24 



severe drought and reforms them promptly when even a sHght 

 rain falls. During the present season the history of the leaf-cov- 

 ering was quite different. In the earlier portion of June these 

 plants were leafless, but owing to the occurrence of a small rain 

 about the middle of the month, they came into leaf and were 

 able to retain their leaves until the beginning of the summer 

 rains. The leaves which were formed in June, as well as those 

 formed later in the summer, remained on the shrubs during July, 

 August, and are still (September 22) to be seen although now 

 they are yellowing and preparing to fall. 



The rainfall of the summer was not sufficient, I believe, to ac- 

 count for this behavior of the leaves of ocotillo since the ground 

 was very dry, as indicated among other things by the failure of 

 many seeds which were planted August 4 to germinate, and had 

 the humidity been low in correspondence with the rainfall the 

 leaves with little doubt would long ago have fallen. In what- 

 ever manner accomplished, it appears to have been mainly, or 

 wholly, the high relative humidity which so changed the life 

 conditions that the ocotillo, like palo verde, was able to retain 

 its leaves during three months of the hot summer weather. 



It is beyond the purpose of this note to discuss the means by 

 which these desert plants were able to keep their leaves through 

 so long a period of apparently unfavorable conditions. But it 

 seems on the surface that aside from the fact that a large amount 

 of moisture in the air would retard the rate of transpiration and 

 thus assist the plant in better conserving the amount of water 

 at its disposal, it is conceivable that the leaves of the plants really 

 absorb atmospheric moisture in sufficient amounts to be of bio- 

 logical importance. This remains to be tested. I have shown, 

 in the paper cited above, that stems of ocotillo absorb both water 

 and atmospheric moisture, and Prof F. E. Lloyd, who kindly 

 permits me to make the announcement in advance of its publi- 

 cation,* has determined that a branch which is without leaves 

 can absorb water in sufficient amount to induce leaf-formation. 

 It will be of much interest, therefore, to learn whether the stems 

 and the leaves of ocotillo especially can absorb moisture from 



* See ToRREYA 5 : 175. 



