Rain forests contain many epiphytes (from the Greek words epi, meaning "upon," and phyton, meaning "plant"), which are plants that grow independently on other plants (fig. 7.19). Common epiphytes include orchids, staghorn ferns (Platycerium bifurcatum), and even some cacti. These plants grow slowly and must absorb their nutrients through aerial roots from sources other than the soil or the plant on which they grow. Aerial roots are adventitious roots formed by aboveground structures such as stems.
Aerial roots have different functions in different kinds of plants:
K Water retention. The epidermis of roots of some epiphytic orchids is almost impermeable to water. The primary functions of the epidermis are mechanical protection of the internal tissues and the retention of water in the root.
K Photosynthesis. In plants such as philodendron, vanilla orchid, and several aquatic plants, the roots are photo-synthetic. In orchids such as Microcoelia smithii, aerial roots are flat and green and look remarkably like shoots.
K Support. Prop roots are supportive aerial roots that grow into the soil; they are common in plants that grow in mudflats, such as banyan trees (Ficus benghalensis), as well as in plants such as corn (see fig. 7.3). Banyan trees can produce thousands of prop roots that grow down from horizontal stems and form pillarlike supports. Roots of the flowering plants called ball moss and English ivy anchor the plant's shoot to the tree on which it grows. Shallow-rooted tropical trees such as Khaya and fig (Ficus) produce remarkable buttress roots at the base of their trunks (fig. 7.20). These planklike roots are often more than 4 meters high and are specialized for support. They contain large amounts of fibers.
Bird's-nest ferns (Asplenium nidus) accumulate rainfall and litter between their closely packed leaves; the ferns get their nutrients by growing roots among these leaves. One of the most specialized epiphytes is Spanish moss (Tillandsia us-neoides), which grows throughout the southeastern United States. These flowering plants lack roots; they absorb water and nutrients through hairs that coat their stems and leaves.
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