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Stem
Stems are formed from the epicotyl, the portion of an embryo above the seed leaves (cotyledons). They usually have leaves, and their growing tip or shoot apex has small stubs on its side, which develop into leaves. The stem has several functions. It acts as a support for the leaves, and transports nutrients to the leaves and food to the roots. In some plants, such as cacti, the stems also photosynthesize and store food.
Stems comprise several parts. In the middle is the pith, a region of undifferentiated cells which, together with the cortex, form a base for the other layers to grow in. Surrounding the pith is either a ring or a series of bundles containing the cells that transport fluids. This is known as the vascular tissue. Outside of this is a layer of cells known as the cortex and then the epidermis, the skin of the stem, which is modified into bark in woody plants.
Vascular System
The vascular system is made up of xylem, which transports water and nutrients from the roots to the leaves, and phloem, which transports food to the roots and the stem.
In herbaceous plants the xylem and phloem are found in bundles running vertically through the stem. In woody plants, however, the stem structure is more elaborate. The xylem and phloem form rings. The xylem is formed yearly as a ring on the inside of the growth layer (called the cambium) and becomes wood. Each yearly ring is called a growth ring. The phloem is formed in rings on the outside of the cambium and is periodically sloughed off as bark.
Bark
Bark is the external skin of a woody stem. It can vary in appearance, but its main functions are to protect the growing stem and at the same time allow air to pass into the living portions of the stem. Because the stem is constantly growing in circumference, the bark is often shed periodically.
Many trees and shrubs can be identified solely by their bark. The following is a short list of some bark types:
- Smooth: Without hair, glands, or roughness. Beech trees (Fagus species) have smooth bark.
- Ridged: With prominent, longitudinal ribs. Chestnut oak (Quercus muehlenbergii) has deep ridges.
- Furrowed: With longitudinal grooves. The American elm (Ulmus americana) has furrowed bark.
- Plated: With shallow, circular depressions. The bark of the red pine (Pinus resinosa) is plated.
- Fibrous: With thread-like strands. Grape vines have fibrous bark.
Stem Modifications [Illustration]
Stems of some plants occur in a modified form to accommodate additional functions. The following are some modified stem forms:
- Corm: An underground, vertical stem that is swollen. A corm looks very much like a bulb but is composed of stem tissue, as opposed to a bulb, which is made up of leaf tissue. Gladioli, crocuses, and freesias are corms.
- Phylloclade: A flattened, green stem with small, scale-like leaves. Phylloclades look like leaves and even function as leaves. But morphologically, they are stems. One plant with phylloclades is Ruscus hypoglossum, the "laurel" of Caesar.
- Rhizome: A horizontal stem found underground. A rhizome can be distinguished from a root by the presence of nodes on the rhizome ends. Ginger (Zingiber officinale) is a rhizome
- Runner: A thin, horizontal stem found above ground with a rosette of leaves at the end. A runner is very similar to a stolon. Both structures allow the plant to move within its environment. Strawberries (Fragaria species) have runners.
- Spine: A modified stem (or leaf stipule, or root) that is sharp. The term thorn is a synonym. A prickle is similar, but is produced from the epidermis of the stem, leaf, or root. Opuntia and other cacti have spines.
- Stolon: A horizontally growing stem at ground level with leaves along its length (not just a rosette at the end, as in a runner) and adventitious roots that form at the nodes. Stolons are similar to runners. White clover (Trifolium repens) has stolons.
- Tendril and hook: Modified stems or leaves that grasp other plants and act like grappling hooks. Tendrils and hooks, which are found on vines, assist the vine in supporting itself. The Virginia creeper is unusual because it has small pads at the ends of its tendrils. These pads attach themselves to walls and tree trunks instead of twining around the trunk.
- Tuber: An underground, swollen stem. A tuber is usually a swollen rhizome. The potato is the classic tuber.