Introduction to Stem
Stem, a plant organ that typically grows above ground and bears leaves, buds, and flowers. Stems conduct water and minerals from the roots to the leaves and also transport food made in the leaves to all other parts of the plant. In addition, stems give support to the plant and may also store food and water, act as reproductive structures, and may themselves manufacture food. .
Stems conduct water and minerals from the roots to the leaves.Stems are found only in vascular plants; that is, plants that have specialized conducting tissues. Seed plants and ferns have stems; mosses do not. Stems provide many useful products, the most important of which is probably wood. (A tree trunk is a stem.) Stems are also the source of tannins, latex, and resins. Edible products that come from stems include sugar from sugarcane, maple syrup, and the white potato. (The potato is an underground stem.) Quinine comes from the bark of the cinchona tree and cinnamon from the bark of the cinnamon tree.
There are basically two different kinds of stemswoody stems and herbaceous stems. Woody stems, found in trees and shrubs, develop substantial amounts of woody tissue and are characteristic of plants that live for many years. Herbaceous stems develop little, if any, woody tissue and are characteristic of plants that live only for one season or plants whose stems die back at the end of a growing season.
Certain types of stems, called modified stems, do not fit the typical description of stems as to form and function. Some modified stems, such as tubers, bulbs, and corms, grow underground. Others, such as stolons and certain kinds of tendrils and thorns, grow aboveground. Woody and herbaceous stems are discussed in the following section, modified stems in the final section.
Structure and Development
Thousands of different kinds of plants have stems and there are many different variations of the typical stem, which grows in an erect manner above ground. Discussed below are the general features found among most of the woody and herbaceous stems.
The most prominent structures found on a stem are buds and leaves. At the tip of each stem is a terminal bud which contains a special tissue called the apical meristem. During the growing season the cells in the apical meristem continually divide to produce new cells, causing the stem to grow in length. Buds found on the stem just above the place where a leaf is attached are called lateral, or axillary, buds. Buds that occur elsewhere (except for terminal buds) are called adventitious buds. Among the structures that grow from buds are new leaves, stems, and flowers. The point on a stem from which a leaf or bud grows is called a node. The region between two successive nodes is called an internode.
Woody StemsThe structure of a woody stem during its first season of growth differs in many respects from that present in subsequent seasons. Most of the tissues formed during the first year are called primary tissues. They include the epidermis, cortex, primary vascular tissues, and pith. The epidermis is the outermost layer that covers and protects the underlying tissues. The cortex, just beneath the epidermis, stores and manufactures food, strengthens, and protects. The central portion of the young stem is made up of the pith, which also stores food. Strips of cells, called pith rays, conduct materials laterally between cortex and pith.
Between the pith and the cortex are the primary vascular tissues, which are usually arranged in bundles. These bundles, called vascular bundles, are composed of primary phloem, primary xylem, and vascular cambium. The major function of the phloem is to conduct foods downward from the leaves. The xylem serves mainly to conduct water and minerals upward from the roots to various plant parts. Cells of the vascular cambium divide laterally to form new cells, causing the stem to grow in diameter. The vascular cambium eventually becomes a continuous layer of cells between the xylem and the phloem.
The tissues formed in a woody stem by the activity of the vascular cambium are called secondary tissues. Some secondary tissues are produced late in the first season's growth, but most are formed after the first season. Secondary tissues consist mainly of secondary xylem (wood) and bark. All the wood is produced by the vascular cambium, which also produces lesser amounts of secondary phloem. Secondary phloem is often called inner bark. The layer of secondary xylem produced during one growing season constitutes the annual ring.
A special secondary growth tissue, called cork cambium, produces cork. As the stem grows, the cork replaces the epidermis as the protective tissue of the stem. The cork, cork cambium, cortex, and secondary phloem make up the bark. As a woody stem grows older, the primary phloem, cortex, and epidermis are completely lost. The pith and primary xylem are always present, though they may be greatly reduced.
Herbaceous StemsThere are two basic kinds of herbaceous stemsthose found among the dicotyledons, or dicots, and those of the monocotyledons, or monocots. Dicots are plants whose seeds have two seed leaves (cotyledons). The seeds of monocots have one cotyledon. Monocots and dicots differ not only in the structure of their seeds, but also in the structure of their leaves, stems, and flowers. Many woody plants are dicots, but no true woody plants are monocots.
As shown in the accompanying illustration, mature stems of herbaceous dicots closely resemble those of young woody plants. Among herbaceous dicots, the vascular tissues may be arranged in bundles or they may appear as continuous layers. The vascular cambium may be a continuous layer or it may be confined to the vascular bundles. The most notable difference between woody and herbaceous dicots is in the activity of the vascular cambium. Whereas in woody dicot stems the vascular cambium is very active and goes on to produce much secondary tissue, in herbaceous dicots the vascular cambium is relatively inactive and little, if any, wood is formed. Under favorable conditions, however, certain herbaceous dicots may develop fairly extensive secondary tissues. In contrast to true woody plants, though, these herbaceous plants are relatively short-lived.
Most monocot plants have no vascular cambium and thus have no secondary growth. In monocots, the xylem and phloem are always arranged in vascular bundles, never in continuous layers. The bundles are often scattered throughout the stem instead of being arranged in a circle. There is no sharp dividing line between the cortex and the pith, and the tissue surrounding the bundles is termed ground parenchyma. An epidermis surrounds the monocot stem.
Modified Stems
Although many of the modified stems do not look or grow like the typical stems described above, their tissues and the patterns of their development are like those of other stems. In most cases, plants with modified stems also have conventional stems.
Underground Modified StemsRhizomes, or rootstocks, are roughly cylindrical stems that grow horizontally underground. They store food and live throughout the year. Under favorable conditions, as in spring, they send up new shoots. Many weeds, such as bind-weed, quack grass, and horse nettle, have rhizomes, thus making them particularly difficult to eradicate. Other plants with rhizomes are iris, asparagus, rhubarb, and most ferns. The chief functions of rhizomes are reproduction and food storage.
Tubers, such as the white potato, are greatly enlarged, short underground stems filled with stored food in the form of starch. The eyes of a potato are actually buds from which conventional, aerial stems and leaves will grow. Potato fields are planted by placing pieces of tuber with one or two eyes into the ground.
Bulbs consist of a small stem surrounded by numerous fleshy leaves. Among the plants with bulbs are onions, daffodils, and tulips. Bulbs store food in the fleshy leaves, usually in the form of sugar. They are also reproductive structures.
Corms superficially resemble bulbs, but, unlike bulbs, they consist mostly of stem tissue bearing a few small, scaly leaves. Like bulbs, corms store food, but in their stem tissue rather than in the leaves. Corms are also reproductive structures.
Aerial Modified StemsIn various climbing plants, such as the grape and the Boston ivy, some parts of stems are modified into climbing organs called tendrils. In some plants, such as the morning glory and sweet potato, the stems grow spirally around a support. Such stems are called twiners.
Stems that grow in a creeping fashion horizontally above the surface of the soil are called runners, or stolons. If the nodes of these stems touch the soil, new plants develop. The most familiar plant with stolons is the strawberry. In some plants, such as honey locust and osage orange trees, portions of stems are modified into thorns. Some cacti and a few other kinds of plants have stems that closely resemble leaves in appearance; that is, they are leaflike in shape and are green and thus manufacture food.