Stem

Stem, part of a vascular plant that commonly bears leaves and buds. It is usually aerial, upright, and elongated, but may be highly modified in structure as in the runners of the strawberry. Subterranean stems include the rhizomes of the iris; the potato is a portion of an underground stem. Some plants, such as the century plant, have very short stems that bear a dense cluster of leaves. Those points on the stem at which leaves or buds arise are called nodes, and the regions of the stem between the nodes are known as internodes.

The chief functions of stems are the production and support of leaves and reproductive structures, conduction of water and nutrients, and occasionally food storage. In some plants, such as cacti, stems also have the specialized functions of storing water and manufacturing food.

Conducting tissues within plant stems are arranged in columns called vascular bundles. These bundles are composed of xylem, which conducts water up the stem, and of phloem, which transports sugars produced by the leaf down the stem. Vascular bundles extend into leaves, in which they are called veins. As the stem grows longer, new cells are added to the vascular system, providing conductive tissue for new leaves and branches.

The arrangement of vascular bundles differs in the stems of the two major groups of angiosperms, or flowering plants: the monocotyledons and the dicotyledons. In monocotyledons, such as maize, the vascular tissue occurs in many scattered bundles throughout the cross-section of the stem. In dicotyledons, such as the pea, the vascular bundles are arranged in a cylindrical ring; the remainder of the stem constitutes the fundamental tissue and is usually divided into the cortex, the part outside the ring of vascular bundles, and the pith, the part inside the cylindrical ring. The outer layer of the stem of a herbaceous plant is called the epidermis.

In the angiosperms, only dicotyledons have true woody growth. Gymnosperms, comprising the conifers and related plants, all have woody stems. Woody stems have a thin layer of cells between the xylem and phloem, called the cambium. During the growing season, these cells divide actively, producing new cells that differentiate into xylem, or wood, towards the inner side of the cambium and phloem towards the outer side. As the cambium grows, the diameter of the stem increases, and the new phloem presses outward upon the soft tissues of the cortex, which become distorted and eventually die. A second layer of dividing cells, however, is formed either in the cortex or, in older stems, in the phloem. This layer of cells produces cork, which is the protective tissue that replaces the dead cells of the epidermis. The bark of tree trunks is thus a complex tissue consisting of phloem and cork.

Herbaceous stems, lacking in woody growth, may derive strength by other means. The monocotyledon bamboo, for example, which does not develop true wood, is strengthened by the presence of numerous fibres in its stem.

The phloem and cortex may contain fibres in addition to soft cells. Such fibres having commercial importance include linen, obtained from flax, hemp, and jute. Wood is the most important economic product of the stem.