30 CORN CROPS 



16. The amount of root. — The amount of root devel- 

 oped is more or less in response to the needs of the plant. 

 When moisture is abundant or excessive, the plant will not 

 develop so much root as when the moisture content is 

 normal or below normal. Also in very dry soil, with a 

 moisture content below the wilting point of plants (about 

 12 per cent in loam soils), the growth of roots is hmited, 

 as is also the case when the soil is very hard. 



17. Functions of the root. — The root functions may be 

 stated as : (1) the absorption of water and of salts in solu- 

 tion; (2) the excretion of organic substances, especially 

 carbon dioxid, and possibly free organic acid, also mineral 

 salts and the salts of organic acids ; (3) the solvent effect 

 of the excretions on soil particles. 



The absorption of water and solutions, as well as the 

 exudations, take place largely through the root-hairs. 

 These root-hairs are constantly produced from the epider- 

 mal cells near the growing root tip. They are forced into 

 close contact with the soil grains ; in fact, the soil grains 

 are more or less embedded in the root-hair tissues. Each 

 soil grain in a moist soil is surrounded by a film of water 

 containing more or less mineral matter dissolved from the 

 soil. This soil water is absorbed by the root-hair, and it 

 seems probable that exudations from the root-hair also aid 

 in freeing less soluble minerals in the soil grains. The 

 process of absorption is by means of osmosis.' 



' Osmosis. — When two solutions of different density are separated by 

 a porous membrane, there mil be first a movement of the weaker solu- 

 tion through the membrane into the stronger, and later a return move- 

 ment, the process continuing until the two solutions have the same den- 

 sity. The contents of a root-hair being denser than the soil solution 

 surrounding it, there is a constant movement of the soil solution into the 

 root-hair. By some means the exosmosis, which would take place in the 

 case of an ordinary membrane (movement of the cell solution outward), 

 seems to be restrained in the root-hair, probably by some functional 



