=Kinds of Roots.=--A strong leading central root, which runs directly downwards, is a =tap-root=. The tap-root forms an axis from which the side roots may branch. The side or spreading roots are usually smaller. Plants that have such a root system are said to be _tap-rooted_. Examples are red clover, alfalfa, beet, turnip, radish, burdock, dandelion, hickory (Figs. 32, 33). A =fibrous root system= is one that is composed of many nearly equal slender branches. The greater number of plants have fibrous roots. Examples are many common grasses, wheat, oats, corn. The buttercup in Fig. 34 has a fibrous root system. Many trees have a strong tap-root when very young, but after a while it ceases to extend strongly and the side roots develop until finally the tap-root character disappears. [Illustration: FIG. 34.--A BUTTERCUP PLANT, with fibrous roots.] =Shape and Extent of the Root System.=--The depth to which roots extend depends on the _kind of plant_, and the _nature of the soil_. Of most plants the roots extend far _in all directions_ and lie comparatively _near the surface_. The roots usually radiate from a common point just beneath the surface of the ground. _The roots grow here and there in search of food_, often extending much farther in all directions than the spread of the top of the plant. Roots tend to spread farther in poor soil than in rich soil, for the same size of plant. _The root has no such definite form as the stem has._ Roots are usually very crooked, because they are constantly turned aside by obstacles. Examine roots in stony soil. _The extent of root surface is usually very large_, for the feeding roots are fine and very numerous. An ordinary plant of Indian corn may have a total length of root (measured as if the roots were placed end to end) of several hundred feet. The fine feeding roots are _most abundant in the richest part of the soil_. They are attracted by the food materials. Roots often will completely surround a bone or other morsel. When roots of trees are exposed, observe that most of them are horizontal and lie near the top of the ground. Some roots, as of willows, extend far _in search of water_. They often run into wells and drains, and into the margins of creeks and ponds. Grow plants in a long narrow box, in one end of which the soil is kept very dry and in the other moist: observe where the roots grow. =Buttresses.=--With the increase in diameter, the upper roots often protrude above the ground and become _bracing buttresses_. These buttresses are usually largest in trees which always have been exposed to strong winds (Fig. 35). Because of growth and thickening, the roots elevate part of their diameter, and the washing away of the soil makes them to appear as if having risen out of the ground. [Illustration: FIG. 35.--THE BRACING BASE OF A FIELD PINE.]