412 Cannon: Transpiratiox of Fouquieria 



plete for the daytime and for a portion of the night. A summary 

 of the experiment is presented in figure 7. 



It will be noticed that the curve of transpiration reaches its 

 highest point about noon and that in a general way it is similar to 

 the curve expressing the temperature and that it opposes that rep- 

 resenting the relative humidity. However, it neither closely fol- 

 lows the one nor flatly opposes the other. The maximum trans- 

 piration precedes the highest temperature of the day and a marked 

 decline in the rate of transpiration takes place while the high tem- 

 perature is maintained. The opposition of the curves represent- 

 ing the relative humidity and the transpiration is more consistent, 

 but even in this case the afternoon decrease in the rate is more 

 rapid than the rise in the relative humidity. It would be of much 

 interest to compare the periodicity with the daily variation of other 

 factors, as the temperature of the ground, and particularly the 

 daily variation of the light, but unfortunately no data w^ere avail- 

 able at the time to make this possible. 



Experiment No. 10, August 26: Maximum rate 



As graphically shown in figure i, the heaviest rain of the 

 year occurred in midsummer at a time when the temperature also 

 was high. Although the rains did not extend over a period 

 exceeding six weeks, the face of the desert was wholly changed 

 because of them. Annuals in great variety and in large numbers 

 appeared on previously barren stretches, and shrubs and trees 

 which were apparently dormant during the droughts became cov- 

 ered with an abundance of large leaves. Thus the summer vege- 

 tation which appeared in consequence of the rains was tropical in 

 its luxuriance. It was with much interest, therefore, that I ob- 

 served the transpiration of plants at this period. 



The following data were obtained about the close of the rainy 

 period from the study of a branch in place : 





Per cent, of 





Amount m 



Time. 



taturation. 



Temperature, 



milligrai^s. 



1:10 P. M. 



33 



98° F. 



263.7 



»:24 58.5 103 540 



The rate of transpiration is 8.25 milligrams per minute for lOO 

 sq. cm. of leaf surface. This is the highest rate observed for 

 Foiiquieria at any time during the year. 



