The plant stem is a component of the shoot system, the portion of the plant body of the angiosperms having phototropic response. Besides the stem, the plant shoot also consists of the leaves and the reproductive organs.
The plant stem has been described as a “central axis” to which all other parts are attached. In most plants the stems are conspicuous aboveground, but in some species they are hidden below the ground. Based on various criteria, there are other more classifications of the stem.
The first stem that develops from a seed arises from the epicotyl, an embryonic shoot within the seed.
Functions of the Plant Stem
The plant stem including its outermost bark has multiple uses ranging from logs, firewood, lumber, source of pulp for paper making, source of food, fiber, medicine, latex, tannin, dye and many more. It is also the most widely used part of the plant for asexual or vegetative propagation through various methods such as by stem cuttings, air layering or marcotting and other forms of layering, budding, and grafting.
In plant growth and development, the plant stem performs the following functions:
1. It supports the leaves, flowers and fruits and connects them with the roots. In trees and shrubs, the main stem or trunk provides a strong columnar structure from which branches are attached, raising the leaves upward to be exposed more fully to the sun.
2. It conducts water, nutrients and the products of photosynthesis to and from roots and leaves. It accommodates the transport system which is necessary for the vertical and lateral movement of water and sap within the plant body.
3. It helps store water, as in cacti, and the products of photosynthesis, as in the trunk of sago palm (Metroxylon sagu) and sweet palm (Arenga pinnata) which store large stock of starch;
4. Young green stem also performs a minor role in the production of food through the process of photosynthesis, but in some species (e.g. cactus) the stem is the chief photosynthesizing organ.
5. The plant stem serves as a means of asexual reproduction in many plant species.
Parts of the Plant Stem
All stems of the angiosperms, including those which are highly modified, are recognizable from other plant organs by their presence of nodes, internodes, buds and leaves. A node is a point on the stem from which leaves or buds arise. The portion between two successive nodes is the internode.
A bud is an embryonic stem which has the potential for further plant growth. It may develop into a leaf, flower, or both. Such buds are called leaf buds, flower buds and mixed buds, respectively. Many buds remain dormant within a certain duration or they may be embedded in the stem tissue as to become hardly visible. A single bud that is found at the apex of the stem is called terminal bud while lateral buds or axillary buds occur in the leaf axils, the upper angle between the stem and leaves. As a result of injury, adventitious buds may be formed also in the internode of the stems, in leaves, or roots. The “eyes” of the potato tuber are buds.
In trees and shrubs, the shoot consists of the main stem which is commonly called trunk or bole, their main ascending axis, with lateral branches to which smaller branches and branchlets or twigs are attached.
Besides the obvious leaves and buds on herbaceous stems, other structures may be present. There may be hairs, which are outgrowths of epidermal cells, and spines, which are either modified twigs, leaves, hairs, or stipules.
In woody stems, the following plant structures may be present in addition to the leaves and buds: leaf scars after the leaves fall, at the point of attachment of the leaves to the stem; bundle scars, the tiny raised dots in the leaf scars which are broken ends of vascular bundles connecting the stem to the petiole of leaves; bud scars which are small, narrow, circular marks around a twig left by the falling away of the bud scales; twig scars left by the falling away of twigs; and fruit scars where fruits are previously attached.
Plant Stem Classifications
Most stems grow above ground and are called aboveground or aerial stems but there are some which grow below the ground, and thus called underground, below-ground or subterranean stems. Plants with no obvious stem above ground, but bear only leaves and flower stalks, are called stemless or acaulescent.
In grass plants like rice, corn and bamboo which belong to the botanical family Gramineae or Poaceae, the main stem is specially called culm. Culms are either hollow or solid stems with pronounced nodes and internodes.
The stem is basically cylindrical. However, there are many modified forms of both aerial stems (e.g. crowns, offshoots, stolons, runners, spurs) and subterranean stems (e.g. bulbs, corms, rhizomes, tubers). (Click to read Starchy Rootcrops, Tuber Crops and Corm Crops)
Offshoot is a collective term for the short, thickened lateral shoot or branches with multiple nodes and having rosette-like appearance, growing out of the main stem in certain plants. They are variously called offsets, suckers, slips, pips, etc. A stolon is a stem that grows horizontally along the ground (e.g. Bermuda grass, zoysia grass, mint). A runner is a stolon having long internodes which originate at the base of the crown of the plant (e.g. strawberry). A spur is a stem in woody plants with greatly shortened internodes and restricted growth. They appear laterally on branches. In apple and grape, the production of fruits is largely confined to the spurs. These modified stems are also used in vegetative propagation. A crown is a compressed stem from which new shoots arise. They are generally found near the surface of the soil (e.g. strawberry, African violet). In trees or shrubs with a single trunk, the crown is that portion of the trunk which marks the transition point from root to stem.
As to texture, plant stems are classified as either herbaceous, suffrutescent, or woody. Herbaceous stems, as in banana, have no permanent woody tissue, with a short life span, and die soon after flowering. Suffrutescent stems, as in eggplant and jute, are more or less woody or half-woody, usually at the basal part. Woody stems, as in all shrubs and trees, form permanent woody tissue which persist indefinitely. The woody portion of the stem is made of secondary xylem.
As to direction, plant stems are either erect, ascending, decumbent, prostrate, creeping, climbing, or twining. Stems are erect when they grow vertically upward in a direction that is perpendicular from the base; ascending when rising obliquely or in slanting direction; decumbent when more or less reclining on the ground at or near the base; prostrate when lying flat on the ground; creeping when growing flat on the ground and rooting on the nodes; climbing or scandent when rising by clinging to other objects by means of tendrils, rootlets, or other specialized organs; and twining when rising by coiling around other stems or objects.
Based on the arrangement of branches and leaves, plant stems are opposite when two are formed at the same node from opposite sides of the stem; whorled or verticillate when three or more arise from the same node in regular arrangement around the stem; fascicled or fasciculate when two or more arise in cluster from the same node on one side of the stem; alternate when one is formed at each node on one side, and the next above or below on the opposite side of the stem; distichous when regularly arranged in two ranks, one above another in two opposite rows; and secund when all are turned to one side.
(Ben G. Bareja 2010)