AZ Master Gardener Manual: Plant Parts and Functions STEMS Stems are structures which support buds and leaves and serve as conduits for carrying water, minerals, and sugars. The three major internal parts of a stem are the xylem, phloem, and cambium. The xylem and phloem are the major components of a plants vascular system. The vascular system transports food, water, and minerals and offers support for the plant. Xylem vessels conduct water and minerals, while phloem tubes conduct food. The vascular systems of monocots and dicots differ. While both contain xylem and phloem, they are arranged differently. In the stem of a monocot, the xylem and phloem are paired into bundles; these bundles are dispersed throughout the stem. But in the stem of a dicot, the vascular system forms rings inside the stem. The ring of phloem is near the bark or external cover of the stem and is a component of the bark in mature stems. The xylem forms the inner ring; it is the sapwood and heartwood in woody plants.

The difference in the vascular system of the two groups is of practical interest to the horticulturist because certain herbicides are specific to either monocots or dicots. An example is 2, 4, -D, which only kills dicots. The cambium is a meristem, which is a site of cell division and active growth. It is located between the xylem and phloem inside the bark of a stem and is the tissue responsible for a stems increase in girth, as it produces both the xylem and phloem tissues. Stems may be long, with great distances between leaves and buds (branches of trees, runners on strawberries), or compressed, with short distances between buds or leaves (fruit spurs, crowns of strawberry plants, dandelions). Stems can be above the ground like most stems with which we are familiar, or below the ground (potatoes, tulip bulbs). All stems must have buds or leaves present to be classified as stem tissue. An area of the stem where leaves are located is called a node. Nodes are areas of great cellular activity and growth, where auxiliary buds develop into leaves or flowers. The area between nodes is called the internode.

Parts of a Stem The length of an internode may depend on many factors. Decreasing fertility will decrease internode length. Internode length varies with the season. Too little light will result in a long internode causing a spindly stem. This situation is known as stretch or etiolation. Growth produced early in the season has the greatest internode length. Internode length decreases as the growing season nears its end. Vigorously growing plants tend to have greater internode lengths than less vigorous plants. Internode length will vary with competition from surrounding stems or developing fruit. If the energy for a stem has to be divided between three or four stems, or if the energy is diverted into fruit growth, internode length will be shortened. Modified Stems Although typical stems are above-ground trunks and branches, there are modified stems which can be found above ground and below ground. The above-ground modified stems are crowns, stolons, and spurs, and the below-ground stems are bulbs, corms, rhizomes, and tubers. A crown is a region of compressed stem tissue from which new shoots are produced, generally found near the surface of the soil.

Crowns (strawberries, dandelions, African violets) are compressed stems having leaves and flowers on short internodes. A spur is a compressed fruiting branch. Spurs are short, stubby, side stems that arise from the main stem and are common on such fruit trees as pears, apples, and cherries, where they may bear fruit. If severe pruning is done close to fruit-bearing spurs, the spurs can revert to a long, nonfruiting stem. A stolon is a horizontal stem that is fleshy or semi-woody and lies along the top of the ground. A runner is a type of stolon. It is a specialized stem that grows on the soil surface and forms a new plant at one or more of its nodes. Strawberry runners are examples of stolons. Remember, all stems have nodes and buds or leaves. The leaves on strawberry runners are small but are located at the nodes which are easy to see. The spider plant also has stolons. A tuber is an enlarged portion of an underground stem like potato tubers, tulip bulbs, and iris rhizomes are underground stems that store food for the plant.

The tuber, like any other stem, has nodes that produce buds. The eyes of a potato are actually the nodes on the stem. Each eye contains a cluster of buds. A rhizome is a specialized stem which grows horizontally at or just below the soil surface. They act as a storage organ and means of propagation in some plants and are similar to stolons. Some rhizomes are compressed and fleshy such as those of iris; they can also be slender with elongated internodes such as bentgrass. Johnsongrass is a hated weed principally because of the spreading capability of its rhizomes. Tulips, lilies, daffodils, and onions are plants that produce bulbs--shortened, compressed, underground stems surrounded by fleshy scales (leaves) that envelop a central bud located at the tip of the stem. If you cut through the center of a tulip or daffodil bulb in November, you can see all the flower parts in miniature within the bulb. Many bulbs require a period of low-temperature exposure before they begin to send up the new plant. Both the temperature and length of this treatment are of critical importance to commercial growers who force bulbs for holidays.

Corms are not the same as bulbs. They have shapes similar to bulbs, but do not contain fleshy scales. A corm is a solid, swollen stem whose scales have been reduced to a dry, leaflike covering. Some plants produce a modified stem that is referred to as a tuberous stem. Examples are tuberous begonia and cyclamen. The stem is shortened, flattened, enlarged, and underground. Buds and shoots arise from the crown and fibrous roots are found on the bottom of the tuberous stem. In addition, some plants such as the dahlia and the sweet potato produce an underground storage organ called a tuberous root, which is often confused with bulbs and tubers. However, these are roots, not stems, and have neither nodes nor internodes. It may sometimes be difficult to distinguish between roots and stems, but one sure way is to look for the presence of nodes. Stems have nodes; roots do not. Stems are commonly used for plant propagation. Above-ground stems can be divided into sections that contain internodes and nodes. They are utilized as cuttings and will produce stems that are good propagative tissues. Rhizomes can be divided into pieces.

Bulbs form small bulblets at the base of the parent bulb. Cormels are miniature corms that form under the parent corm. Tubers can be cut into pieces containing eyes and nodes. All of these will produce new plants. Types of Stems A shoot is a young stem with leaves present. A twig is a stem which is less than one year old and has no leaves since it is still in the winter-dormant stage. A branch is a stem which is more than one year old and typically has lateral stems. A trunk is a main stem of a woody plant. Most trees have a single trunk. Trees are perennial woody plants, usually have one main trunk, and are usually more than 12 feet tall at maturity. Shrubs are perennial woody plants that may have one or several main stems, and are usually less than 12 feet tall at maturity. A vine is a plant which develops long, trailing stems that grow along the ground unless they are supported by another plant or structure. Some twining vines circle their support clockwise while others circle counter clockwise. Climbing vines are supported by aerial roots, slender tendrils which encircle the supporting object, or tendrils with adhesive tips.

Texture and Growth of Stems Woody stems contain relatively large amounts of hardened xylem tissue in its core, and are typical of most tree fruits and ornamental trees and shrubs. A cane is a stem which has a relatively large pith and usually lives only one or two years. Examples of plants with canes include rose, grape, and blackberry. Herbaceous or succulent stems contain only small amounts of xylem tissue and usually live for only one growing season. If the plant is perennial, it will develop new shoots from the root. Life Cycles of Plants Plants are classified by the number of growing seasons required to complete a life cycle. Annuals pass through their entire life cycle from seed germination to seed production in one growing season and then die. Biennials are plants which start from seeds and produce vegetative structures and food storage organs the first season. During the first winter a hardy evergreen rosette of basal leaves persists. During the second season flowers, fruit, and seeds develop to complete the life cycle. The plant then dies. Carrots, beets, cabbage, celery, and onions are biennial plants.

Hollyhock, Canterbury Bells, and Sweet William are biennials which are commonly grown for their attractive flowers. Plants which typically develop as biennials may, in some cases, complete the cycle of growth from seed germination to seed production in only one growing season. This situation occurs when drought, variations in temperature or other climatic conditions are experienced. These cause the plant to physiologically pass through the equivalent of two growing seasons, in a single growing season. This phenomenon is referred to as bolting. Perennial plants live for many years, and after reaching maturity, typically produce flowers and seeds each year. Perennials are classified as herbaceous if the top dies back to the ground each winter and new stems grow from the roots each spring. They are classified as woody if the top persists, as in shrubs or trees. Stems as Food The edible portion of cultivated plants such as asparagus and kohlrabi is an enlarged succulent stem. The edible parts of broccoli are composed of stem tissue, flower buds, and a few small leaves. The edible part of potato is a fleshy underground stem called a tuber. Although the name suggests otherwise, the edible part of the cauliflower is proliferated stem tissue.

Next Untitled Document stem: modified Stems/modified: Plants often modify their stems for special functions. These modifications can have unusual and even bizarre shapes, and can grow above as well as below the ground. Modified stems that grow above ground: Stolons or runners-are horizontally oriented stems that grow along the soil surface. Their function is vegetative production. Example: number of grasses have stolons. Spider also plant has many stolons, each with a plantlet forming at the end of the stolon. Strawberries are normally propagated asexually by two different methods: runners (stolons) and crown division. Tendrils and twining shoots-coil around objects and help support the plant. Searcher shoots-are stems with long internodes that move in circles through the air seeking a support. Cladodes or cladophylls-are flat, leaflike stems modified for photosynthesis. Succulent stems-of plants have a low surface to volume ration and store large amounts of water and are common in desert plants. Thorns-are modified stems that protect plants from grazing animals. Spines are modified leaves. How can you tell the difference?

Short ( spurs) and long shoots-long shoots have long internodes, while short shoots are rosettes and have short internodes. Apple spurs One of the simplest modified stems is the fruiting spur of an apple tree. Once the plant reaches maturity it begins to develop short branches (spurs) that grow very slowly. Spur growth may be limited to a few millimeters per year versus several inches of vegetative growth on other branches. Offshoots Applies to lateral branches on stems of monocots. Date palm, pineapple, banana, and orchids produce offshoots. Bamboo also propagate from offshoots, if left unchecked they will form whole forests of plants... Modified stems that grow below ground Examples include tulips, hyacinths Bulbs-are rosette stems surrounded by fleshy leaves that store nutrients. A bulb consists of a stem from which modified fleshy leaves develop. The fleshy leaves are circular and hollow; new leaves develop within the older leaves. Leaves are attached to the 'stem' at nodes and spaces between nodes are called internodes. The presence of nodes and internodes means that the bulb is actually a modified stem. The papery covering is called a tunica.

Note the basal plate and the lack of roots. Tunicate bulbs produce a new (adventitious--not pre-formed) root system annually from the basal plate. Roots appear soon after planting the bulb. Rhizomes-are underground stems that grow near the soil surface. They typically have short internodes and scale leaves, and produce roots along their lower surface. They store food for renewing growth of the shoot after periods of stress. Example: iris, ginger, canna Corms The corm is a relatively solid modified stem that has a few fleshy leaves.. The node is the point where the leaf attaches to the stem. The presence of nodes and internodes makes the corm a modified stem. The papery tunic on the outside of a crocus corm is the dead petiole from last year's leaf. Corms also have a nonpersistent root system that lives only one season .....In addition to normal fibrous roots, corms and some nontunicate bulbs have special roots (contractile roots) .Because new corms develop on top of old, they are closer to the soil surface. Corms too close to the surface could suffer winter injury.

Contractile roots are capable of becoming deeply anchored and contracting to pull the corm deeper into the soil. Thus corms can change their depth in the soil. Example: crocus. gladiolas Tubers-are swollen regions of stems that store food for subsequent growth. The potato is an example. It is a stem because it has many nodes called eyes with spaces between eyes known as internodes. Potato tubers develop at the end of swollen underground stem structures, rhizomes. Eyes of potatoes are really axillary buds which contain several small buds at each site. These buds can expand to form shoots which grow on to make whole plants. For fun see the potato play site : Although the common potato is a stem, the sweet potato is a modified root! Crown. Generally that part of a plant at the surface of the ground from which new shoots are produced. see also the stolens/strawberries above for another illustration Spider plants, African violets are propagated by pulling shoots off of the crown.

Asparagus spears develop from a crown that grows underground : NameThatPlant.net: Glossary - principal parts of a grass plant Natural Communities Glossary of Technical Terms Principal Parts of a Grass Plant, reprinted from 100 Native Forage Grasses in 11 Southern States by Horace L. Leithead, Lewis L. Yarlett, and Thomas N. Shiflet, range conservationists. US Department of Agriculture, Soil Conservation Service, Agriculture Handbook No. 389. (Technical information developed by the Federal government is generally in the public domain and is not copyrighted.) To see photographic examples of a term, click the camera next to it in the list of botanical terms. The vegetative parts of a grass plant are roots, stems, and leaves. The flowering (reproductive) part is the seedhead. Brief descriptions and drawings of these parts follow : Roots Grass roots are fibrous. Size of the root system depends on genetic and environmental factors and on management. In general, grasses that are capable of producing the greatest top growth are also capable of producing the greatest root growth.

The ratio of roots to tops by weight ranges from about 0.8 :1 to 1.5 :1 for most native grasses. Soil factors influencing root growth are moisture, temperature, structure, depth, fertility, and chemical reaction. Soil moisture, generally, has the greatest effect on root depth. In shallow soils, root systems are shallow; in moist, deep, well-developed soils, root systems are deep. Grasses differ widely in their ability to grow in saturated, poorly aerated soil. Excessive wetness inhibits root growth of most grasses except those adapted to marshes. Soil temperature affects the growth of grass roots. In general, native perennial grasses have active root growth in the fall until frost, very slow growth through winter, and active growth again in spring from about 10 to 15 days before green leaves appear until flowering. After flowering, growth slows down and stops during summer when soil temperatures are high. The amount and frequency of top-growth removal by grazing or mowing have a marked influence on root growth. According to research (Crider, 1955), the proportion of roots that stop growing varies according to the proportion of top growth that is removed.

Stems The jointed stem (culm) of a grass plant consists of nodes and internodes. The nodes (joints) are solid and usually larger than the rest of the stem. The internodes, the part of the stem between two nodes, are usually hollow. Branching at the base of the main stem may produce erect shoots; horizontal, above-ground stolons (stoloniferous) ; or horizontal, below-ground rhizomes (rhizomatous). Both rhizomes and stolons bear roots at the underside of the nodes. Some species of grasses branch only at or near the base from axillary buds at the basal nodes. Others branch and rebranch from buds produced at upper nodes, giving the appearance of a dense bouquet. Stolons are stems or runners that originate at the base of the main stem and grow along the surface of the ground. They have nodes and scales or well-developed leaves. Roots borne at the nodes help to establish and spread the plant and to produce new plants if stolons are broken. Rhizomes are stems that originate at the base of the main stem and grow horizontally below the ground surface. They have nodes, internodes, and scalelike leaves.

Roots grow from the underside of the nodes; shoots (stems and leaves) grow from the topside. Stems of most perennial grasses die back to the approximate base of the stem each year. However, in the basal part of the stem, there are from several to many basal nodes with axillary buds capable of initiating new growth. Leaves Grass leaves are borne at nodes along the stem in two ranks. The newest leaf is always on the opposite side of the stem from the leaf just below it. Leaves are parallel-veined. The grass leaf consists of three principal parts: Blade, sheath, and ligule. Other parts are collar and auricle. The blade  the expanded part of the leaf  may be (1) flat, (5) keeled (boat shaped). but it may grade from one form to another. The sheath is borne at the node and surrounds the stem like a tube. It is characteristically split down one side, making it possible to separate the sheath from the stem without tearing the sheath. In some grasses, the sheath is open  the margins do not come together; in others, it is closed-the margins overlap.

Some sheaths are flattened; others are rounded. Because of their different forms and shapes, they are important in identifying a grass. The ligule, meaning little tongue, usually clasps the stem firmly on the inside of the leaf at the junction of the sheath and blade, preventing dirt and water from getting between them. In the absence of a seedhead, the ligule is often used to identify a grass. Some ligules are membranous or papery; some are only a ring of hair. The collar is on the outside of the leaf at the junction of sheath and blade. Some grasses have two earlike lobes or appendages, called auricles, which are borne, one on either side, at the base of the blade. Seedhead The seedhead (inflorescence) is the flowering (reproductive) part of the grass plant. Generally, the seedhead has no leaves. On some grasses, a sheathlike bract, called spathe, encloses or partly encloses the seedhead The spikelet is the basic unit of the seedhead. It may be pediceled (on a pedicel or footstalk) or sessile (without a pedicel).

The spikelet consists of a rachilla (jointed stem or axis), one to several florets, and two glumes. The florets are borne in two ranks on the rachilla. Below them are the glumes  two bracts without flowers. Each floret consists of one flower or seed enclosed in two papery membranes called lemma and palea. The lemma is borne on the rachilla above the pair of glumes and the palea at the base of the flower or seed. Both the glumes and lemma have nerves or veins that run from the base to the tip. If the center nerve is extended, it is called an awn. Because awns are of different lengths, shapes, and colors, they are often used to identify a particular grass. The basic forms of grass seedheads are spike, raceme, and panicle but they may grade from one form to another and may have specialized forms. Spike is a seedhead in which one or more sessile spikelets are borne on the main axis (rachis). Raceme is a seedhead in which the spikelets are borne on individual footstalks (pedicels) growing directly on the main axis (rachis).

Panicle is a seedhead with a main axis and subdivided branches. It may be compact and spikelike or open. Plants: An Introduction to Vegetative Reproduction | Biology article by D G Mackean and Tropical Examples Vegetative reproduction is a form of asexual reproduction in plants. It does not involve flowers, pollination and seed production. Instead, a new plant grows from a vegetative part, usually a stem, of the parent plant. However, plants which reproduce asexually almost always reproduce sexually as well, bearing flowers, fruits and seeds. Vegetative reproduction from a stem usually involves the buds. Instead of producing a branch, the bud grows into a complete plant which eventually becomes self-supporting. Since no gametes are involved, the plants produced asexually have identical genomes and the offspring form what is known as a clone. In some cases of vegetative reproduction, the structures involved also become storage organs and swell with stored food, e.g. potatoes. The principal types of vegetative reproduction structures are bulbs, corms, rhizomes and runners. Bulbs Bulbs consist of very short stems with closely packed leaves arranged in concentric circles round the stem. These leaves are swollen with stored food e.g.

onion. A terminal bud will produce next year’s flowering shoot and the lateral (axillary) buds will produce new plants. Corms Corms also have a short stem but in this case it is the stem itself which swells and stores food. The circular leaves form only papery scales. As with bulbs, the terminal bud grows into a flowering shoot and the lateral buds produce new plants. Rhizomes Rhizomes are stems which grow horizontally under the ground. In some cases the underground stems are swollen with food reserves e.g. iris. The terminal bud turns upwards to produce the flowering shoot and the lateral buds may grow out to form new rhizomes. Runners Runners are also horizontal stems growing from the parent plant, but they grow above ground. When their terminal buds touch the ground they take root and produce new plants. Advantages of vegetative reproduction Since food stores are available throughout the year and the parent plant with its root system can absorb water from quite a wide area, two of the hazards of seed germination are reduced. Buds are produced in an environment where the parent is able to flourish, but many seeds dispersed from plants never reach a suitable situation for effective germination.

Vegetative reproduction does not usually result in rapid and widespread distribution of offspring in the same way as seed dispersal, but tends to produce a dense clump of plants with little room for competitors between them. Such groups of plants are very persistent and, because of their buds and underground food stores, can still grow after their foliage has been destroyed by insects, fire, or cultivation. Those of them regarded as weeds are difficult to eradicate, since even a small piece of rhizome bearing a bud can give rise to a new colony (clone). Bulbs - Snowdrop In the snowdrop and daffodil, the bulb is formed by the leaf bases which completely encircle the short, conical stem. The part of the leaf above ground makes food by photosynthesis and sends it to the leaf bases which swell as they store the food. In the following year the stored food is used for the early growth of the bulb. Life cycle of Daffodil In the spring, adventitious roots grow out of the stem, and the leaves begins to grow above ground, making use of the stored food in the fleshy leaf bases which consequently shrivel.

During late spring some of the food made in the leaves in the daffodil is sent to the leaf bases which swell and form a new bulb inside the old one. Life cycle of Tulip and Onion In these bulbs, the food is not sent to the leaf bases but to the lateral buds. As these buds enlarge they form two or more ‘daughter’ bulbs inside the old bulb. The leaves of the old bulb shrivel and dry out forming the dry scales which surround the daughter bulbs. In both cases, when the daughter bulbs grow, they form a clump, together with the parent bulb. Corms - Crocus Plants with bulbs store food in special leaves or leaf bases. Plants with corms store food in the stem, which is very short and swollen. When the foliage has died off, the leaf bases, where they encircle the short stem, form protective scaly coverings. A familiar corms is that of the crocus, and the wild arum corm is illustrated on p.1. Since the corm is a stem, it has lateral buds which can grow into new plants. The stem remains below ground all its life, only the leaves and flower stalk coming above ground.

Life Cycle of Corm In Spring, the food stored in the corm enables the terminal bud to grow rapidly and produce leaves and flowers above ground. Later in the year, food made by the leaves is sent back, not to the old corm, but to the base of the stem immediately above it. This region swells and forms a new corm on top of the old, now shrivelled, corm. Some of the lateral buds on the old corm have also grown and produced new plants with corms. Contractile Roots The formation of one corm on top of another tends to bring the successive corms nearer and nearer to the soil surface. Adventitious roots develop from the base of the new corm. Once these have grown firmly into the soil, a region near their junction with the stem contracts and pulls the new corm down, keeping it at a constant level in the soil. Wrinkles can be seen on these contractile roots where shrinkage has taken place. Bulbs also have contractile roots which counteract the tendency in successive generations to grow out of the soil. Rhizomes In plants with rhizomes, the stem remains below ground but continues to grow horizontally.

The old part of the stem does not die away as in bulbs and corms, but lasts for several years. In the iris, the terminal bud turns up and produces leaves and flowers above ground. The old leaf bases form circular scales round the rhizome, which is swollen with food reserves. Lateral buds grow into new rhizomes. Life Cycle of Rhizome The annual cycle of a rhizome is similar to that of a corm. In late spring/early summer, food from the leaves passes back to the rhizome, and a lateral bud uses it, grows horizontally underground, and so continues the rhizome. Other lateral buds produce new rhizomes which branch from the parent stem. The terminal buds of these branches curve upwards and produce new leafy shoots and flowers. Contractile, adventitious roots grow from the nodes of the underground stem and keep it at a constant depth. Runners Plants such as the strawberry have a very short stem, called a rootstock, with thin scale leaves,. Foliage leaves and flowers grow from the buds in the axils of the scale leaves. Some of the lower buds produce shoots which grow horizontally over the surface of the ground and bear scale leaves and buds.

The terminal buds of these runners turn up and produce daughter plants some distance away from the parent, the new plants developing adventitious roots. Later, the runner shrivels away. The runner does not store food but conducts it from the parent plant to the daughters, until they are well developed. Stem tubers - potato In the potato plant, lateral buds at the base of the stem produce shoots which grow laterally at first and then down into the ground. These are comparable to rhizomes, as they are underground stems with tiny scale leaves and lateral buds. They do not swell evenly along their length with stored food. Annual cycle - potato Food made in the leaves passes to the ends of these rhizomes, which swell and form the tubers we call potatoes. Since the potato tuber is a stem, it has leaves and lateral buds; these are the familiar ‘eyes’. Each one of these can produce a new shoot in the following year, using the food stored in the tuber. The old tubers shrivel and rot away at the end of the season Stolons Blackberry stems form a rather different type of runner in which the main shoot forms the new individual.

When the growing end of a shoot arches over and touches the ground, the terminal bud curves up, producing a new shoot which soon develops adventitious roots. Grafting A bud or shoot from one plant is inserted into a cleft or under the bark on the stem of a closely related variety. The rooted portion is called the stock; the bud or shoot being grafted is the scion. The stock is obtained by growing a plant from seed then cutting away the shoot. The scion is a branch or a bud cut from a cultivated variety with the required characteristics of flower colour, fruit quality, etc. Rose plants grown from seed would produce a wide variety of plants, only a few of which would retain all the desirable features of the parent plant. Most of them would be like wild roses. Similarly, most of the apple trees grown from seed would bear only small, sour ‘crab-apples'. By taking cuttings and making grafts, the inbred characteristics of the plant are preserved and you can guarantee that all the new individuals produced by this kind of artificial propagation will be the same. Cuttings It is possible to produce new individuals from certain plants by putting the cut end of a shoot into water or moist earth.

Roots grow from the base of the stem into the soil while the shoot continues to grow and produce leaves. In some cases the cut end of the stem may be treated with a rooting 'hormone' to promote root growth. Evaporation from the shoot is reduced by covering it with polythene or a glass jar. Carnations, geraniums and chrysanthemums are commonly propagated from cuttings. Tissue culture Once a cell has become part of a tissue it usually loses the ability to reproduce. However, the nucleus of any plant cell still holds all the 'instructions' (genes) for making a complete plant and in certain circumstances they can be brought back into action. In laboratory conditions single plant cells can be induced to divide and grow into complete plants. One technique is to take a small piece of plant tissue from a root or stem and treat it with enzymes to separate it into individual cells The cells are then provided with particular plant 'hormones’ which induce cell division and, eventually the formation of roots, stems and leaves. An alternative method is to start with a small piece of tissue and place it on a nutrient jelly (agar).

Cells in the tissue start to divide and produce many cells forming a shapeless mass called a callus. If the callus is then provided with the appropriate ‘hormones’ it develops into a complete plant. Stems Of Tropical Plants Home > Nature In Africa > Stems Of Tropical Africa Stems Of Tropical African Plants WHEN a seed starts to grow into the young plant two parts appear-the root and the shoot. The shoot comes up above the ground, grows towards the light and consists of stem and tiny leaves. In this chapter we shall see what the stem is like and what it does for the plant. The stem is the part of the plant between the root and the leaves; it bears the leaves on it and holds them up to where they can get plenty of air and sunlight. The food material taken in by the root is manufactured into plant food in the leaves; this material soaks up through very small tubes or pipes in the stem; these tubes can be seen if the stem is cut across. This, then, is the work that the stem does, but stems are by no means all alike. Some are always soft and green, while others become woody as in trees. "Herbaceous stems" is the name given to soft green stems.

Stems may be of many shapes: round, as those of most grasses and trees; square, as those of the granadilla or of dead-nettle, a common herb on abandoned farms; or ribbed, as those of the balsams Some of the stems are strong and stand erect' but many are not strong enough to stand alone, and these either climb up some other plant or creep along the ground. Climbing plants climb in several ways; they may twist themselves round a stick or another plant, as the yam does, or the morning-glory; or the plant may have some special climbing growths called tendrils, which stick to or twist round any support they can find, as in Antigonon, passion-lower, cucumber, gourd etc. Creeping plants neither climb nor can they stand erect, but have to creep along the ground, as sweet potato, bindweed, etc. Some plants do this from choice, such as those growing on exposed places or on very loose soil. Woody stems are found in all trees and most shrubs; the wood is formed in the stem to strengthen it.

Woody stems are not all alike, and the stems of the mango, the iroko, or the cotton, are different from those of the palms or bamboos. If you the end of a cut stem of, say, a teak tree, you will see a number of circular rings; if you will count the number of rings this number will be the age of the tree; for the tree grows by adding one of these rings each year. Trees which shed their leaves for part of the year show these annual rings, as they are called, best. The trees with stems like a palm (i.e., the same thickness all the way up) never have these annual rings. So far all the stems we have considered grow above the ground; but there are quite a number of stems of very important plants which grow underground. These, although at first sight they may look like roots, must never be mistaken for roots; for they bear buds and often leaves, which a root never does. Underground stems are useful to the plant for the storing of food or for the producing of young. Yams and coco-yams are underground stems that are swollen with stored food; such stems are called tubers.

On the tubers are to be seen numbers of small buds, which will grow into new plants the following season, making use at first of the stored food. In some cases the bases of the leaves get swollen and closely surround a short swollen stem. What is called a bulb is formed. The onion is a typical example of a bulb. if an onion is cut through lengthways in hail; the stem, the swollen leaves, and probably one or two buds will be seen, and also the flower of the plant. It is for the use of the food found in these underground swollen stems that we cultivate a great many of our best-known vegetables. Another type of underground stem is found in the caladium, the cobra-lily, and the gloriosa; this type looks like a bulb in shape, but if you cut it down the centre you will find that it is not composed of swollen leaves but of one solid piece of stem. In this case the new plant grows from the top of the corm, as the swollen stem is called. In some cases the stem grows horizontally just below the surface of the ground, and gives off branches which grow up to the surface.

These stems grow zigzag for many feet in all directions; they are swollen and contain plant food so that if they get broken the loose portion can form into a new plant entirely. At the nodes where the branches are given off there are generally a number of roots. Such stems are called rhizomes, and common examples of these are cannas, ginger, plantains, ekong grass, ordinary river sedges, etc. Dictionary of Gardening and Plant Growing Terms and Phrases - Garden Helper, Gardening Questions and Answers Plants whose life cycle lasts only one year, from seed to blooms to seed. arboretum A garden with a large collection of trees and shrubs cultivated for scientific or educational purposes. aquatic plants Plants which grow in, live in, or live on the water. bare root Plants offered for sale which have had all of the soil removed from their roots. bedding plant Plants (mainly annuals), nursery grown and suitable for growing in beds. Quick, colorful flowers. biennial A plant that usually only lives two years, normally producing flowers and seed the second year. bipinnate Compound leaf structures with a feather-like formation of leaflets arranged in pairs, with each leaflet also pinnately divided into pairs.

bolting Vegetables which quickly go to flower rather than producing the food crop. Usually caused by late planting and too warm temperatures. bonsai A method of propagation using sections of stems, roots or leaves. damping off A fungus, usually affecting seedlings and causes the stem to rot off at soil level. It can also rot seeds before they even germinate. Sterilized seed starting mixes and careful sanitation practices can usually prevent this. Use care not to over-water. Provide good air circulation. deadheading The process of pinching or snipping off used or spent blooms to keep the plants well groomed and to prevent them from setting seed. This will promote continued bloom. dibble stick A pointed tool used to make holes in the soil for seeds, bulbs, or young plants. dethatch Process of removing dead stems that build up beneath lawn grasses. dioecious plant Plants bear male flowers on one plant and female flowers on another. In order to produce fruit and viable seeds, both a female and male plant must be present The process of splitting up plants, roots and all that have began to get bound together. This will make several plants from one plant, and usually should be done to mature perennials every 3 to 4 years.

dormancy The yearly cycle in a plants life when growth slows and the plant rests. Fertilizing should be withheld when a plant is in dormancy. Preparing the soil by systematically digging an area to the depth of two shovels. double flower A flower with many overlapping petals which gives it a very full appearance. drip line The circle which would exist if you drew a line below the tips of the outer most branches of a tree or plant. epiphyte A plant which grows on another plant but gets its nourishment from the air and rainfall. They do no damage to the host plant. erosion The wearing away, washing away, or removal of soil by wind, water or man. espalier The process of training a tree or shrub so that its branches grow in a flat, exposed pattern by tying, pinching and pruning the branches. evergreen A plant which never loses all of it's leaves at one time. eye An undeveloped bud growth which will ultimately produce new growth. evaporation The sticky secretion produced by sucking insects such as aphids. humus The brown or black organic part of the soil resulting from the partial decay of leaves and other matter. hybrid The offspring of two plants of different species or varieties of plants.

Hybrids are created when the pollen from one kind of plant is used to pollinate and entirely different variety, resulting in a new plant altogether. hydroponics The science of growing plants in mineral solutions or liquid, instead of in soil. inflorescence The structure that carry the flowers on a plant. J K lath In gardening, an overhead structure of evenly spaced slats of wood or other materials used to create shade. layering A method of propagation, by which a branch of a plant is rooted while still attached to the plant by securing it to the soil with a piece of wire or other means.. leaching The removal or loss of excess salts or nutrients from soil. The soil around over fertilized plants can be leached clean by large quantities of fresh water used to 'wash' the soil. Areas of extremely high rainfall sometimes lose the nutrients from the soil by natural leaching. leaf mold Partially decomposed leaf matter, used as a soil amendment. loam A rich soil composed of clay, sand, and organic matter. manure Hmmmm...... Organic matter, excreted by animals, which is used as a soil amendment and fertilizer. Green manures are plant cover crops which are tilled into the soil.

microclimate Variations of the climate within a given area, usually influenced by hills, hollows, structures or proximity to bodies of water. (i.e. when it's raining at your house, and the sun is shining on the other side of the street) micro nutrients Mineral elements which are needed by some plants in very small quantities. If the plants you are growing require specific 'trace elements' and they are not available in the soil, they must be added. monoecious plant Plants bearing separate male and female flowers on the same plant mulch Any loose material placed over the soil to control weeds and conserve soil moisture. Usually this is a coarse organic matter, such as leaves, clippings or bark, but plastic sheeting and other commercial products can also be used. native plant Any plant that occurs and grows naturally in a specific region or locality. naturalize To plant randomly, without a pattern. The idea is to create the effect that the plants grew in that space without man's help, such as you would find wild flowers growing. node The point on a stem where one or more leaves, branches or flowers start to grow. organic gardening The method of gardening utilizing only materials derived from living things. (i.e.

composts and manures) organic material Any material which originated as a living organism. (i.e. peat moss, compost, manure) palmate Shaped like a hand, with segments radiating from a central point, like fingers panicle A loose, airy, multi-branched cluster of flowers parasitic plant A plant which lives on, and acquires it's nutrients from another plant. This often results in declined vigor or death of the host plant. peat moss The partially decomposed remains of various mosses. This is a good, water retentive addition to the soil, but tends to add the acidity of the soil pH. A non-woody plant which grows and lives for more than two years. Perennials usually produce one flower crop each year, lasting anywhere from a week to a month or longer. perlite A mineral, which when expanded by a heating process forms light granuals. Perlite is a good addition to container potting mixes, to promote moisture retention while allowing good drainage. Any insect or animal which is detrimental to the health and well being of plants or other animals. photosynthesis The internal process by which a plant turns sunlight into growing energy.

The formation of carbohydrates in plants from water and carbon dioxide, by the action of sunlight on the Chlorophyll within the leaves. pinching back Utilizing the thumb and forefinger to nip back the very tip of a branch or stem. Pinching promotes branching, and a bushier, fuller plant pinnate A compound leaf structure with a feather-like formation of leaflets arranged in pairs or alternating along the main stem. pistil The seed-bearing organ of a flower, consisting of the ovary, stigma, and style. pollination The transfer of pollen from the stamen (male part of the flower) to the pistil (female part of the flower), which results in the formation of a seed. Hybrids are created when the pollen from one kind of plant is used to pollinate and entirely different variety, resulting in a new plant altogether. potting soil A soil mixture designed for use in container gardens and potted plants. Potting mixes should be loose, light, and sterile. propagation Various methods of starting new plants ranging from starting seeds to identical clones created by cuttings or layering. pH Basically, pH is a measure of the amount of lime (calcium) contained in your soil.

A soil with a pH lower than 7.0 is an acid soil, a soil pH higher than 7.0 is alkaline soil. Soil pH can be tested with an inexpensive test kit. Q raceme A single stemmed inflorescence with flowers on individual stalks along a stem. The bottom flowers open first as the raceme continues to elongate. Snapdragon and Delphinium flowers grow on racemes. A modified plant stem which grows horizontally, under the surface of the soil. New growth then emerges from different points of the rhizome. Irises and some lawn grasses are rhizome plants. root ball rootbound A condition which exists when a potted plant has outgrown its container. The roots become entangled and matted together, and the growth of the plant becomes stunted. When repotting, loosen the roots on the outer edges of the root ball, to induce them to once again grow outward. rooting hormone A powder or liquid growth hormone, used to stimulate a plant cutting to send out new roots from a stem node. rosette A slender stem growing out from the base of some plants, which terminates with a new offset plant.

The new plant may be severed from the parent after it has developed sufficient roots. relative humidity The measurement of the amount of moisture in the atmosphere. scarification Scratching or nicking of a seed's shell to facilitate germination. scion A short length of stem, taken from one plant which is then grafted onto the rootstock of another plant. single flower soil pH Basically, pH is a measure of the amount of lime (calcium) contained in your soil. A soil with a pH lower than 7.0 is an acid soil, a soil pH higher than 7.0 is alkaline soil. Soil pH can be tested with an inexpensive test kit. sphagnum A bog moss which is collected and composted. Most peat moss is composed primarily of sphagnum moss. This moss is also packaged and sold in a fresh state, and used for lining hanging baskets and air layering. spore Spores are the reproductive cell structure of ferns, fungi and mosses. Fern spores develop inside small green capsules on the underside of the fronds, called sporangia. These types of plants do not form flowers nor produce seeds.

staking The practice of driving a stake into the ground next to, and as a support for a plant. When attaching the plant to the stake, be sure that it is tied loosely so it doesn't strangle the stem. When staking a potted plant, the stake should be set into the planter before the plant is added. stolon An above ground spreading stem or runner that often produces a new plant at the tip. (example: Strawberry plants ) A process used to break the dormancy of a seed. This usually requires that the seeds be placed in a moistened rooting medium and kept in the refrigerator or freezer for a designated length of time sucker A growth originating from the rootstock of a grafted plant, rather than the desired part of the plant. Sucker growth should be removed, so it doesn't draw energy from the plant. systemic A chemical which is absorbed directly into a plants system to either kill feeding insects on the plant, or to kill the plant itself. tap root plant stem plant stem Figure 1. Cross-section of a mature plant stem. Figure 2. Stages in the germination and early growth of a pea. Figure 3. Section of the tip of a stem. Figure 4.

Passage of a stem from embryonic structure to mature structure with differentiated structures. Figure 5. Stems of butcher's broom, organ, and prickly pear. Figure 6. Types of stems. Plant stems perform two basic functions: they support the leaves and flowers and they carry water and food from place to place within the plant . A typical stem is cylindrical and may be soft (herbaceous) or woody. It is usually branched and leafy. The point at which a leaf joins the stem is called the node. There are several nodes on a stem, each separated by leafless internodes. Each node has one or more leaves, each of which has a bud in its basal angle ( axil ). These axillary buds often remain dormant but may produce a branch, especially if the main stem is damaged. Buds are in fact miniature shoots (stems) which have not yet elongated. The leaves are clustered together around the tip (the growing point). It is in the winter that buds are most noticeable. The outer leaves are modified as protective scales around the young stem. When these scale-leaves fall off in spring they leave rings of scars on the stem.

The age of a woody twig can be established by counting these scar patches. The main axis of a plant may continue to grow throughout life, giving off branches laterally. This is the racemose branching pattern, well illustrated in larches and "Christmas trees." The cymode branching pattern differs in that the main stem apex does not continue through life. It ends in a flower or dies away and a lateral bud assumes the main position each year. Internal structure of a stem The internal structure of the stem reflects the function (see Figure 1). There are conducting and supporting tissues. These are basically the same in all parts of the plant and the description given here will cover roots and leaves as well. It is in the arrangement of the tissues that roots and stems mainly differ. The internal structure is studied by cutting very thin sections and examining them under a microscope. A suitable, easily obtained dicotyledon stem for study is that of the sunflower. A section of an internode cut a short distance from the tip shows the mature primary structure. The epidermal cells are regular and have a waxy cuticle on the outside. The cortex cells are simple and of the type known as parenchyma .

In the outer cortical region the cells have thicker walls for extra support and are known as collenchyma cells. The inner layer of the cortex is the starch sheath whose cells have starch deposits within them. Inside this sheath is the stele, made up of vascular tissue and pith. The conducting (vascular) tissue is arranged in vascular bundles. Each bundle has on the outside a number of sclerenchyma fibers. These are elongated cells whose walls have been impregnated with various chemicals known collectively as lignin . The fibers have no cytoplasm and are therefore not living cells. They are tough and elastic, enabling the plant to stand bending by the wind. The phloem is the tissue through which manufactured food is transported. It consists of elongated sieve-tubes with large vacuoles . The ends of the tubes are perforated (sieve-plates), allowing passage of the food solutions. Each sieve tube has a companion cell alongside it which regulates its functioning. There is usually some ordinary parenchyma tissue too. The dicotyledon bundles are said to be open (i.e., they contain cambium). Cambium is the name given to the actively dividing cells or meristems in the center of the bundle.

These play an important part in secondary growth. The inner part of the bundle consists of the water-conducting tissue called xylem . These are elongated vessels and shorter tracheids , although the latter are not common. Xylem tissues are dead and have lignified walls. They are not evenly thickened however. The thickness is spiral or ringed in the first-formed xylem (protoxylem). The later vessels have numerous unthickened pits through which the water passes to neighboring vessels. The pith and the medullary rays are normally of simple parenchyma cells. Monocotyledon stems differ considerably from the foregoing. The bundles are not arranged in any regular pattern and are of the closed type, i.e., there is no cambium. Strengthening tissue may occur, especially near the outside. The bundles are frequently surrounded by sclerenchyma fibers. Plant growth When a seed planted in the soil germinates, the embryo that is formed is elongate in shape. At the tip of the part which grows upwards there is an apical shoot, whose growth increases the height of the stem. The downward-growing tip will penetrate the soil and branch to form a network of roots.

In both cases, actual growth takes place just behind the tip not along the whole length of the stem and root (see Figure 2). The growing point is surrounded and protected by the developing leaves. In the axils of the leaves axillary buds develop into new growing points. The growing point in a stem is situated right at the tip. It consists of rapidly dividing meristematic cells which are without vacuoles (see Figure 3). Growth takes place just behind the tip. The cells produced by the meristem develop vacuoles and lengthen rapidly, producing an elongation of the stem. As the cells lengthen they begin to differentiate, or change in character. The outer cells develop into epidermal cells and very quickly tiny leaves arise and cover the growing point. Other cells give rise to the cortex, while certain columns of cells retain their ability to divide and are known as pro-cambium strands (see Figure 4). In the elongating region the first conducting tissues (protoxylem and protophloem) are formed from the procambial strands. The vascular tissues lengthen very rapidly to form the conducting strands.

At about the same time the first strengthening tissues (sclerenchyma fibers) develop on the outside of the phloem. The procambial strand continue to produce vascular tissue beyond the elongating region until only a narrow band of cambium remains. Secondary thickening occurs to some degree in most dicotyledons and is especially marked in those with perennial (persistent) aerial parts (i.e. woody plants). It involves the laying down of extra vascular and strengthening tissue necessitated by the increased size of the plant. Some of the cells of the medullary rays become active again and join with the cambium of the vascular bundles to form a meristematic ring. Secondary xylem and phloem are then formed. The xylem tissues build up inside the cambium and force it and the phloem further from the center. A tree trunk consists mainly of xylem tissue, with a thin layer of phloem near the outside. In areas with well-marked seasons the xylem shows annual rings. In the spring, when the sap is rising, the xylem consists mainly of large vessels, but in autumn there is a high proportion of fibers. In cross section spring and and autumn wood are very different.

producing the ringed effect. Secondary phloem does not build up a thick layer because the cells are unthickened and are sooner or later crushed by the pressure of the expanding xylem tissue. This tension usually breaks down the epidermis too and its protective function is taken over by the phellum. This is a corky tissue formed from a layer of cortex which becomes meristematic. The phellum is a dead tissue and cuts off food from all tissues outside it. These tissues die and as a whole they form the bark of a tree. Modified stems Stems frequently perform other functions in addition to or instead of their basic ones. The only thing to remember about a stem is that it is any part of the plant from which leaves, flowers, or buds grow, so that they can be very diverse indeed and may often quite strange (see Figures 5 and 6). The chief modifications are concerned with food storage and vegetative reproduction . Runners are long thin stems which grow over the ground surface and produce new plants some distance from the parent. Strawberries reproduce rapidly in this way. Horizontally-growing underground stems are termed rhizomes.

They may be food stores, as in irises, or merely reproductive shoots, as in some grasses and weeds, such as couch-grass and bindweed. The 'eyes' of a potato are buds, so the potato is also a stem. Potatoes are swellings (tubers) arising on the underground parts of the stem. They serve as food storage and reproductive organs. These underground stems can be distinguished from roots by the presence of scale leaves and buds. Corms are special types of underground stems. They are swollen with food reserves and carry a number of scale leaves. One or more buds are developed in the axils of the leaves and these buds produce the flowering shoot. The base of each new shoot forms a new corm. Corms are distinct from bulbs. The latter are small underground stems with a number of fleshy scales in which food material is stored. In both corms and bulbs the flowering shoot is fully formed underground and grows rapidly when environmental conditions are suitable. Next year's shoot develops in the axil of one of this year's leaves. Climbing stems are fairly common.

They may climb by coiling round a support (e.g., runner beans), by using tendrils (e.g., vines), or by using hooked prickles to scramble over the vegetation (e.g., blackberry). Protective spines often occur as stem modifications (hawthorn) or as stem outgrowths (rose). Stems are in some cases modified to perform as leaves. Butcher's broom has very leaf-like branch stems and its leaves are tiny scales. The stems of cacti also act as leaves (and water stores too). Stems acting as leaves are called phylloclades. Strange stems A great variety of plant structures which grow above the ground must be regarded as stems, although they do not look like them. The illustrations below show a few of the more striking and curious of these; most strange stems have grown in these shapes for a definite reason, such as adapting the plant to local conditions. Stems with suckers In the American vine (Vitis inconstans) some shoots are transformed into tendrils with flattened suckers at the ends. These have the power of sticking to any surface they meet.

The suckers enable the plant to cling to trees or rocks and so to climb up into the light and air. Spine-like stems If we examine a hawthorn bush or an anchor plant (Colletia) we find it has many sharp thorns. These have tiny scale-like leaves, which show that they are really stems. The thorns are a good protection against browsing herbivorous (plant-eating) animals. Round or spherical stems In some plants of the cactus family, such as Echinocactus and mammillaria, the whole plant is spiny and shaped like a ball. Here the spines are leaves whose shape has altered and the 'ball' is a stem. This inflated stem serves to store water in the very dry deserts where these cactus grow. The leaves are reduced to thorns to prevent loss of water by evaporation and, of course, to give protection too. Ribbon-shaped stems In the curious plant called Muhelembeckia the stem has the shape of a long ribbon, from which sprout a few scale-like leaves. This is another example of stems being altered in structure in order to reduce loss of water.

Spatulate and columnar stems The flattened, bat-shaped stems of the prickly pear (Opuntia) and the thick, column-like stems of the organ cactus (Cereus) are both designed to store water, as these are desert plants. The leaves are reduced to spines as in other cacti. Leaf-like stems Anyone looking at a plant of the butcher's broom (Ruscus aculeatus) would suppose that it has the normal arrangement of leaves growing out of a stalk. But if you look closely at the 'leaves' you will see that little white flowers grow out of the centers of them. They cannot therefore be leaves - they are stems transformed to look like leaves and to perform the function of both stems and leaves. Where is the chlorophyll? We know that plants make their food and live by means of a process called photosynthesis that is carried out with the help of the green substance chlorophyll in their leaves. In plants in which the leaves are lost or reduced to spines the stem is always green and takes over this essential function from the leaves.

Question: What name is given to a plant that has a stem that grows horizontally below the surface of the soil?
Answer: RHIZOME