406 Cannon : Transpiration of Fouquieria 



^ 



Although this experhnent and others not reported indicate 

 that under certain conditions leafless branches of Fouquieria can 

 and in fact do absorb appreciable amounts of water and of water- 

 vapor, they do not show that the water or the vapor is of physio- 

 logical significance. Whether such is true I have not been able 

 to demonstrate ; since all the evidence thus far accumulated is 



negative. 



Experiments 4.-6 : Effect of rains 



The immediate effect of rains upon the rate of transpiration was 

 repeatedly observed. When they resulted in an actual increase in 

 the water-supply of the plant, the rate became greater, and later, 

 as the available water-supply was decreased, the rate of transpira- 

 tion became smaller. This was shown by the experiments on 

 February 24 and March 4, 10, and 17, when leaves were develop- 

 ing, when they were mature, and when they may be termed senile. 

 The rates for these experiments were respectively 47. 5? 120, 24.4, 

 and 13.7 milligrams per hour. 



The intimate connection of the rate with the rains is also 

 shown, and more strikingly, in the experiments which were per- 

 formed soon after the rains of May. Between the experiments 



March 



May 



the drought The rains of May were distributed as follows : May 

 10, .47 in.; May 11, .75 in. (the latter fell between 1:40 and 2:15 

 P.M.) ; May 12, ,02 in. and May 16, ,06 in. As a result of this 

 sudden and considerable rainfall the mountain upon which the 

 Desert Botanical Laboratory is situated, which had been very dry 

 all winter, was thoroughly wet, and the mesa between it and the 

 Santa Cruz River was covered with water. This rain had a very 

 marked effect upon all vegetation. Trees and shrubs which had 

 been without leaves all winter put out leaves promptly, the cacti 

 which had become badly shriveled from loss of water directly be- 

 came plump, but perhaps the most noticeable response was given 

 by the fouquierias. Up to the time of this storm Fougtderia 

 No. I and other ocotillos were leafless. On Friday, May 13, 4^ 

 hours after the heaviest fall of rain, Fouquieria No. I was rapidly 

 coming into leaf, and by the afternoon of the following day these 

 were of nearly mature size. The three succeeding experiments 



