48 FOUNDATIONS OF BOTANY 



been carefully studied to ascertain the total weight and 

 length of the roots. Those of winter wheat have been 

 found to extend to a depth of seven feet. By weighing 

 the whole root-system of a plant and then weighing a 

 known length of a root of average diameter, the total 

 length of the roots may be estimated. In this way the 

 roots of an oat plant have been calculated to measure 

 about 154 feet; that is, all the roots, if cut off and strung 

 together end to end, would reach that distance. 



Single roots of large trees often extend horizontally to 

 great distances, but it is not often possible readily to trace 

 the entire depth to which they extend. One of the most 

 notable examples of an enormously developed root-system 

 is found in the mesquite of the far Southwest and Mexico. 

 When this plant grows as a shrub, reaching the height, 

 even in old age, of only two or three feet, it is because the 

 water supply in the soil is very scanty. In such cases 

 the roots extend down to a depth of sixty feet or more, 

 until they reach water, and the Mexican farmers in dig- 

 ging wells follow these roots as guides. Where water is 

 more plenty, the mesquite forms a good-sized tree, with 

 much less remarkably developed roots. 



60. The Absorbing Surface of Roots. Such aerial roots 

 as are shown in Fig. 13 are usually covered with a spongy 

 absorbent layer, by means of which they retain large 

 quantities of the water which trickles down them during 

 rain-storms. ' This water they afterwards gradually give 

 up to the plant. Most water-roots (not however those of 

 tradescantia) have no special arrangement for absorbing 

 water except through the general surface of their epidermis. 

 But some water-roots and most soil-roots take in water 



