BOOKS AND CURRENT LITERATURE 45 



In the study of the perennial root systems the plants were examined 

 in situ and the excavated roots carefully mapped. Among the plates 

 there are a number of these root system surveys represented and they 

 are, perhaps, the most striking of any of the features of this interest- 

 ing paper. The perennials studied comprised a number of the larger 

 cactus forms as well as other types, such as ocotillo, creosote bush, yucca 

 and so forth. The root systems of these perennials are summed up 

 under three heads, a generalized type and two other categories, one of 

 which has a prominent tap root and the other prominent laterals. One 

 of the notable facts brought forward in the paper is the great diversity 

 in the extent and position of the roots of these desert perennials. Far 

 from having, as a rule, deeply penetrating roots of great length, many 

 have exceedingly shallow roots, notably the barrel cactus, the roots of 

 which are so close to the surface as not infrequently to be exposed by 

 the erosion of rain storms. In fact it appears that among the cacti 

 the extremely long shallow laterals form the absorbing system and the 

 relatively short tap root and in some cases the modified bases of the 

 laterals constitute the anchoring apparatus of the plant. The deeper 

 root systems of the more generalized type were not found in the plants 

 growing on the detrital slopes, but rather in those which inhabit the 

 flood-plains and washes. Thus the mesquite was found to penetrate 

 to the depth of 5 meters in the flood plain of the Santa Cruz where it 

 grows to the best advantage and the author is inclined to doubt if the 

 smaller, stunted bush-like trees of this plant which grow at slightly 

 greater elevations from the river, ever reach the water table with their 

 roots. The ability of some of the perennials to form seasonal, decidu- 

 ous roots during the rainy seasons must be of no small importance. 



In the matter of competition among individuals for water supply the 

 plottings of root systems, previously referred to, are of especial interest. 

 On the slopes, among plants of the same species, the competition is 

 often of the keenest, as is shown strikingly in one of the maps illustrat- 

 ing the distribution of the roots of a creosote bush (Covillea tridentata) . 

 Among the cacti there does not appear to be so sharp a rivalry. 



There are many other facts of interest to which it is impossible to 

 call attention within the limits of a review. The results in general 

 modify in no small measure our previous conceptions of root distribu- 

 tion in desert plants and well illustrate how an extended study of diverse 

 plant forms is apt to modify generalizations either drawn from relatively 

 few instances or argued on teleological grounds. — H. M. R. 



