136 ROOTS 



the shoot, for the larger the shoot, the larger the root system nec- 

 essary to supply the adequate amount of water and mineral mat- 

 ter, and to furnish sufficient anchorage. As to the size of the 

 roots of a plant, that depends upon the size of the shoot, the num- 

 ber of roots, and the distances of roots from the stem. The re- 

 lation of the size of roots to that of the shoot is well shown in case 

 of trees, where the roots directly connected with the stem and 

 known as main roots increase in diameter from a few millimeters 

 often to a foot or more as the shoot passes from the seedhng to the 

 mature stage. Where there is only one main root, as in Alfalfa 

 and the DandeUon, its size is directly in proportion to the size of 

 the shoot, usually being as large or even larger in diameter than 

 the short stem of the crown. On the other hand, when the roots 

 leading from the stem are numerous, as in the Grasses and 

 numerous other plants, all are relatively small. As to the size 

 of a branch root, that depends much upon its distance from the 

 stem or main root, for all roots branch and rebranch until the 

 branches are fibrous-like, usually being a millimeter or less in 

 diameter at their tips. It is in connection with these fiber-like 

 branches, which are the absorptive regions, that roots show most 

 uniformity; for the roots of all plants taper down to these fiber- 

 like branches, which are practically uniform in size for all plants. 

 This uniformity in size is probably due to the fact that only 

 roots with a very small diameter are efficient absorbers. 



The texture of rqots is always soft at the tips where the cells 

 active in division, elongation, and absorption have thin cellulose 

 walls, which readily yield to pressure or strains. But not far back 

 of the absorptive region there are formed strengthening fibers, 

 which afford a toughness that enables the root to endure the 

 strains in connection with its anchorage function. Furthermore, 

 roots, in their older regions, are covered with cork which adds 

 firmness to the texture. In shrubs and trees the roots, in their 

 older regions, become as woody and just as hard as the stems. 



As to duration, roots may be short-lived, serving the plant only 

 in the seedhng stage, as in case of temporary roots, or they may 

 last as long as the plant, as in case of permanent roots. The life 

 of permanent roots is one, two, or many years according to 

 whether or not the plant is annual, biennial, or perennial. 



Interdependence of Shoot and Root. — Upon the roots the 

 shoot depends for water, mineral matter, and anchorage, while 



