62 REPORTS ON INVESTIGATIONS AND PROJECTS. 



of all components, but neither type penetrates the soil to a greater depth than 

 20 cm. (8 inches). Perennials have three types of roots, namely, the gen- 

 eralized type, with the tap root and the laterals both well developed, and the 

 specialized forms of which one type has a prominent tap root and the other 

 prominent laterals. Covillea tridentata and Prosopis velutina are repre- 

 sentatives of plants with generalized roots, while Kceberlinia spinosa and a 

 few other plants have specialized roots of the second type. 



The roots of perennials growing on Tumamoc Hill and on the bajada do 

 not penetrate as a rule more deeply than 30 cm., the depth of the available 

 soil ; while those on the flood-plains attain a depth of 2 to 5 meters. 



The most shallowly rooted perennials observed were Opuntia arbuscula 

 (whose roots frequently do not lie more than 2 cm. below the surface) and 

 Echinocactus wislizeni. The arborescent opuntias have roots which ap- 

 proach the generalized type. 



Most cacti have two divisions to the root-system: an anchoring and an 

 absorbing system. 



Fleshiness in the roots of the opuntias is a condition resulting directly 

 from an abundant supply of water; this is true of young plants also. 

 Whether some species exhibit greater tendency toward fleshiness than others 

 was not determined. A similar reaction was seen in Brodicea capitata. 



Perennials with the generalized type of root-system have the widest local 

 distribution, and those with a pronounced development of the tap root have 

 the most limited distribution. Plants with laterals well developed, the cacti 

 especially, are most abundant on bajadas or detrital slopes, as on Tumamoc 

 Hill, where the soil is shallow, and occur seldom on the flood-plains. 



Germination and Establishment of Desert Perennials (by Dr. F. Shreve) : 



Laboratory experiments have been conducted to determine the best condi- 

 tions of temperature and soil-moisture for the germination of a selected 

 group of desert perennials. All the forms used germinate at the higher 

 temperatures of the summer growing-season, and it is found, in general, that 

 the sclerophyllous species germinate at lower percentages of soil-moisture 

 than do the succulents. The development of the root-system of the seedling 

 and the changes in minimum water requirement with advancing age have 

 been tested with the result that atmospheric factors are found to be quite as 

 important as those of the soil in determining the behavior of seedlings. Fur- 

 ther investigations have been carried into the field, where germination, 

 growth, and fate of seedlings have been followed under uncontrolled condi- 

 tions, simultaneously with the measurement of the atmospheric and soil 

 conditions to which they are subjected. Striking differences have been found 

 in the manner of establishment of the different classes of perennials, and 

 survival has been found due not so much to chance variation in the seedlings 

 as to small differences in the immediate environment of different individuals. 



