40 



THE AMERICAN NATURALIST [Vol. XL 



The transpiration of the cactus was taken September 9, and 

 as the high relative humidity at the beginning of the experiment 

 shows, the effects of the rains of August were still manifest. The 

 rate which is 0.2 milligrams per minute for 100 sq. cm. of tran- 

 spiring surface, may be considered a high one, since without ex- 

 ception the greatest rate of transpiration of all the plants, whose 

 seasonal variation in rate has been observed, has been after or 

 at the time of the summer rains. 



Experiment 2. Echinocactus wislizeni 



This experiment took place on March 19, at the Desert Botan- 

 ical Laboratory. 



The rate of transpiration of the entire plant per hour is 3.4 

 milligrams. 



On September 3 the experiment was repeated when the fol- 

 lowing data were derived: — 



Experiment 3. Echinocactus wislizeni 



12: 24 p. M. 35 104° F. * 60 



3: 14 p. M. 45 109° F. 89 



The rate for the entire plant is 9.6 milligrams per hour. 



This specimen of Echinocactus is growing on a westerly slope 

 on the Laboratory Mountain. In its vicinity are found a few 

 giant cacti and F.nrrlid jarino.sa, Li/muji and P(u-k'n>s,>nia 



microphylla. Because of tlu' dcsiralnlitv of prcscjn itiii; this s{H>ci- 

 men the surface was not coniputt'd and therefore the rate cannot 

 be comparecl directly with that of Crrcus ^/njanlniy a. ^iven in the 

 preceding experiment. However, it liappcne l that the two cacti 

 were of nearly the same size, — 10 cm. in height,— - and a gen- 

 eral comparison between the two can be made. The rate of 



