10 FIELD CROPS FOR THE COTTON-BELT 



limited to a zone not far behind the growing point, 

 or the apex, of the young roots. Root-hairs are very 

 short-lived. As the young root grows in length, the 

 root-hairs farthest from the growing tip perish, more 

 being formed continually at about the same distance from 

 the apex. 



8. Functions of the root-system. In the main, the 

 functions of the root-system are: (a) 'to obtain food and 

 water for the plant, (b) to excrete carbon dioxide and 

 possibly organic acids that render plant-food available, 

 and (c) to anchor the plant to the soil, and thus afford a 

 firm support for the aerial portion. 



The primary function of the tap-root is probably that of 

 aiding the plant to secure moisture. During periods of 

 drouth it is very helpful in this respect. This is evidenced 

 by the fact that it grows faster and deeper in a relatively 

 dry soil than in a wet soil. The lateral roots, by their 

 extensive growth and copious branching, are the means of 

 producing the infinite number of root-hairs. The inter- 

 spaces of the soil are penetrated by the young growing 

 portions of the roots in such a way as to bring them into 

 close contact with the soil particles. The delicate root- 

 hairs stand out at right angles to the surface of the true 

 root. Consequently they are brought into very intimate 

 relations with the surface of the particles. A film of 

 capillary water surrounds each soil particle and contains, 

 in solution, mineral plant-food which has been dissolved 

 from the soil. Thus the acid juices in the root-hair and 

 the solution of minerals surrounding the soil particle are 

 separated only by the thin porous wall of the root-hair. 

 This relationship makes it easy for the root-hairs to per- 

 form their functions, namely, to absorb the water and 

 soluble food in the soil, and also to excrete into the soil- 



