by light, and root distribution is much wider in a lateral direction than that of the branches. 2d, In light rains the leaf surface holds back practically all the rain which is then evaporated into the air and lost to the root systems. 3d, In heavy and long-continued rains the water breaks through the leaf system to such an extent that roots under the tree would be as well supplied as those outside, and the ground outside being saturated anyway, the roots do not need the small additional water which may have been shed outward. 4th, It is the habit of plants where left undisturbed (except in rare cases), to grow in more or less dense formations or societies. Here there is no opportunity for any appreciable centrifugal distribution of rainfall and yet the root distribution is practically the same, except that the root systems of adjacent plants are interlaced. II. Kinds of Roots. =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.