Establishment Behavior of the Palo Verde. 289 



It needs also to be stated here that tlie new atmometer now- 

 described — and indeed the porous cup forms as well — are to be 

 largely attributed to financial aid from The Department of Bo- 

 tanical Research of the Carnagie Institution of Washington. 

 The Johns Hopkins University, 

 Baltimore, Md. 



ESTABLISH MEiNT BEHAVIOR OF THE PALO VERDE. 



Forrest Shri: vk 



One of the most striking cha-actevistics of the deserts of 

 southern Arizona is the diversified assemblage of ''vegetation 

 forms" or "life forms" which they exhibit. These fall sharply 

 into two classes, — the succulents and the sclerophylls, — the former 

 of which is represented by fewer species than the latter but is 

 of quite as great importance in determining the physiognomy 

 of the vegetation. Among the succulents ma^" be distinguished 

 the leafless stem-succulent cacti, greatly diversified in size 

 and form, the leaf-succulent Agave, the leafy stem-succulent 

 Yucca and Dasylirion, and the root-succulent Ttimanwca. 

 Among the sclerophylls there are less striking differences of 

 gross form, but equally important distinctions in character of 

 foliage and seasonal habits. As examples may be 



noted Covin ca, with small evergreen leaves, Celtis pallida, 

 with broad, evergreen leaves, Prosopis and Acacia, with 

 deciduous dissected leaves, Jatropha cardiophylla with 

 broad deciduous leaves, F ouqiiieria , with ephemeral broad, thin 

 leaves, Parkinsonia, with minute deciduous leaves and chloro- 

 phyll-bearing bark, and Ephedra, with leafless chlorophyll- 

 bearing stems. 



Spalding has shown * for this region that a number of the 

 characteristic types are limited to certain topographic sites, 

 and that many occur abundantly in certain situations and only 

 sparingly in others. However, on the higher mesas, the basaltic 

 hills and the foothills of the larger mountain ranges may be 



*Spaldiag, V M. — Distribution and Movements of Desert Plants. Pub. Cam. Inst., 1909. 



