ROOTS AND THEIR FUNCTIONS 81 



hoe are separated from each other, and thus 

 the continuity of their surfaces with that of 

 the lower soil is largely interrupted. At the 

 same time the broken, loose soil serves to 

 check evaporation, inasmuch as it shelters 

 the lower unbroken (therefore continuous) soil 

 both from the sun and from the drying 

 influence of currents of air. It is a matter 

 of common experience that if plants are 

 grown in unwatered soil long enough, they 

 begin to droop and wilt. This means that 

 the root-hairs are not able to extract enough 

 water from the ground to keep pace with that 

 which is lost by the plant. Wilting takes 

 place when the water contents of the soil fall 

 below a certain amount, and this varies greatly 

 in different soils, but is fairly constant for 

 each particular kind. Thus, in sand a plant 

 may utilise all the water down to about 1-2%; 

 while in heavy clay the water ceases to be 

 available as soon as its content sinks below 

 about 25 %. It is evident that there is 

 probably some relation between the physical 

 state of the soil, and its physiologically avail- 

 able water content. And this turns out to 

 be the case. The fine particles of clay, with 

 their relatively enormous surface, retain far 

 more water than sand with its large particles 

 and relatively small surface. Ingenious ex- 

 periments on soil in centrifugal machines have 

 shown that approximately the same amount 

 of force is required to clear out water from 

 clay so as to leave 25 % remaining as is 



