96 SUMMARY. 



8. Most cacti have two divisions of the root-system : an anchoring and 

 an absorbing system. In Carnegiea gigantea the anchoring roots, in old 

 plants, are assisted in their mechanical function by the enlarged bases 

 of the laterals. This to a certain extent is true of the arborescent cacti 

 as well, but the lack of secondary development of this character in Echino- 

 cactus wislizeni is the chief reason why plants in age lean sharply, or fall. 



9. Fleshiness in the roots of the opuntias is a condition resulting directly 

 from an abundant supply of water. Whether some species exhibit greater 

 tendency toward fleshiness than others was not determined. A similar 

 reaction was seen in Brodicea capitata. 



10. The most superficial roots of the generalized type of root-systems 

 and a few of the specialized type of the cactus-form bear, on the larger lat- 

 erals, filamentous roots in groups of 6 or more. These are formed during 

 the rainy season, especially in the summer, and die with the drying out 

 of the soil. The deciduous rootlets are of great advantage to the plants 

 in that they greatly and quickly increase the absorption surface, without 

 at the same time increasing the distance of water transport — a factor of 

 undoubted importance where the transpiration rate is high. 



1 1 . Competition as evidenced by the relationship of the roots of neigh- 

 boring plants may be summarized as follows : The roots of annuals inter- 

 mingle and often occupy the same horizon. Where perennials of a single 

 species occur together on the bajada, the roots of one plant ma}^ reach to 

 and intrude upon the root-area of its neighbor. Thus in studying Covillea 

 on the bajada, 60 roots of neighboring Covilleas were encountered which 

 either were in contact with the roots of the plant studied or were in the 

 same horizon. On the flood-plain competition among the roots may not 

 be so keen as on the bajada. The roots of annuals growing on the bajada 

 reach as deep as most perennials in the same habitat, and since they 

 occur in large numbers competition with them must be an important 

 causal factor contributing to the sparseness of the perennial vegetation of 

 the bajada. The annuals also come into competition with the shallow 

 rooted perennials through the laterals which are developed on the tap root 

 of the annuals 4 to 5 cm. beneath the surface of the ground. 



12. Perennials with the generalized type of root-system have the widest 

 local distribution, and those with a pronounced development of the tap root 

 have the most limited distribution. Plants with laterals well developed, 

 the cacti especially, are most abundant on the bajada and on Tumamoc 

 Hill, where the soil is shallow, and seldom occur on the flood-plains. 





