90 ROOT HABITS OF DESERT PLANTS. 



Santa Cruz is sufficiently near the surface to permit its being reached by 

 the roots of the larger plants, as Prosopis vclutma. It has been learned* 

 that the Santa Cruz is at present lower than the water-table, so that the 

 river acts as a ditch. In times not very remote there was no well-defined 

 channel of the river, as at present, but the water spread over the plain, 

 which might indicate that formerly the water-table was nearer the surface 

 than it is now and thus more easily reached by the roots of plants. The 

 changes in the position of the water-table have probably operated to modify 

 profoundly the flora of this habitat. 



Not only do the sources of water vary in the different habitats, but also 

 the periods during which it is available for the use of plants. Tumamoc 

 Hill and the bajada, partly for reasons of differences in water supply, are 

 the most arid, and the flood-plain of the vSanta Cruz the least so. The 

 character of the soil and its depth are also important factors in this con- 

 nection. The soil of the bajada and Tumamoc Hill is usually less than 50 

 cm. deep, while the soil by West Wash is over 2 meters and that of the flood- 

 plain of the Santa Cruz is 5 meters, more or less. Thus the arid condi- 

 tions of the bajada and Tumamoc Hill, with the least water supply, are 

 greatly increased by the slight soil covering. The length of time elapsing 

 after a through wetting before the soil of the bajada, at a depth of 15 cm., 

 becomes air-dry, is about three weeks, but moisture at this depth on the flood- 

 plain of the Santa Cruz remains for six weeks. In this instance, however, 

 in addition to the fact that more water is present in the habitat, the char- 

 acter of the soil as regards its fineness is an important factor. 



Annuals are directly affected by the rains of the particular habitats 

 where they are placed, while the perennials are in part directly and in part 

 indirectly dependent on obtaining water by this means. On the bajada 

 and on Tumamoc Hill, water is mostly derived directly from the rains, and 

 only such perennials as are provided with water-storage organs, or are the 

 most resistant, can live or attain the best development in these habitats. 



The depth to which the roots of annuals penetrate the ground is directly 

 controlled bv the depth of the penetration of the rains of the season and 

 the persistence of the annuals is mainly directly dependent on the length 

 of time the water remains in the soil where they are growing. As above 

 stated, within a period of three weeks following the rains, the superficial 

 soil on the bajada, to a depth of 15 cm., may become air-dry, while that 

 of Tumamoc Hill and of the flood-plain may retain moisture for a period 

 exceeding six weeks. Since the largest part of the roots of most annuals do 

 not reach deeper than 20 cm. it is seen that the period of their activity is 

 definitely fixed and comparatively brief. 



Annuals having the most deeply penetrating root-systems, and which 

 are provided with checks against excessive water loss, survive longest after 

 the wet season has passed. KaUstrcemia grandiflora, for example, a sum- 



*Communicated to the writer by Prof. G. E. P. Smith, of the Arizona Experiment 

 Station. 



