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. 
