VEGETATIVE ORGANS OF PLANTS. 261 



a. Tlie Delicacy of its Structure, especially that of the 

 newer portions, the cells of which are very soft and ab- 

 sorbent, as may be readily shown by immersing a young 

 seedling bean in solution of indigo, when the roots 

 shortly acquire a blue color from imbibing the liquid, 

 while the stem is for a considerable time unaltered. 



It 'is a common but erroneous idea that absorption 

 from the soil can only take place through the ends of the 

 roots through the so-called spongioles. On the con- 

 trary, the extreme tips of the rootlets cannot take up liq- 

 uids at all. (Ohlerts, loc. cit., see p. 270.) All other 

 parts of the roots, which are still young and delicate in 

 surface-texture, are constantly active in the work of im- 

 bibing nutriment from the soil. 



In most perennial plants, indeed, the larger branches 

 of the roots become after a time coated with a corky or 

 otherwise nearly impervious cuticle, and the function of 

 absorption is then transferred to the rootlets. This is 

 demonstrated by placing the old, brown-colored roots of 

 a plant in water, but keeping the delicate and unindu- 

 rated extremities above the liquid. Thus situated, the 

 plant withers nearly as soon as if its root-surface were all 

 exposed to the air. 



b. Its Rapid Extension in Length, and the vast Sur- 

 face which it puts in contact with the soil, further adapts 

 the root to the work of collecting food. The length of 

 roots in a direct line from the point of their origin is 

 not, indeed, a criterion by which to judge of the effi- 

 ciency wherewith the plant to which they belong is nour- 

 ished ; for two plants may be equally flourishing be 

 equally fed by their roots when these organs, in one 

 case, reach but one foot, and in the other extend two feet 

 from the stem to which they are attached. In one case, 

 the roots would be fewer and longer ; in the other, 

 shorter and more numerous. Their aggregate length, 

 or, more correctly, the aggregate absorbing surface, 

 would be nearly the same in both. 



