Cross-Section of a Stem
Stems are structures which support buds and leaves and serve as conduits for carrying water, minerals, and sugars. The three major internal parts of a stem are the xylem, phloem, and cambium. The xylem and phloem are the major components of a plant’s vascular system.
The vascular system transports food, water, and minerals and offers support for the plant. Xylem vessels conduct water and minerals, while phloem tubes conduct food. The vascular systems of monocots and dicots differ. While both contain xylem and phloem, they are arranged differently. In the stem of a monocot, the xylem and phloem are paired into bundles; these bundles are dispersed throughout the stem. But in the stem of a dicot, the vascular system forms rings inside the stem. The ring of phloem is near the bark or external cover of the stem and is a component of the bark in mature stems. The xylem forms the inner ring; it is the sapwood and heartwood in woody plants. The difference in the vascular system of the two groups is of practical interest to the horticulturist because certain herbicides are specific to either monocots or dicots. An example is 2, 4, -D, which only kills dicots.
The cambium is a meristem, which is a site of cell division and active growth. It is located between the xylem and phloem inside the bark of a stem and is the tissue responsible for a stem’s increase in girth, as it produces both the xylem and phloem tissues.
Stems may be long, with great distances between leaves and buds (branches of trees, runners on strawberries), or compressed, with short distances between buds or leaves (fruit spurs, crowns of strawberry plants, dandelions). Stems can be above the ground like most stems with which we are familiar, or below the ground (potatoes, tulip bulbs). All stems must have buds or leaves present to be classified as stem tissue.
An area of the stem where leaves are located is called a node. Nodes are areas of great cellular activity and growth, where auxiliary buds develop into leaves or flowers. The area between nodes is called the internode.
|Parts of a Stem
The length of an internode may depend on many factors. Decreasing fertility will decrease internode length. Internode length varies with the season. Too little light will result in a long internode causing a spindly stem. This situation is known as stretch or etiolation. Growth produced early in the season has the greatest internode length. Internode length decreases as the growing season nears its end. Vigorously growing plants tend to have greater internode lengths than less vigorous plants. Internode length will vary with competition from surrounding stems or developing fruit. If the energy for a stem has to be divided between three or four stems, or if the energy is diverted into fruit growth, internode length will be shortened.
A crown is a region of compressed stem tissue from which new shoots are produced, generally found near the surface of the soil. Crowns (strawberries, dandelions, African violets) are compressed stems having leaves and flowers on short internodes.
A spur is a compressed fruiting branch. Spurs are short, stubby, side stems that arise from the main stem and are common on such fruit trees as pears, apples, and cherries, where they may bear fruit. If severe pruning is done close to fruit-bearing spurs, the spurs can revert to a long, nonfruiting stem.
A stolon is a horizontal stem that is fleshy or semi-woody and lies along the top of the ground. A runner is a type of stolon. It is a specialized stem that grows on the soil surface and forms a new plant at one or more of its nodes. Strawberry runners are examples of stolons. Remember, all stems have nodes and buds or leaves. The leaves on strawberry runners are small but are located at the nodes which are easy to see. The spider plant also has stolons.
A tuber is an enlarged portion of an underground stem like potato tubers, tulip bulbs, and iris rhizomes are underground stems that store food for the plant. The tuber, like any other stem, has nodes that produce buds. The eyes of a potato are actually the nodes on the stem. Each eye contains a cluster of buds.
A rhizome is a specialized stem which grows horizontally at or just below the soil surface. They act as a storage organ and means of propagation in some plants and are similar to stolons. Some rhizomes are compressed and fleshy such as those of iris; they can also be slender with elongated internodes such as bentgrass. Johnsongrass is a hated weed principally because of the spreading capability of its rhizomes.
Tulips, lilies, daffodils, and onions are plants that produce bulbs--shortened, compressed, underground stems surrounded by fleshy scales (leaves) that envelop a central bud located at the tip of the stem. If you cut through the center of a tulip or daffodil bulb in November, you can see all the flower parts in miniature within the bulb. Many bulbs require a period of low-temperature exposure before they begin to send up the new plant. Both the temperature and length of this treatment are of critical importance to commercial growers who force bulbs for holidays.
Corms are not the same as bulbs. They have shapes similar to bulbs, but do not contain fleshy scales. A corm is a solid, swollen stem whose scales have been reduced to a dry, leaflike covering.
Some plants produce a modified stem that is referred to as a tuberous stem. Examples are tuberous begonia and cyclamen. The stem is shortened, flattened, enlarged, and underground. Buds and shoots arise from the crown and fibrous roots are found on the bottom of the tuberous stem. In addition, some plants such as the dahlia and the sweet potato produce an underground storage organ called a tuberous root, which is often confused with bulbs and tubers. However, these are roots, not stems, and have neither nodes nor internodes. It may sometimes be difficult to distinguish between roots and stems, but one sure way is to look for the presence of nodes. Stems have nodes; roots do not.
Stems are commonly used for plant propagation. Above-ground stems can be divided into sections that contain internodes and nodes. They are utilized as cuttings and will produce stems that are good propagative tissues. Rhizomes can be divided into pieces. Bulbs form small bulblets at the base of the parent bulb. Cormels are miniature corms that form under the parent corm. Tubers can be cut into pieces containing eyes and nodes. All of these will produce new plants.
Types of Stems
Trees are perennial woody plants, usually have one main trunk, and are usually more than 12 feet tall at maturity. Shrubs are perennial woody plants that may have one or several main stems, and are usually less than 12 feet tall at maturity.
A vine is a plant which develops long, trailing stems that grow along the ground unless they are supported by another plant or structure. Some twining vines circle their support clockwise while others circle counter clockwise. Climbing vines are supported by aerial roots, slender tendrils which encircle the supporting object, or tendrils with adhesive tips.
Texture and Growth of Stems
A cane is a stem which has a relatively large pith and usually lives only one or two years. Examples of plants with canes include rose, grape, and blackberry.
Herbaceous or succulent stems contain only small amounts of xylem tissue and usually live for only one growing season. If the plant is perennial, it will develop new shoots from the root.
Life Cycles of Plants
Stems as Food