=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. [Illustration: FIG. 35.—THE BRACING BASE OF A FIELD PINE.] =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. =Aërial Roots.=—Although roots usually grow underground, _there are some that naturally grow above ground_. These usually occur on climbing plants, the roots becoming _supports_ or fulfilling the office of tendrils. These aërial roots _usually turn away from the light_, and therefore enter the crevices and dark places of the wall or tree over which the plant climbs. The trumpet creeper (Fig. 36), true or English ivy, and poison ivy climb by means of roots. [Illustration: FIG. 36.—AËRIAL ROOTS OF TRUMPET CREEPER OR TECOMA. ] [Illustration: FIG. 37.—AËRIAL ROOTS OF AN ORCHID.] In some plants all the roots are aërial; that is, _the plant grows above ground_, and the roots gather food from the air. Such plants usually grow on trees. They are known as _epiphytes_ or _air-plants_. The most familiar examples are some of the tropical orchids which are grown in glass-houses (Fig. 37). Rootlike organs of dodder and other parasites are discussed in a future chapter.