Page:First course in biology (IA firstcourseinbio00bailrich).pdf/66

 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.

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