be expressed or not. Gene expression can also be controlled by repressor proteins that attach to silencer regions of the DNA and prevent that region of the DNA code from being expressed. Epigenetic marks may be added or removed from the DNA during programmed stages of development of the plant, and are responsible, for example, for the differences between anthers, petals and normal leaves, despite the fact that they all have the same underlying genetic code. Epigenetic changes may be temporary or may remain through successive cell divisions for the remainder of the cell's life. Some epigenetic changes have been shown to be heritable, while others are reset in the germ cells.

Epigenetic changes in eukaryotic biology serve to regulate the process of cellular differentiation. During morphogenesis, totipotent stem cells become the various pluripotent cell lines of the embryo, which in turn become fully differentiated cells. A single fertilised egg cell, the zygote, gives rise to the many different plant cell types including parenchyma, xylem vessel elements, phloem sieve tubes, guard cells of the epidermis, etc. as it continues to divide. The process results from the epigenetic activation of some genes and inhibition of others.

Unlike animals, many plant cells, particularly those of the parenchyma, do not terminally differentiate, remaining totipotent with the ability to give rise to a new individual plant. Exceptions include highly lignified cells, the sclerenchyma and xylem which are dead at maturity, and the phloem sieve tubes which lack nuclei. While plants use many of the same epigenetic mechanisms as animals, such as chromatin remodelling, an alternative hypothesis is that plants set their gene expression patterns using positional information from the environment and surrounding cells to determine their developmental fate.

Epigenetic changes can lead to paramutations, which do not follow the Mendelian heritage rules. These epigenetic marks are carried from one generation to the next, with one allele inducing a change on the other.

## Plant evolution

The chloroplasts of plants have a number of biochemical, structural and genetic similarities to cyanobacteria, (commonly but incorrectly known as "blue-green algae") and are thought to be derived from an ancient endosymbiotic relationship between an ancestral eukaryotic cell and a cyanobacterial resident.

The algae are a polyphyletic group and are placed in various divisions, some more closely related to plants than others. There are many differences between them in features such as cell wall composition, biochemistry, pigmentation, chloroplast structure and nutrient reserves. The algal division Charophyta, sister to the green algal division Chlorophyta, is considered to contain the ancestor of true plants. The Charophyte class Charophyceae and the land plant sub-kingdom Embryophyta together form the monophyletic group or clade Streptophytina.

Nonvascular land plants are embryophytes that lack the vascular tissues xylem and phloem. They include mosses, liverworts and hornworts. Pteridophytic vascular plants with true xylem and phloem that reproduced by spores germinating into free-living gametophytes evolved during the Silurian period and diversified into several lineages during the late Silurian and early Devonian. Representatives of the lycopods have survived to the present day. By the end of the Devonian period, several groups, including the lycopods, sphenophylls and progymnosperms, had independently evolved "megaspory" – their spores were of two distinct sizes, larger megaspores and smaller microspores. Their reduced gametophytes developed from megaspores retained within the spore-producing organs (megasporangia) of the sporophyte, a condition known as endospory. Seeds consist of an endosporic megasporangium surrounded by one or two sheathing layers (integuments). The young sporophyte develops within the seed, which on germination splits to release it. The earliest known seed plants date from the latest Devonian Famennian stage. Following the evolution of the seed habit, seed plants diversified, giving rise to a number of now-extinct groups, including seed ferns, as well as the modern gymnosperms and angiosperms. Gymnosperms produce "naked seeds" not fully enclosed in an ovary; modern representatives include conifers, cycads, Ginkgo, and Gnetales. Angiosperms produce seeds enclosed in a structure such as a carpel or an ovary. Ongoing research on the molecular phylogenetics of living plants appears to show that the angiosperms are a sister clade to the gymnosperms.

## Plant physiology

Plant physiology encompasses all the internal chemical and physical activities of plants associated with life. Chemicals obtained from the air, soil and water form the basis of all plant metabolism. The energy of sunlight, captured by oxygenic photosynthesis and released by cellular respiration, is the basis of almost all life. Photoautotrophs, including all green plants, algae and cyanobacteria gather energy directly from sunlight by photosynthesis. Heterotrophs including all animals, all fungi, all completely parasitic plants, and non-photosynthetic bacteria take in organic molecules produced by photoautotrophs and respire them or use them in the construction of cells and tissues. Respiration is the oxidation of carbon compounds by breaking them down into simpler structures to release the energy they contain, essentially the opposite of photosynthesis.

Molecules are moved within plants by transport processes that operate at a variety of spatial scales. Subcellular transport of ions, electrons and molecules such as water and enzymes occurs across cell membranes. Minerals and water are transported from roots to other parts of the plant in the transpiration stream. Diffusion, osmosis, and active transport and mass flow are all different ways transport can occur. Examples of elements that plants need to transport are nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. In vascular plants, these elements are extracted from the soil as soluble ions by the roots and transported throughout the plant in the xylem. Most of the elements required for plant nutrition come from the chemical breakdown of soil minerals. Sucrose produced by photosynthesis is transported from the leaves to other parts of the plant in the phloem and plant hormones are transported by a variety of processes.

### Plant hormones

Plants are not passive, but respond to external signals such as light, touch, and injury by moving or growing towards or away from the stimulus, as appropriate. Tangible evidence of touch sensitivity is the almost instantaneous collapse of leaflets of Mimosa pudica, the insect traps of Venus flytrap and bladderworts, and the pollinia of orchids.

The hypothesis that plant growth and development is coordinated by plant hormones or plant growth regulators first emerged in the late 19th century. Darwin experimented on the movements of plant shoots and roots towards light and gravity, and concluded "It is hardly an exaggeration to say that the tip of the radicle . . acts like the brain of one of the lower animals . . directing the several movements". About the same time, the role of auxins (from the Greek auxein, to grow) in control of plant growth was first outlined by the Dutch scientist Frits Went. The first known auxin, indole-3-acetic acid (IAA), which promotes cell growth, was only isolated from plants about 50 years later. This compound mediates the tropic responses of shoots and roots towards light and gravity. The finding in 1939 that plant callus could be maintained in culture containing IAA, followed by the observation in 1947 that it could be induced to form roots and shoots by controlling the concentration of growth hormones were key steps in the development of plant biotechnology and genetic modification.

Cytokinins are a class of plant hormones named for their control of cell division (especially cytokinesis). The natural cytokinin zeatin was discovered in corn, Zea mays, and is a derivative of the purine adenine. Zeatin is produced in roots and transported to shoots in the xylem where it promotes cell division, bud development, and the greening of chloroplasts. The gibberelins, such as gibberelic acid are diterpenes synthesised from acetyl CoA via the mevalonate pathway. They are involved in the promotion of germination and dormancy-breaking in seeds, in regulation of plant height by controlling stem elongation and the control of flowering. Abscisic acid (ABA) occurs in all land plants except liverworts, and is synthesised from carotenoids in the chloroplasts and other plastids. It inhibits cell division, promotes seed maturation, and dormancy, and promotes stomatal closure. It was so named because it was originally thought to control abscission. Ethylene is a gaseous hormone that is produced in all higher plant tissues from methionine. It is now known to be the hormone that stimulates or regulates fruit ripening and abscission, and it, or the synthetic growth regulator ethephon which is rapidly metabolised to produce ethylene, are used on industrial scale to promote ripening of cotton, pineapples and other climacteric crops.

Another class of phytohormones is the jasmonates, first isolated from the oil of Jasminum grandiflorum which regulates wound responses in plants by unblocking the expression of genes required in the systemic acquired resistance response to pathogen attack.

In addition to being the primary energy source for plants, light functions as a signalling device, providing information to the plant, such as how much sunlight the plant receives each day. This can result in adaptive changes in a process known as photomorphogenesis. Phytochromes are the photoreceptors in a plant that are sensitive to light.

## Plant anatomy and morphology

Plant anatomy is the study of the structure of plant cells and tissues, whereas plant morphology is the study of their external form. All plants are multicellular eukaryotes, their DNA stored in nuclei. The characteristic features of plant cells that distinguish them from those of animals and fungi include a primary cell wall composed of the polysaccharides cellulose, hemicellulose and pectin, larger vacuoles than in animal cells and the presence of plastids with unique photosynthetic and biosynthetic functions as in the chloroplasts. Other plastids contain storage products such as starch (amyloplasts) or lipids (elaioplasts). Uniquely, streptophyte cells and those of the green algal order Trentepohliales divide by construction of a phragmoplast as a template for building a cell plate late in cell division.

The bodies of vascular plants including clubmosses, ferns and seed plants (gymnosperms and angiosperms) generally have aerial and subterranean subsystems. The shoots consist of stems bearing green photosynthesising leaves and reproductive structures. The underground vascularised roots bear root hairs at their tips and generally lack chlorophyll. Non-vascular plants, the liverworts, hornworts and mosses do not produce ground-penetrating vascular roots and most of the plant participates in photosynthesis. The sporophyte generation is nonphotosynthetic in liverworts but may be able to contribute part of its energy needs by photosynthesis in mosses and hornworts.

The root system and the shoot system are interdependent – the usually nonphotosynthetic root system depends on the shoot system for food, and the usually photosynthetic shoot system depends on water and minerals from the root system. Cells in each system are capable of creating cells of the other and producing adventitious shoots or roots. Stolons and tubers are examples of shoots that can grow roots. Roots that spread out close to the surface, such as those of willows, can produce shoots and ultimately new plants. In the event that one of the systems is lost, the other can often regrow it. In fact it is possible to grow an entire plant from a single leaf, as is the case with plants in Streptocarpus sect. Saintpaulia, or even a single cell – which can dedifferentiate into a callus (a mass of unspecialised cells) that can grow into a new plant. In vascular plants, the xylem and phloem are the conductive tissues that transport resources between shoots and roots. Roots are often adapted to store food such as sugars or starch, as in sugar beets and carrots.

Stems mainly provide support to the leaves and reproductive structures, but can store water in succulent plants such as cacti, food as in potato tubers, or reproduce vegetatively as in the stolons of strawberry plants or in the process of layering. Leaves gather sunlight and carry out photosynthesis. Large, flat, flexible, green leaves are called foliage leaves. Gymnosperms, such as conifers, cycads, Ginkgo, and gnetophytes are seed-producing plants with open seeds. Angiosperms are seed-producing plants that produce flowers and have enclosed seeds. Woody plants, such as azaleas and oaks, undergo a secondary growth phase resulting in two additional types of tissues: wood (secondary xylem) and bark (secondary phloem and cork). All gymnosperms and many angiosperms are woody plants. Some plants reproduce sexually, some asexually, and some via both means.

Although reference to major morphological categories such as root, stem, leaf, and trichome are useful, one has to keep in mind that these categories are linked through intermediate forms so that a continuum between the categories results. Furthermore, structures can be seen as processes, that is, process combinations.

## Systematic botany

Systematic botany is part of systematic biology, which is concerned with the range and diversity of organisms and their relationships, particularly as determined by their evolutionary history. It involves, or is related to, biological classification, scientific taxonomy and phylogenetics. Biological classification is the method by which botanists group organisms into categories such as genera or species. Biological classification is a form of scientific taxonomy. Modern taxonomy is rooted in the work of Carl Linnaeus, who grouped species according to shared physical characteristics. These groupings have since been revised to align better with the Darwinian principle of common descent – grouping organisms by ancestry rather than superficial characteristics. While scientists do not always agree on how to classify organisms, molecular phylogenetics, which uses DNA sequences as data, has driven many recent revisions along evolutionary lines and is likely to continue to do so. The dominant classification system is called Linnaean taxonomy. It includes ranks and binomial nomenclature. The nomenclature of botanical organisms is codified in the International Code of Nomenclature for algae, fungi, and plants (ICN) and administered by the International Botanical Congress.

Kingdom Plantae belongs to Domain Eukaryota and is broken down recursively until each species is separately classified. The order is: Kingdom; Phylum (or Division); Class; Order; Family; Genus (plural genera); Species. The scientific name of a plant represents its genus and its species within the genus, resulting in a single worldwide name for each organism. For example, the tiger lily is Lilium columbianum. Lilium is the genus, and columbianum the specific epithet. The combination is the name of the species. When writing the scientific name of an organism, it is proper to capitalise the first letter in the genus and put all of the specific epithet in lowercase. Additionally, the entire term is ordinarily italicised (or underlined when italics are not available).

The evolutionary relationships and heredity of a group of organisms is called its phylogeny. Phylogenetic studies attempt to discover phylogenies. The basic approach is to use similarities based on shared inheritance to determine relationships. As an example, species of Pereskia are trees or bushes with prominent leaves. They do not obviously resemble a typical leafless cactus such as an Echinocactus. However, both Pereskia and Echinocactus have spines produced from areoles (highly specialised pad-like structures) suggesting that the two genera are indeed related.

Judging relationships based on shared characters requires care, since plants may resemble one another through convergent evolution in which characters have arisen independently. Some euphorbias have leafless, rounded bodies adapted to water conservation similar to those of globular cacti, but characters such as the structure of their flowers make it clear that the two groups are not closely related. The cladistic method takes a systematic approach to characters, distinguishing between those that carry no information about shared evolutionary history – such as those evolved separately in different groups (homoplasies) or those left over from ancestors (plesiomorphies) – and derived characters, which have been passed down from innovations in a shared ancestor (apomorphies). Only derived characters, such as the spine-producing areoles of cacti, provide evidence for descent from a common ancestor. The results of cladistic analyses are expressed as cladograms: tree-like diagrams showing the pattern of evolutionary branching and descent.

From the 1990s onwards, the predominant approach to constructing phylogenies for living plants has been molecular phylogenetics, which uses molecular characters, particularly DNA sequences, rather than morphological characters like the presence or absence of spines and areoles. The difference is that the genetic code itself is used to decide evolutionary relationships, instead of being used indirectly via the characters it gives rise to. Clive Stace describes this as having "direct access to the genetic basis of evolution." As a simple example, prior to the use of genetic evidence, fungi were thought either to be plants or to be more closely related to plants than animals. Genetic evidence suggests that the true evolutionary relationship of multicelled organisms is as shown in the cladogram below – fungi are more closely related to animals than to plants.

In 1998, the Angiosperm Phylogeny Group published a phylogeny for flowering plants based on an analysis of DNA sequences from most families of flowering plants. As a result of this work, many questions, such as which families represent the earliest branches of angiosperms, have now been answered. Investigating how plant species are related to each other allows botanists to better understand the process of evolution in plants. Despite the study of model plants and increasing use of DNA evidence, there is ongoing work and discussion among taxonomists about how best to classify plants into various taxa. Technological developments such as computers and electron microscopes have greatly increased the level of detail studied and speed at which data can be analysed.

## Symbols

A few symbols are in current use in botany. A number of others are obsolete; for example, Linnaeus used planetary symbols (Mars) for biennial plants, (Jupiter) for herbaceous perennials and (Saturn) for woody perennials, based on the planets' orbital periods of 2, 12 and 30 years; and Willd used (Saturn) for neuter in addition to (Mercury) for hermaphroditic. The following symbols are still used:

♀ female

♂ male

⚥ hermaphrodite/bisexual

⚲ vegetative (asexual) reproduction

◊ sex unknown

☉ annual

⚇ biennial

♾ perennial

☠ poisonous

🛈 further information

× crossbred hybrid

\+ grafted hybrid

## See also

- Branches of botany
- Evolution of plants
- Glossary of botanical terms
- Glossary of plant morphology
- List of botany journals
- List of botanists
- List of botanical gardens
- List of botanists by author abbreviation
- List of domesticated plants
- List of flowers
- List of systems of plant taxonomy
- Outline of botany
- Timeline of British botany

Science of Plant Life