58 ROOT HABITS OF DESERT PLANTS. 



abruptly. Several slender roots left the main root between 5 and 10 cm. 

 from the surface of the ground and either extended fairly horizontally or 

 dipped downward to a depth of 45 cm. and even deeper, where they pene- 

 trated the caliche. The root-system of Covillca, so far as its position was 

 concerned, was similar to that of Parkins onia; that is, it extended down- 

 ward through the malpais and the caliche, and ran on the surface of the 

 bed rock. 



The relation of the root-systems of the three species is shown graphically 

 in the cuts of the horizontal and vertical extension. It will be seen that, 

 although growing in close proximity to one another (the Covillca was only 

 50 cm. from the cactus), the roots are not in physical contact anywhere, so 

 that competition for water in the soil is probably not keen in this instance, 

 notwithstanding the proximity of the plants. 



CoviLivEA Tridentata. 



Perhaps the most widely distributed perennial in the vicinity of the 

 Desert Laboratory is Covillea tridentata, which occurs not only on the bajada, 

 where it is the most characteristic shrub, but on the flood-plain of the 

 Santa Cruz river and West Wash, and on Tumamoc Hill. It attains its 

 most luxuriant growth on the flood-plain, or elsewhere where the soil is 

 deep and the water conditions relatively favorable. The specimens spe- 

 cially studied were situated on the flood-plain of West Wash and on the 

 bajada at the northern base of Tumamoc Hill. In these locations the 

 extremes of soil conditions, as far as concerns its depth, were met, together 

 with the greatest differences in water relations to be found in the four 

 habitats under consideration. 



The upper soil at the bajada habitat is of adobe clay, with an admixture 

 of rock fragments and larger stones, about 20 cm. in thickness, below which 

 is a layer of rotten caliche, also about 20 cm. thick, and the latter is under- 

 laid by hard caliche of undetermined depth. A section of similar soil con- 

 ditions, made from a photograph of a cut not farfrom the habitat inquestion, 

 is shown in plate i, and illustrates the conditions which obtain here. 



The larger plants of the vicinity were all of the genus Covillca, of which 

 those nearest the specimen were 1.5 meters northwest, 2.6 meters southwest, 

 and 3.1 meters south. The roots of each of these plants, and probably the 

 roots of others also, invaded the root-area of the specimen specially studied. 



The Covillea examined was 95 cm. high and was composed of numerous 

 branches. It was a typical and vigorous plant. The leading characters 

 of its root-system were found to be as follows: A main root went down 

 to the hard caliche, about 35 cm., where it forked. One branch was traced 

 about 3 meters northwest and lay from 35 to 53 cm. beneath the surface 

 of the ground. The other branch was followed about i meter and lay from 

 35 to 45 cm. deep. These were the longest roots of the plant and 

 extended to pockets in the hard caliche. 



