54 ROOT HABITS OF DESERT PLANTS. 



Opuntia discata, they are much branched. Besides the more prominent 

 roots, such as those just referred to, there are filamentous ones which occur 

 in groups and which are usually about i cm. long. Such filamentous roots 

 aje to be seen throughout the most of the course of the superficial larger 

 laterals. 



Growing within the compass of the roots of the cactus, as described in an 

 earlier paragraph, there were several perennials whose position would indi- 

 cate, if other conditions were favorable, that they competed with the cactus 

 for water. An examination of the roots of these plants indicated that this 

 might be the case in some species, but that it probably was not true in 

 others. Menodora, for example, has a tap root which penetrates deep 

 but has little extension of laterals, and the roots of Parkinsonia also pene- 

 trate the ground more deeply than those of the cactus. These plants, there- 

 fore, did not enter into close competition with the cactus. 



Of the other aliens, Echinocactiis would be supposed to have superficial 

 roots which would be placed similarly to the corresponding roots of versi- 

 color, but owing to the fact that the specimen was young, the anchoring 

 roots, and not the widel}- extending ones, were developed, and competition 

 between the two species of cacti had not yet begun. The case was different, 

 however, with the specimen oijatropha and the neighboring 0/)«M^ia discata. 

 The distal roots of one lateral of the discata, as the plate indicates, occupied 

 the same horizon as those of versicolor, and were mingled with the roots of 

 the latter cactus. There must have been active competition for water, 

 therefore, between the roots of the two cacti. The root-system of Jatropha 

 was of the sort that, had the plant been more fortunately placed, active 

 competition with it must also have occurred. 



The root-system of Jatropha is entirely superficial and placed fairly close 

 to the surface of the ground. As shown in plates 12 and 13, the root sys- 

 tem of the plant was composed of a long and fleshy main root, which ran 

 horizontally, and several shorter roots, some of which were fleshy also. 

 There were also two relatively short fleshy roots which arose from the 

 enlarged shoot-base and ran horizontally. At a point 10 cm. from its base, 

 the main root was 2.5 cm. in diameter. The terminal rootlets of Jatropha 

 were placed at about the depth of the small superficial roots of the cactus, 

 and the plant would have competed with the cactus actively had the roots 

 of the two species chanced to occupy the same territory. Thus it seems 

 that despite the large population of the area the roots of Opuntia versicolor 

 were in close relation with those of but one alien, and that one with its 

 central axis relatively remote. However, this statement would undoubtedly 

 be modified if consideration were given to the relation of the roots of the 

 cactus and those of the numerous summer and winter annuals that also 

 occupy the area, as the roots of many of them must be in close relation, if 

 not in physical contact, with those of the cactus. 



