56 



THE RELATION OF DESERT PLANTS TO 



perature, as far as the observations show, occurred very early, at 9 h 30 m 

 p.m. The temperatures at which the two minima occur are much more 

 nearly in agreement than are the corresponding evaporation rates. 

 Obviously light intensity can play no important role in the response. 



On the whole this curve of transpiration agrees fairly well with that 

 of Experiment IV, and points to the same general conclusion. The 

 variation in relative transpiration in the middle portion of the period 

 of this experiment was from a maximum of 0.237 to a minimum of 



0.029, or from unity to about 

 one-eighth, which is the same 

 as in the other specimen of this 

 form used in the last experi- 

 ment. The present plant had 

 more young leaves than did the 

 previous one, and this fact may 

 explain why its hourly rate of 

 water loss per unit area was 

 higher than in the former case. 

 Experiment VI. The sub- 

 ject was a plant of Allionia 

 incarnata, consisting of three 

 shoots, each about 15 cm. long, 

 coming from a single root. 

 Flowers were opening during 

 the experiment. The plant was 

 sealed at Il h 30 m a.m., August 

 15, and the record of weighings 

 was continued until 3 h 30 m p.m., 

 August 17, when wilting en- 

 sued. The plant was exposed 

 to bright sunshine during the 

 daytime. The evaporimeter 

 for use in sunshine was not 

 available until 3 h 30 m p. m., 

 August 16, so that relative 

 transpiration was not obtained 

 till after that time. The total 

 leaf area was 119.44 sq. cm., 

 and the average hourly rate of 

 transpiration from the entire plant for the whole period of the experiment 

 was 0. 8396 gram, or 0. 007 gram per unit of leaf surface. Since the aver- 

 age daily rate of transpiration remains practically uniform throughout 

 the experiment, and does not fall toward its end, this hourly rate may 



FIG. 11. Curve of relative transpiration for a plant 

 of Allionia incarnata, August 16-17, 1904. 



