106 PLANT LIFE 



or bending stress. An axial cord of mechan- 

 ical tissue, however strong, would be useless 

 to resist such a stress on the lee side, while 

 the arched form of the roots would minimise 

 the value of an axile strand of a root on the 

 windward side. This special form of mechan- 

 ical stress is overcome by the Indian corn 

 roots in a remarkable way. The extra- 

 terrestrial arched parts of the roots have a 

 thick ring of mechanical tissue specially 

 differentiated from the cells of the outer rind, 

 whilst at the same time they retain the cord- 

 like axile strand. Thus these roots are 

 excellently adapted to withstand both crush- 

 and pulling strains from whichever quarter 

 they may come. Beneath the ground only 

 the pulling strains, of course, are operative, 

 and we find that the peripheral thick tissue 

 ring is not formed in the subterranean parts 

 of the root system. 



