Stem

Stem, portion of vascular plants that commonly bears leaves and buds. It usually is aerial, upright, and elongate, but may be highly modified in structure. Subterranean stems include the rhizomes of the iris and the runners of the strawberry; 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 food storage. In some plants, such as cactus, 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 (see Dicots; Monocots). In monocotyledons, such as corn, 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, or portion outside the ring of vascular bundles, and the pith, the portion inside the cylindrical ring. The outer layer of the stem of herbaceous plants is called the epidermis.

In the angiosperms, only dicotyledons have true woody growth. Gymnosperms, comprising the conifers and related plants, all have woody stems. Such 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, toward the inner side of the cambium and phloem toward 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, thus, is 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 wood, is strengthened by the presence of numerous fibers in its stem.

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

Plant Organs

The body of a vascular plant is organized into three general kinds of organs: roots, stems, and leaves. These organs all contain the three kinds of tissue systems mentioned above, but they differ in the way the cells are specialized to carry out different functions.

Roots
The function of roots is to anchor the plant to its substrate and to absorb water and minerals. Thus, roots are generally found underground and grow downward, or in the direction of gravity. Unlike stems, they have no leaves or nodes. The epidermis is just behind the growing tip of roots and is covered with root hairs, which are outgrowths of the epidermal cells. The root hairs increase the surface area of the roots and serve as the surface through which water and nutrients are absorbed.

Internally, roots consist largely of xylem and phloem, although many are highly modified to carry out specialized functions. Thus, some roots are important food and storage organs—for example, beets, carrots, and radishes. Such roots have an abundance of parenchyma tissue. Many tropical trees have aerial prop roots that serve to hold the stem in an upright position. Epiphytes have roots modified for quick absorption of rainwater that flows over the bark of the host plants.

Roots increase in length through the activity of apical meristems and in diameter through the activity of lateral meristems. Branch roots originate internally at some distance behind the growing tip, when certain cells become meristematic.

Stems
Stems usually are above ground, grow upward, and bear leaves, which are attached in a regular pattern at nodes along the stem. The portions of the stem between nodes are called internodes. Stems increase in length through the activity of an apical meristem at the stem tip. This growing point also gives rise to new leaves, which surround and protect the stem tip, or apical bud, before they expand. Apical buds of deciduous trees, which lose their leaves during part of the year, are usually protected by modified leaves called bud scales.

Stems are more variable in external appearance and internal structure than are roots, but they also consist of the three tissue systems and have several features in common. Vascular tissue is present in bundles that run the length of the stem, forming a continuous network with the vascular tissue in the leaves and the roots. The vascular tissue of herbaceous plants is surrounded by parenchyma tissue, whereas the stems of woody plants consist mostly of hard xylem tissue. Stems increase in diameter through the activity of lateral meristems, which produce the bark and wood in woody plants. The bark, which also contains the phloem, serves as a protective outer covering, preventing damage and water loss.

Within the plant kingdom are many modifications of the basic stem, such as the thorns of hawthorns. Climbing stems, such as the tendrils of grapes and Boston ivy, have special modifications that allow them to grow up and attach to their substrate. Many plants, such as cacti, have reduced leaves or no leaves at all, and their stems act as the photosynthetic surface. Some stems, including those of many grasses, creep along the surface of the ground and create new plants through a process called vegetative reproduction. Other stems are borne underground and serve as food-storage organs, often allowing the plant to survive through the winter; the so-called bulbs of the tulip and the crocus are examples.

LeavesThe leaf is the primary photosynthetic organ of most plants. Leaves are usually flattened blades that consist, internally, mostly of parenchyma tissue called the mesophyll, which is made up of loosely arranged cells with spaces between them. The spaces are filled with air, from which the cells absorb carbon dioxide and into which they expel oxygen. The mesophyll is bounded by the upper and lower surface of the leaf blade, which is covered by epidermal tissue. A vascular network runs through the mesophyll, providing the cell walls with water and removing the food products of photosynthesis to other parts of the plants.

The leaf blade is connected to the stem through a narrowed portion called the petiole, or stalk, which consists mostly of vascular tissue. Appendages called stipules are often present at the base of the petiole.

Many specialized forms of leaves occur. Some are modified as spines, which help protect plants from predators. Insectivorous plants possess highly modified leaves that trap and digest insects to obtain needed nutrients. Some leaves are brightly colored and petal-like, serving to attract pollinators to otherwise small, unattractive flowers. Perhaps the most highly modified leaves are flowers themselves. The individual parts of flowers—carpels, stamens, petals, and sepals—are all modified leaves that have taken on reproductive functions.