48 ROOT HABITS OI^ DESERT PLANTS. 



Figure 3 shows the horizontal and vertical extensions of the root-system 

 of Encelia farinosa. 



The specimen of Encelia studied had very little competition from its 

 perennial neighbors. No other perennial grew within reach of its roots, 

 but the roots of two plants, Opuntia discata and Parkinsonia microphylla, 

 reached into or passed through the area occupied. The roots of the former 

 were more shallow than those of Encelia, while those of Parkinsonia ran 

 beneath the roots of Encelia. It should not be concluded, however, that 

 the plant is without competitors for ground water since during two seasons, 

 covering three months or more, annuals with deeply penetrating roots 

 occur in large numbers and probably extract more moisture from the soil 

 than Encelia itself does. The relative thickness of the 3 upper soil layers, 

 adobe, rotten caliche, hard caliche, are shown diagrammatically in fig. 36. 



Fig. 3. — Encelia farinosa. 



Opuntia Discata. 



Several types of flat-stemmed opuntias are native on Tumamoc Hill, and 

 certain of them, as far as the habitats considered in connection with the 

 present paper are concerned, are restricted to the Hill. Among these is 

 Opuntia discata, a plant of varying habit, with prostrate branches when 

 young, and with branches free of the ground when older. Two specimens 

 of this plant were selected for study, one mature and the other young. 

 The latter was growing on the north face of Tumamoc and the former on 

 the west side. The soil conditions where the cacti were placed were appar- 

 ently alike ; the upper soil to a depth of 1 5 to 20 cm. is malpais, with rotten 

 caliche beneath, all lying on the bed rock. 



The plant from the northern exposure had two shoots composed of two 

 and three ' 'joints " each. Both branches were lying on the ground, so that 

 the main stem bore none of their weight. The root-system consisted of 

 several main roots, which either ran straight downward from the base of 

 the main axis of the plant, or at an acute angle, and also about 8 laterals, 



