Although most roots absorb water and minerals and anchor the plant in place, the following are a few of the most common functions of modified roots; that is, roots for which other major functions have evolved:
K Storage. In plants such as beets, turnips, radish, dandelion, and cassava, roots store large amounts of starch. Sugar produced in leaves during photosynthesis moves throughout the plant via the phloem and is transformed into starch in the roots. Roots of other plants store carbohydrates as sugars; for example, the roots of sweet potato (Ipomoea batatas) contain 15% to 20% sucrose. Roots also can store large amounts of water; the taproots of some desert plants store more than 70 kilograms of water. In the desert of South Africa, a plant called Hottentot bread (Dioscorea elephantipes) is named after the people of the region who use the huge (350 kilograms; over 770 pounds) underground root as a source of starch during famines. This member of the yam family, however, also stores water for its own use during long periods between rainfalls.
K Vegetative reproduction. The roots of plants such as cherry, pear, apple, and teak produce shoots called suckers that emerge from the soil. When separated from the parent plant, suckers become new individuals. The production of suckers is a relatively common form of vegetative reproduction in plants. For example, most groups of creosote bushes (Larrea tridentata)
are clones (genetically identical individuals) derived from a single plant. Some of these clones are more than 12,000 years old; this means the seeds that started these clones germinated approximately 4,000 years before humans began to communicate through writing. The most massive plant in the world is believed to be a quaking aspen (Populus tremuloides) that has grown thousands of suckers from the same root system (fig. 7.16). This plant, which grows in the Wasatch Mountains of Utah, consists of more than 47,000 tree trunks, each with the usual complement of leaves and branches. It covers almost 43 hectares (over 100 acres), and its mass has been estimated to be about 6 million metric tons. The discoverers of this aspen named it Pando, a Latin word meaning "I spread." What evidence would you need to determine that this is actually only one plant?
K Aeration. Many plants grow in stagnant water and mud that contains much less oxygen than does the air. In this environment, the roots of most plants would suffer a similar fate to that of any person without oxygen—although roots can tolerate anaerobic conditions for much longer before dying. Plants such as black mangrove (Avicennia germi-nans) that grow in low-oxygen environments avoid suffocation because specialized roots have evolved that import oxygen from the atmosphere (fig. 7.17). These specialized roots contain as much as 80% aerenchyma (tissue containing large air spaces) and grow up into the air. These roots function like snorkels through which oxygen diffuses to submerged roots.
Epiphytic bromeliad growing on a tree.
Mistletoe (Phoradendron flavescens), a parasitic plant. This oak tree is heavily infected by mistletoe.
K Movement. Contractile roots evolved on plants such as lily, gladiolus, ginseng, and dandelion. These roots contract as cells shrink in the cortex; this shrinkage twists the xylem cells into a corkscrew and wrinkles the surface of the root. As a result, contractile roots can shrink more than 50% in only a few weeks. Contractile roots are important because they pull a plant into the more stable environment of deeper soil.
K Nutrition. Several plants are parasites, meaning that they are attached to a host plant by specialized root structures and harm these plants by siphoning nutrients from them (fig. 7.18). For example, cancer root (Orobanche) and mistletoe (Phoradendron) parasitize hardwood trees, whereas trees such as sandalwood (Santalum) obtain their nutrients from nearby grasses. Many parasitic plants lack chlorophyll and therefore depend entirely on their host for their sugar as well as water and minerals. However, the presence of chloro
Epiphytic bromeliad growing on a tree.
phyll in a parasite does not guarantee that the plant is photosynthetic; witchweed (Striga) is green yet grows only as a parasite.
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