50 



Guayule. 



in turn terminate in inflorescences and, by ending their growth, give stim- 

 ulus to the growth of branches of higher orders, each in its turn. Thus 

 the plant becomes profusely branched, and this habit contributes mate- 

 rially to the amount of secretion, which is proportional to the number of 

 branches. 



Fig. io. The inflorescence of the guayule. 



THE MATURE PLANT. 



ROOT-SYSTEM. 

 The root-system of the guayule consists, in a plant derived from a 

 seedling, of a strong tap-root extending to a considerable but undeter- 

 mined depth in the soil. The lower end, which branches more or less, 

 draws upon the water-supply of the deeper layers of the soil, especially 

 in younger plants. Just below the surface of the soil a number of strong 

 lateral roots are given off, which in many instances are of extraordinary 

 length, reaching a distance of 150 to 200 cm. or more from the plant 

 (plate 9, fig. A). These serve to take up the water in the shallower lay- 

 ers of the soil, derived from rains sufficient to wet the soil to this depth. 

 Such far-reaching, shallow-placed roots are characteristic of many desert 

 plants. Cannon (1909) has studied and mapped the root-systems of a 

 number of such, and has further shown that competition between juxta- 

 posed plants may be eliminated by the difference in the type of root- 

 system, the one going deeply, while the other is chiefly shallow. The 

 development of two differently placed parts of the same root-system, the 

 one drawing on the deeper, the other on the shallower layers of the soil, is 

 of very great importance biologically, and is well exemplified by the little 

 cactus Ariocarpus kotschubeyanus , which grows in the alluvial plains of the 

 mesa central. The shallow roots arise from the top of the tap-root and 

 ascend as nearly vertically upward as may be, till they reach to within a 

 few millimeters of the surface of the soil, when they suddenly take a hori- 



