793. The root system.—From the foregoing, it will be understood that the roots of a plant taken together form the root system of that plant. In soil-roots in general we usually recognize two kinds of root systems.
794. The fibrous-root system.—Roots which are composed of numerous slender branching roots resembling “fibers,” are termed fibrous, or the plant is said to have a fibrous-root system. The bean, corn, most grasses, and many other plants have fibrous-root systems.
795. The tap-root system.—Plants with a recognizable central shaft-like root, more or less thickened and considerably stouter than the lateral roots, are said to have tap roots, or they have a tap-root system. The dandelion, beet, carrot (see crown tuber) are examples. The hickory, walnut, and some other trees have very prominent tap-roots when young. The tap-root is maintained in old age, but the lateral roots often become finally as large as the tap-root. Besides tap-roots and fibrous-roots, which include the larger number, several other kinds of roots are to be enumerated.
796. Aerial roots.—Aerial roots are most abundantly developed in certain tropical plants, especially in the orchids and aroids. Many examples of these plants are grown in conservatories. The amount of moisture is so great in these tropical regions that the roots are abundantly supplied without the soil relation. Certain of the roots hang free in the air and are provided with a special sheath of spongy tissue called the velamen, through which moisture is absorbed from the air. Other roots attach themselves to the trunk or branches of the tree on which the orchid is growing, and furnish the support to the epiphyte, as such plants are often called. Among the tangle of these clinging roots falling leaves are caught. Here they decay and nourishing roots grow from the clinging roots into this mass of decaying leaves and supply some of the plant food. Aerial roots sometimes possess chlorophyll.
There are a number of plants, however, in temperate regions which have aerial roots. These are chiefly used to give the stem support as it climbs on trees or on walls. They are sometimes called clinging roots. A common example is the climbing poison-ivy (Rhus radicans), the trumpet creeper, etc. Such aerial roots are called adventitious roots.
797. Bracing roots, or prop roots.—These are developed in a great variety of plants and serve to brace or prop the plant where the fibrous-root system is insufficient to support the heavy shoot system, or the shoot system branches so widely props are needed to hold up the branches. In the common Indian corn several whorls of bracing roots arise from the nodes near the ground and extend outward and downward to the ground, though the upper whorls do not always succeed in reaching the ground. The screw-pine so common in greenhouses affords an excellent example of prop roots. The roots are quite large, and long before the root reaches the soil the large root cap is evident. The banyan tree of India is a classic example of prop roots for supporting the wide-reaching branches. The mangrove in our own subtropical forests of Florida is a nearer example.
Fig. 448.
Bracing roots of Indian corn.
Fig. 449.
Buttresses of silk-cotton tree,
Nassau.