ROOTS AND THEIR USES 29 



which can be obtained from the roots decreases, or the air 

 conditions are such that water is drawn from the plant more 

 rapidly than it can be supplied by the roots, these stomata 

 gradually close, thus checking transpiration and tending to 

 maintain the proper quantity of water in the leaves and 

 tissues of the plant. Some plants possess the power of re- 

 taining their water content to a marked degree and are able 

 to live with a very small water supply. Cacti and other 

 desert plants have this characteristic, and the sorghums 

 are among the most drought-resistant of cultivated crops. 

 The quantity of water transpired by plants during their 

 period of growth is enormous. It is estimated that corn 

 gives off 270 pounds of water for every pound of dry matter 

 which is produced, while nearly twice this quantity is tran- 

 spired by oats in the making of a pound of dry matter. The 

 quantity transpired varies with the kind of plant and the 

 climatic conditions. Where evaporation is very rapid, the 

 quantity of water required by plants is greatly increased. 



THE ROOTS AND THEIR USES 

 27. What the Root Is. The root is the portion of the 

 plant below the surface of the ground, by which the plant 

 maintains its position. The roots hold firmly to the soil 

 and prevent the plant from being blown from place to place. 

 Some plants, like clover and alfalfa, have taproots which 

 extend straight down into the soil, though they may be 

 changed somewhat in direction by obstacles or by supplies 

 of air, water, or plant food. From these taproots, branches 

 are sent out which spread through the upper portions of 

 the soil. Other plants, like wheat and corn, send out several 

 fibrous roots with many branches which extend into the 

 soil in all directions. Roots are of many kinds and shapes, 

 from the fibrous ones of the grasses to the long, slender tap- 

 root of alfalfa and the heavy, thickened root of the mangel 

 and sugar beet. 



