44 



ROOT HAIUTS OP DUSURT I'l.ANTS. 



PLANTS FROM TUMAMOC HILL. 

 Of the areas under consideration in this paper, Tiimamoc 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. Opuniia discata, and Opiintia 

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

 moc Hill, such as Carnegica gigantea, but which are in other areas also, were 

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

 not necessarily on the Hill exclusively. 



ECHINOCACTUS WiSUZENI. 



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

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

 Tumamoc Hill about 60 meters north of the Laboratorv building. 



^ 



^O" 



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

 are represented: by broken lines, Acacia constricta; by large dots, Menodora scabra; and 

 Opuntia discaia by dotted lines. 



