PHYSIOLOGICAL FEATURES OF ROOT-SYSTEMS. 93 



lateral extent may be less than 2 meters. In the latter case there is the 

 additional fact that the root-system of Covillea is, as a whole, deeper than 

 on the bajada. Competition between annuals and perennials on the flood- 

 plain, therefore, may be considered a negligible quantity, but competition 

 between the two classes of plants on the bajada during the vegetative 

 seasons is undoubtedly very keen, and the presence of annuals on the 

 bajada is probably an important factor in bringing about the sparse char- 

 acter of the perennial flora of the habitat. 



Since the roots of the specialized type of the Ephedra form penetrate 

 deep and reach laterally to but a small extent, the roots of the annuals do not 

 come into close relation wuth them, but the relation of the roots of annuals 

 and of the specialized roots of the cactus type is peculiar in that usually 

 the roots of such perennials are more shallowl}^ placed than those of the 

 annuals. Owing, however, to the fact that in the root-system of annuals, 

 laterals are ordinarily prominent 4 to 5 centimeters beneath the surface, 

 during the rainy season, the roots of the two classes occupy the same horizon 

 and undoubtedly enter into keen competition for water. Soon after the close 

 of the rainy period the superficial roots of the annuals and the deciduous 

 rootlets of the perennials, owing to the progressive desiccation of the soil, 

 can no longer remain functional. With the drying of the surface layers the 

 more superficial roots of the plants having the generalized root type also 

 cease absorbing, and the entire amount taken in by the root-system of 

 such plants comes from the deeper levels. At this period the most deeply 

 placed roots of such annuals as mentioned above are placed in close rela- 

 tion with the deeper absorbing roots of the perennials with generalized 

 roots, and the competition is transferred from the more superficial to the 

 more deeply placed roots both of annuals and of perennials. Finally the 

 annuals mature, their roots absorb in decreasing amounts, and competi- 

 tion with the roots of other plants ceases. 



Where the growth of annuals is most dense, the roots intermingle and 

 occupy the soil fairly completely. This, however, occurs only in the case 

 of perennials when, as on the bajada, plants of one species, or plants having 

 similar types of root-systems, are growing in proximity. The most strik- 

 ing instance observed of the encroaching of the roots of one species on the 

 territory occupied by the roots of another plant of the same species was 

 that of Covillea, where 60 roots of neighboring Covilleas were seen either in 

 physical contact or lying very near the roots of the plant being specially 

 studied. 



The roots of plants with specialized root-systems of the cactus type do 

 not occupy the same horizon as the roots of perennials having the general- 

 ized forms of roots, and therefore do not compete with them for water. 

 This, however, is true only of such cacti as have the most superficial roots ; 

 those like Opuntia fulgida, with a root-system which approaches the gen- 

 eralized type in character, probably approach this type in their mode of 

 distribution in the soil, particularly where the soil is shallow. 



