Although most higher plants have an erect shoot system, many species have specialized stems that are modified for various functions (Fig. 6.14). The overall appearance of specialized stems may differ markedly from that of the stems discussed so far, but all stems have nodes, inter-nodes, and axillary buds; these features distinguish them from roots and leaves, which do not have them. The leaves at the nodes of these specialized stems are often small and scalelike. They are seldom green, but full-sized functioning leaves are also produced. Descriptions of some of the specialized stems follow.
Rhizomes (Fig. 6.14) are horizontal stems that grow below ground, often near the surface of the soil. They superficially resemble roots, but close examination will reveal scalelike leaves and axillary buds at each node, at least during some stage of development, with short to long internodes in between. Adventitious roots are produced all along the rhizome, mainly on the lower surface. As indicated in Chapter 5, the word adventitious refers to structures arising at unusual places, such as roots growing from stems, or leaves or buds appearing at places other than leaf axils and tips of stems. A rhizome may be a relatively thick, fleshy, food-storage organ, as in irises, or it may be quite slender, as in many perennial grasses or some ferns.
Runners are horizontal stems that differ from rhizomes in that they grow above ground, generally along the surface; they also have long internodes (Fig. 6.14). In strawberries, runners are usually produced after the first flowers of the season have appeared. Several runners may radiate out from the parent plant, and within a few weeks may grow up to 1 meter (3 feet) or more long. Adventitious buds appear at alternate nodes along
In Irish or white potato plants, several internodes at the tips of stolons become tubers as they swell from the accumulation of food (Fig. 6.14). The mature tuber becomes isolated after the stolon to which it was attached dies. The "eyes" of the potato are actually nodes formed in a spiral around the modified stem. Each eye consists of an axillary bud in the axil of a scalelike leaf, although this leaf is visible only in very young tubers; the small ridges seen on mature tubers are leaf scars.
Bulbs (Fig. 6.14) are actually large buds surrounded by numerous fleshy leaves, with a small stem at the lower end. Adventitious roots grow from the bottom of the stem, but the fleshy leaves comprise the bulk of the bulb tissue, which stores food. In onions, the fleshy leaves usually are surrounded by the scalelike leaf bases of long, green, above-ground leaves. Other plants producing bulbs include lilies, hyacinths, and tulips.
Corms resemble bulbs but differ from them in being composed almost entirely of stem tissue, except for the few papery scalelike leaves sparsely covering the outside (Fig. 6.14). Adventitious roots are produced at the base, and corms, like bulbs, store food. The crocus and the gladiolus are examples of plants that produce corms.
The stems of butcher's broom plants are flattened and appear leaflike. Such flattened stems are called cladophylls (or cladodes orphylloclades) (Fig. 6.14). There is a node bearing very small, scalelike leaves with axillary buds in the center of each butcher's broom cladophyll. The feathery appearance of asparagus is due to numerous small cladophylls. Cladophylls also occur in greenbriers, certain orchids, prickly pear cacti (Fig. 6.15), and several other lesser-known plants.
Other Specialized Stems
The stems of many cacti and some spurges are stout and fleshy. Such stems are adapted for storage of water and food. Other stems may be modified in the form of thorns, as in the honey locust whose branched thorns may be more than 3 decimeters (1 foot) long, but all thornlike objects are not necessarily modified stems. For example, at the base of the petiole of most leaves of the black locust are a pair of spines (modifed stipules; stipules were mentioned in the discussion of twigs and are discussed further in Chapter 7). The prickles of raspberries and roses, both of which originate from the epidermis, are neither thorns nor spines. Tiger lilies produce small aerial bulblets in the axils of their leaves.
Climbing plants have stems modified in various ways that adapt them for their manner of growth. Some stems, called ramblers, simply rest on the tops of other plants, but many produce tendrils (see Fig. 6.14). These are specialized stems in the grape and Boston ivy but are modified leaves or leaf parts in plants like peas and cucumbers. In Boston ivy, the tendrils have adhesive disks. In English ivy, the stems climb with the aid of adventitious roots that arise along the sides of the stem and become embedded in the bark or other support material over which the plant is growing.
Figure 6.16 Continued.
Figure 6.16 Continued.
104 Chapter 6
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