VEGETATIVE ORGAifS OF PLANTS. 261 



a. The 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. 370.) 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^ne 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. 



