112 



GENERAL BOTANY 



is not occupied by pith or a pith cavity. This is due to the fact, 

 already mentioned, that the xylem of a young root occupies the 

 center of the root, excluding the pith. 



Functions.. Roots serve various functions, such as anchorage 

 in the soil for the aerial portions of the plant body, the transpor- 

 tation of food and water, storage, and absorption. Of these 

 various functions the most important for our consideration is 

 that of absorption, since the other functions are more or less 

 apparent after our discussion of similar functions in stems. 



We have already learned 

 that the root system is 

 able to adjust itself by 

 tropistic movements so 

 as to place its members 

 in favorable relations to 

 the soil particles and soil 

 water. This adjustment, 

 together with the great 

 extent of the root sys- 

 tem, enables both herba- 

 ceous and woody plants 

 to absorb the large quan- 

 tities of water necessary 

 to meet the great demands made upon them by the evaporation 

 of water vapor from the leaves. Some plants, like the clovers, 

 have deeply penetrating roots (Fig. 59), while others (for 

 example, many of the cereal grains and some trees) have sur- 

 face roots. In alfalfa the root system is said to penetrate to 

 a depth of 31 feet, and in mesquite, a desert plant, to that 

 of 60 feet. Corn (Fig. 60) has been estimated to have a root 

 system which reaches a total length of over 1300 feet, and the 

 roots of a single plant of oats may be as much as 150 feet in 

 length. In many trees the root system quite equals the branch 

 system in horizontal extension, and its absorbing root tips are 

 placed where they receive the drip from the outer leaf surface 

 of the tree. In order to understand the very important absorptive 

 function of roots it will be necessary to discuss in some detail 



FIG. 60. Root system of the corn plant 

 (Zea mays) 



Note the uniform distribution of the roots in 

 the soil 



