ZEA 159 



This is quite likely a response to oxygen supply, as well as to 

 moisture supply. The amount of oxygen in the soil decreases 

 as the depth increases. Moreover, the rate of decrease is 

 greater in heavy or wet soils than in light or dry soils. When 

 it is understood that every living root cell derives its oxygen 

 for respiration from the soil air immediately surrounding 

 and that the oxygen does not diffuse to any extent from the 

 aerial parts of the plant down through the stem to the roots, 

 we see the probable explanation of the fact that a shallow 

 root system is peculiar to a heavy or wet soil. In this con- 

 nection, it should be stated that an important result of tillage 

 is the loosening of the soil so that oxygen may more easily 

 diffuse to the roots of the plant. All main roots give off 

 numerous finer branches and these in turn branch, so that at 

 maturity there is an interlacing mass of roots in the soil. 

 Fully two-thirds of the entire root system occurs in the first 

 4 inches of soil. This statement is based on records of 

 a number of observers (Sturtevant, Hunt, Newman, Ten 

 Eyck, Hays). Ten Eyck has observed that, although the 

 main laterals are several inches below the soil surface, they 

 may send upward finer branches to within }4, inch of the sur- 

 face. The depth of the corn roots determines the depth of 

 cultivation. If it is so deep as to destroy roots, the yield 

 is decreased. 



"Prop" or "Brace" Roots. — In addition to the ordinary 

 underground roots, corn develops aerial roots, the so-called 

 "prop^' or "brace" roots (Fig. 56). These arise in whorls 

 at successive levels above the. surface, extending obliquely 

 downward. They are covered with a mucilaginous sub- 

 stance which protects them from drying out. As aerial 

 roots, they are unbranched, but they branch profusely when 

 they strike the soil. They have the r61e of absorption, then, 

 as well as anchorage. 



