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 
A 
