ROOTS AND RHIZOIDS 



497 



pp. 678 and 696. The penetration of roots into new soil regions takes 

 place without injury to the delicate growing root tip, because the latter 

 is protected by a root cap, consisting of older and less delicate cells, 

 which slough off and become slimy, as they are pushed ahead of the 

 elongating tip (fig. 552). 



Roots play an important role in anchorage and in nutrition. In many 

 species all roots take part in both processes, but there are some plants 



FIG. 707. A sand-binding grass (Calamovilfa longifoiia) holding a mound of sand 

 in place by its numerous long and slender but tough roots ; to the left are exposed roots 

 and a trough blown out by the wind; note the small herbs among the grass culms, by 

 which they are protected; Dune Park, Indiana. Photograph supplied by MEYERS. 



in which anchoring and nutritive roots are in sharp contrast. The 

 former have a central vascular strand composed of thick-walled cells of 

 small caliber; nutritive roots, however, have prominent pith and vas- 

 cular elements that are relatively thin-walled and of large caliber. If 

 growing roots are subjected to tension, they develop into anchoring 

 roots, while roots not so subjected develop into nutritive roots (figs. 737, 

 738). Many cacti have strong descending anchoring roots and weak 



