The stem is one of two main structural parts of a plant. It is the support structure that is usually seen above the ground. It works with the plant roots to support the buds and leaves of the plant and also plays an important role in the transport of fluids throughout the body of the plant. The stem is made up of various parts that change as the plant grows and matures.
Stems are generally cylindrical in shape. They can be hard, as they are in woody plants, or soft (typical of herbaceous plants). In most cases, a stem grows above the ground and is branched and leafy. Some stems, such as potatoes, do grow underground. Stems come in varying lengths and have several nodes sporting one or more leaves. (The nodes is the point at which a leaf joins the stem). Each leaf has a bud at its base, which could potentially become another stem. All stems must have buds or leaves present to be classified as a stem.
Parts and Basic Functions
A stem generally consists of three tissues--dermal tissue, ground tissue and vascular tissue. The dermal tissue covers the outer surface of the stem and serves as a waterproofing and protection agent. It also controls gas exchange. The ground tissue, mostly consisting of parenchyma (bulk) cells, surrounds the vascular tissue and sometimes functions in the process of photosynthesis. Vascular tissue provides nutrient transport and structural support. There may exist some variations of function and arrangement of vascular tissues among different plant species.
The Vascular System
The stem contains a system of vascular tissue that transports water, minerals and sugars throughout the body of the plant. The phloem, with its sieve tubes, is responsible for food transport. The xylem transports water up from the roots of the plant. The cambium is a meristem (a site of cell division and growth) located between the xylem and phloem responsible for producing more phloem and xylem tissues. The more of these tissues produced, the greater the girth of the stem.
Stems sometimes perform other functions in addition to their basic plant functions. Underground stems (mainly of the monocot plant families) can serve as food storage organs for the use of new plant growth. These large, fleshly organs, called modified stems, come in the form of bulbs, tubers, corms and rhizomes. They produce a variety of perennial plants. Perennials utilize the food stored in the underground stem during the winter months to send forth new plants shoots in the spring. Underground stems can also used for the asexual reproduction of the plant.
Stems as Food
Plants have been a valuable food source for thousands of years. Although the seeds, fruit, flowers and roots are commonly consumed, the stems of many plants can also provide good nutrition. Potatoes and radishes are tubers and provide calcium, iron, phosphorous, magnesium and vitamin C. Garlic and onions are layered bulbs that contain properties to help protect against a number of health ailments, including cardiovascular disease and diabetes. Other edible stems include asparagus, broccoli, cauliflower, celery and figs.