9-7 



fusion of annuals, and the shrubs and trees, also, were covered 

 with foliage and renewed their growth with great vigor. But in 

 the present year (1905), the rainfall of summer was scant and the 

 desert has quite another appearance. The annuals are wholly 

 wanting and the manifestations of vegetative activity on the part 

 of the longer-lived plants are very meager. Certain of the shrubs 

 and the trees, however, show that the conditions obtaining this year 

 are far removed from those of a drought. It is in fact to these 

 conditions and to their apparent influence on the plants that I 

 wish by this note to direct attention. 



The rainfall and the relative humidity data for the summers of 

 1904 and 1905 show that the relative humidity for the two 

 seasons was approximately the same but that the rainfall was 

 very unlike. In fine, not one half the normal rainfall was re- 

 corded during the summer of 1905. The peculiar condition of a 

 small rainfall accompanied by high relative humidity is accounted 

 for by the occurrence of rains in plenty in all of the country 

 adjacent to Tucson. The climate of the summer of 1905 was 

 therefore not only different from that of the preceding summer 

 but was so striking as to merit attention. 



So lon.^ as the annuals failed to appear, as has been mentioned 

 in an earlier paragraph, it naturally happened that the only plants 

 which exhibited the effects of the summer's climate were those 

 with a variable transpiring surface, that is, plants which increase 

 their transpiring surface and decrease it in accordance with the 

 fluctuation of the available water. Under the usual atmospheric 

 conditions obtaining during a rainless period such plants would 

 be leafless, or at least nearly so. As an example of these, Park- 

 insonia inicropJiylla, the " palo verde " of the Mexicans, and 

 Foiiqjiieria splcndens, the " ocotillo," may be selected. How did 

 these forms respond to the anomalous climate of summer? 



Palo verde is so called not only because it is a green tree 

 from its foliage like other trees but chiefly because it is green 

 even when the leaves have fallen. The twigs, branches and stem 

 are green and all perhaps capable of emitting watery vapor from 

 their surface and capable of carbon assimilation. Besides this, 

 the leaves are so small (they average about 14.3 leaflets per 



