Ch. V, 8] SPECIAL FUNCTIONS OF ROOTS 



251 



such roots is insignificant in comparison with the corre- 

 sponding diversity in leaves and stems. The explanation 

 is found no doubt in the fact that the conditions of life 

 underground are much more uniform than conditions of life 

 in the air. 



The commonest additional function of roots is storage, 

 mostly of food, but partly of water, which occurs in all 

 degrees, from so little as not to affect perceptibly the root 

 shape, up to the pro- 

 duction of a rotundly- 

 swollen organ. The 

 storage is oftenest in 

 tap roots (which per- 

 haps originated in this 

 way), as familiar in 

 Carrots, Beets, and 

 Turnips. In other 

 cases side roots are 

 specialized, as exempli- 

 fied in Dahlia and Sweet 

 Potato, where they be- 

 come tuberous (Fig. 

 175), and would be 

 hard to distinguish 

 from stem tubers, were 

 it not for the absence 



of eyes. Anatomically, as functionally, stem and root 

 tubers are closely ahke. As with stems, when starch is 

 principally the food the texture is hard and white (e.g. 

 Turnip) : when sugar, it is softer and translucent (e.g. Beet) : 

 and when water, as in some desert plants, it is markedly 

 succulent and almost transparent. 



Those flowering plants which have returned in their evolu- 

 tion to a Ufe in the water {e.g. Water Lihes and Pondweeds) 

 exhibit naturally a marked reduction in the root system. 

 Their roots are smaller in size and usually lack both hairs 



Fig. 175. 

 Dahlia ; 



— Typical tuberous roots, of 

 X 5. (From Strasburger.) 



