Support and Food Storage in Stems
Investigating the Functions of Stems
Stems come in a wide variety of forms varying from the soft, spongy stalks of aquatic plants to the hard, towering trunks of giant forest trees hundreds of feet tall. Some stems even grow underground.
Stems Provide Support
Support in plants happens at a different level than it does in animals. Complex animals with internal skeletons are supported at the organ/organ system level – bones, joints, tendons, and ligaments all working in concert. Plants on the other hand, are supported at the cellular level. The cell walls of each of billions to trillions of cells meshed together side-by-side, front-to-back, and top-to-bottom combined with the turgor pressure within each cell hold the plant firmly but flexibly upright.
In low-growing plants and those adapted for a horizontal life such as vines and creepers, vertical support by the stem is a nonissue. However, in the majority of plants, it means survival itself. The efficiency of stem support can best be appreciated in those plants that grow not only tall but incredibly tall. The tallest gymnosperms (evergreen trees) are the North American coast redwoods also known as giant redwoods. They measure in at over 110 meters (360 feet) tall, the height of a 30-story building.
The angiosperms (flowering plants) are led in the height competition by the Australian mountain ash. (While certainly Australian, it is not an ash but in the eucalypt family.) The current record-holder is said to be one in the Styx Valley of Tasmania which has been measured at 99 meters (325 feet). Back in 1880 a surveyor measured a mountain ash in Victoria that stood 114 meters (375 feet) high. Around the same time, an official inspector of forests reported a fallen trunk of one of these giant trees that was 132 meters (435 feet) long. That may well have been the largest tree ever measured.
According to the "National Register of Big Trees", published by the American Forestry Association, the tallest angiosperm tree in America is a valley oak located in Gridley, California measuring in at 49 meters (161 feet) tall. However, it takes more than height to make a tree a true champion. Trees must be viewed and compared in their totality – height, girth (width), and crown spread to be truly appreciated. To walk a grove of giant trees is to truly tread in nature’s cathedrals. It is up to us to do all we can to protect these ancient giants from the nearsightedness of profit that has destroyed so many of their brethren, if for no other reason than the very magnificence of their being.
Stems as Food Storage
Most plants have stems that are adapted to store some food but in some which are highly modified, such as the potato, onion, and tulip, food storage is a major function.
Plants often store food in their stems during their growth period. Plants need to do this in order to survive dormancy. During dormancy, a plant’s growth and activity slows or stops. Dormancy occurs during cold winters or long dry periods that may last for months or even years. When conditions once again support activity, the dormant plant begins to grow, using food previously stored until new growth can begin to produce enough food.
Categories of Food Storage Stems
Stems modified for food storage are categorized as rhizomes, tubers, bulbs, or corms.
- Rhizomes are thick, fleshy, creeping stems that grow either along or just beneath the surface of the soil. Along the length of the rhizomes are buds from which leaves and stems can grow. When frost kills the above-ground portion of the plant, the rhizomes survive and slumber on until they burst forth with new growth in the spring. Garden plants such as irises, canna lilies, and many species of grasses are examples of plants with rhizome stems.
- Tubers are modified underground stems that are swollen with stored food, usually in the form of starch. Tubers have one or more prominent “eyes,” which are actually lateral (side) buds. From these buds grow new above-ground stem and leaves. The potato is an example of a well-known tuber.
- Bulbs are underground stems with the small stem at the center of the bulb. Most of the food stored in this type of stem is located in the layers of short, thick leaves that wrap around and protect the stem. Tulips and daffodils are familiar spring flowers that have bulbs.
- Corms are similar to bulbs but they have much thinner leaves. Most of the stored food in corms is located in the stem itself. Thin, scalelike leaves surround the stem and serve mainly to protect it. Crocuses and gladioli are common garden plants that produce corms.
Without stems supporting leaves so that they might make food for the plant and without stems storing that food within themselves for winter use, plants could not survive.
Stems Support Leaves and Flowers
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