44 



ROOT HABITS OF DUSERT PLANTS. 



PLANTS FROM TUMAMOC HILL. 

 Of the areas under consideration in this paper, Tumamoc Hill is the richest 

 in the number of species of perennials and the bajada in annuals. The same 

 species frequently occur on all of the habitats, but to this there are numerous 

 exceptions. Of the plants studied on Tumamoc Hill the following do not 

 grow in the other areas : Jatropha cardiophylla, Opuntia discata, and Opuntia 

 versicolor. On the other hand, some forms which are most typical of Tuma- 

 moc Hill, such as Carnegiea gigantea, but which are in other areas also, w^ere 

 studied where the soil conditions were most favorable for excavating, and 

 not necessarily on the Hill exclusively. 



ECHINOCACTUS WiSLIZENI. 

 Two specimens of Echinocactus were selected for special study, one of 

 which was growing by West Wash and the other on the north shoulder of 

 Tumamoc Hill about 60 meters north of the Laboratory building. 



^r 



Fig. 2. — Root-system of Echinocactus wislizeni, horizontal and vertical extension. Intruders 

 are represented : bv broken lines. Acacia conslricta; bv large dot%, Menodora scabra; and 

 Opuntia discata by dotted lines. 



