SUMMARY. 



1 . On Tumamoc Hill there is sufficient moisture in the soil at a depth of 

 30 to 40 cm. to be available to plants all of the year. The soil of Tumamoc 

 Hill and the flood -plain of the vSanta Cruz at a depth of 15 cm. may be air- 

 dry within six weeks following rains and that of the bajada within three 

 weeks. 



2. The highest soil temperatures are reached in July just preceding the 

 summer rains; the lowest temperatures of the soil, at a depth of 15 cm., 

 are reached in midwinter and a depth of 30 cm. in late winter. 



3. The roots of most annuals do not penetrate the soil deeper than 20 

 cm. The largest development of laterals of annuals takes place 4 to 5 cm. 

 from the surface of the ground. Rudimentary roots of the first order 

 were seen in Aiiisinckia, Erodium, Eritrichium, Harpagonella, Malva, and 

 Pectocarya among the winter annuals, and Amaranthus, Boerhaavia, Clado- 

 tlirix, and Triantlicma of the summer forms. These rudiments are further 

 developed only under favorable moisture conditions, such as out-of-season 

 rains, when they become of great importance to the plant. 



4. The root-systems of the summer annuals, which are distinct from the 

 winter annuals, are especialh' well developed. This characteristic is due 

 to the great luxuriance of shoots of the summer forms, owing to the favor- 

 able vegetative conditions of this season, not least of which is the warm soil. 

 In winter the soil at the depth attained by the roots of the annuals is 

 colder than the superficial la3'ers and does not present the most favorable 

 conditions for water absorption. In winter, also, the shoot development 

 of annuals is less than in summer. 



5. Perennials have three t3'pcs of roots, namely, the generalized type, 

 with the tap root and the laterals both well developed, and two specialized 

 forms, of which one type has a prominent tap root and the other promi- 

 nent laterals. Covillea tridcntata and Prosopis vehitina are representatives 

 of plants with generalized roots, while Koerberlinia spinosa and a few other 

 plants have specialized roots of the first type, and most of the cacti have 

 specialized roots of the second kind. 



6. The roots of perennials growing on Tumamoc Hill and on the bajada 

 do not penetrate as a rule deeper than 30 cm., the depth of the available 

 soil ; while those on the flood-plain of West Wash attain a depth of 2 meters, 

 and those by the Santa Cruz may reach as deep as 5 meters or more. 



7. The most shallow rooted perennials observed were Opuntia arbuscula 

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

 and EchinocciCtus wislizcni. The arborescent opuntias have roots which 

 approach the generalized type. 



