THE ABSORBING SURFACE OF ROOTS 



27 



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 



O O o <~> 



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 



digging wells follow these 



roots as guides. Where water 



is more abundant, the mesquite 



forms a good-sized tree, with 



much shorter roots. 



35. The absorbing surface 

 of roots. The soil roots of 

 most seed plants are provided 

 with a highly efficient means 

 for absorbing water in the 

 shape of a coating of root hairs, 

 with which their younger por- 

 tions are thickly covered. 

 Some idea of their abundance 

 may be gathered from the 

 estimate that on the hair- 

 bearing portions of the roots 

 of the common pea about 1437 

 hairs occur on every hundredth of a square inch of surface. 



A root hair is an extremely thin-walled tube, springing from 

 an epidermal cell, into which it opens. The way in which the 

 cells give rise to hairs is well shown in Figs. 21 and 23. ^ 



n-- 



n 



B 



FIG. 23 



A, a very young root hair; B, an older 

 one (both greatly magnified) ; e, cells 

 of the epidermis of the root: n, nu- 

 cleus; s, watery cell sap; p, proto- 

 plasm lining the cell wall. After 

 Frank 



