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Stems

Stems

The body of a land plant is composed of a shoot system and a root system. The shoot includes the stem, leaves, and other reproductive systems. The stem is the axis or supporting column of the shoot system, providing support for the leaves and the reproductive structures. It is the channel through which water and mineral elements derived from the soil flow from the roots to the upper parts of the plant. It is also the route taken by organic products synthesized in the leaves when they are transported to other plant parts.

Anatomy

The structure of the stem is organized around the vascular or conducting system, which in seed plants consists of an interconnected pattern of bundles extending lengthwise in the stem. At each node, or site of leaf attachment, one or more of the bundles bends outward into the petiole (leaf stalk) to provide the vascular supply of the leaf, the leaf trace. Despite this constant loss of bundles into the leaves, the number remains constant because of branching within the stem. Each bundle consists of two tissues, the inner xylem, which is the channel for water and dissolved minerals, and the outer phloem, which transports the organic products of photosynthesis.

In dicotyledorous plants the bundles are arranged in a ring around a central pith composed of thin-walled parenchyma . The bundles themselves are surrounded by the cortex, which consists of parenchyma often with additional supporting tissues. The entire structure is bounded by the epidermis, which prevents excessive water loss. In many herbaceous plants the cells of the cortex contain chloroplasts and are photosynthetic, and the surrounding epidermis includes the stomata necessary for gas exchange. In the monocotyledons, such as lilies and grasses, the vascular bundles are not arranged in a ring but are distributed through the central region of the stem inside the cortex.

Growth

Unlike animals, which ordinarily reach a final stable size, plants continue to grow throughout their lives. This indeterminate growth is accomplished by means of meristems. Growth of the stem, and of the entire shoot, is accomplished through the activity of the shoot apical meristem located at the tip of the stem. This region of continued cell division gives rise to the tissues of the stem and also to the primordia of the leaves that the stem bears. The leaves are initiated in a regular pattern and develop to mature size along with the tissues of the stem. Although the tissues of the stem are initiated by the apical meristem, much of the growth takes place below the meristem as the tissues are acquiring their mature size and functional properties through the process of differentiation. When initiated by the apical meristem, the leaf primordia are very close together; the growth that occurs below the meristem is by expansion of the internodes (regions between the leaf attachment points). Elongation of the internodes separates the leaves to their final positions. In some plants the expansion of the internodes is limited so that the mature leaves are close together and the stem is very short. This is seen, for example, in rosette plants such as the dandelion.

One further aspect of growth is very important for trees or shrubs; this is the formation of a woody secondary body. A meristem known as the vascular cambium develops between the xylem and the phloem in the bundles. Divisions of this cambium form a layer of xylem to the inside and of phloem to the outside. This secondary production of tissue may be limited if the life of the stem is short, but in long-lived trees it may build up a trunk of extensive dimensions. The development of the secondary body expands the stem laterally. If continued for any length of time, this lateral growth ruptures the protective epidermis. Continued protection is provided by the formation of the cork cambium, which lays down a layer of largely impervious cork tissue.

Branching

In many of the lower vascular plants the stem branches by the subdivision of the apical meristem into two portions each of which forms a branch stem. In the seed plants, however, branches are formed through the development of buds formed in leaf axils , that is, on the stem just above the point of attachment to the leaf. Typically a small portion of the apical meristem is detached, or left behind, in the axillary position as the apical meristem advances and this forms a daughter apical meristem that initiates leaf primordia and forms an axillary bud . These often become arrested after a certain degree of development is attained, remaining suppressed by the apical meristem. When released, such a bud expands to form a branch, a replica of the main axis. In addition to this regular pattern of branching, many plants form buds capable of developing into new shoots in various locations other than leaf axils, including roots. Such buds are called adventitious .

Modified Stems

There are many ways in which the basic form of the stem deviates from the typical pattern just described, and some of these are of economic significance. In the strawberry, for example, branches called runners extend over the ground, take root at nodes and establish new plants. This natural means of reproduction is used to propagate the plant commercially. The same kind of structure but extending underground is called a stolon; when it is the main axis of the plant it becomes a rhizome. Rhizomes may become enlarged and serve as repositories for stored nutrients that may be economically important.

Another similar structure develops when the terminal portion of a stolon enlarges to form a tuber, as in the potato. A corm, seen in the gladiolus, is an upright underground stem that is greatly swollen with stored nutrients. The superficially similar bulb, as in the onion, has a much reduced stem and the stored materials are in fleshy leaf bases. In some cases stems may become broad and flattened to resemble leaves, and are called cladophylls. Many plants of desert or salty environments become very succulent or fleshy and bear reduced leaves. Stems may also assume a protective function by developing sharp, hardened tips as spines, while others form tendrils, which may help climbing stems to attach to their support.

see also Anatomy of Plants; Germination and Growth; Meristems; Phyllotaxis; Potato; Shape and Form of Plants.

Taylor A. Steeves

Bibliography

Esau, Katherine. Anatomy of Seed Plants, 2nd ed. New York: John Wiley & Sons, 1977.

Mauseth, James D. Plant Anatomy. Menlo Park, CA: Benjamin/Cummings Publishing Company, 1988.

Rudall, Paula. Anatomy of Flowering Plants, 2nd ed. Cambridge: Cambridge University Press, 1992.

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