SUMMARY. 



i . 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 Santa Cruz at a depth of 15 cm. may be air- 

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

 weeks. 



2. The highest soil temperatures are reached in Jul)' jusl ]iieceding 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 sfiil 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 Aiiisiiickia. Eyodiuin, Kntrichinm. Ilaipagniiella, Malva, and 

 Pcctocarya among the winter annuals, and Aiuaiinilhus, Bociluiavia, Clado- 

 thrix, and Tnanllictna of the summer forms. These rudiments are further 

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

 rains, when the}- become of great imjjortance to the plaiU. 



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

 winter annuals, are especially 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 b\' the roots of the annuals is 

 colder than the superficial layers 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 types of roots, namelv, 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. Covillca friJentata and Prosi}pis vcliiiina are representatives 

 of plants with generalized roots, while Kavbrrlinia sf^hiosa and a few other 

 plants have specialized roots of the first ty]3e, and most of the cacti have 

 s])ecialized roots of the second kind. 



6. The roots of perennials growing on Tvunamoc 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 3 meters or more. 



7. The most shallow rooted perennials observed were ('piinlia iiihiisciila 

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

 and hlchinocadiis wislizcni. The arborescent opuntias have roots which 

 approach the generalized type. 



