SOIL MOISTURE AND TO EVAPORATION. 



63 



11 a.m., August 19. The plant began to wilt at the last-named hour. 

 The total leaf area was 50.376 sq. cm. During the entire period the 

 plant transpired at the average rate of 1.3620 grams per hour, or 0.027 

 gram per unit leaf surface. Owing to the short period no curve was 

 constructed for this plant. 



Experiment XV. This test was performed with three seedling 

 squash plants (Cucurbita pepo), each having two leaves besides the 



cotyledons. The soil about them 

 had been kept moist since germi- 

 nation and they had grown with 

 exceedingly great rapidity. The 

 three plants together possessed a 

 total leaf surf ace of 238. 2 sq. cm. 

 They were sealed August 16 at 

 12"30 m p.m., and had begun to 

 wilt at 3"30 m p.m. During this 

 period of three hours they trans- 

 pired at the rate of 1.9 grams per 

 hour, or 0.008 gram per square 

 centimeter of leaf surface. For 

 this period of three hours their 

 rate of relative transpiration was 

 0.131. 



GENERALIZATIONS FROM THE 

 EXPERIMENTS. 



It appears from the data just 

 presented that Euphorbia, Trib- 

 ulus, Allionia, and Boerhavia all 

 show a periodic fluctuation in 

 their relative transpiration. The 

 highest relative transpiration ob- 

 served was 0.785 (Experiment 

 VIII) and the lowest was 0.008 

 (Experiment IV) . They all have 

 some form of regulative response 

 whereby transpiration begins to 

 be checked between 6 h 30 m a.m. 

 and 1 p.m., the check being generally removed between 6 and 8 p.m. 

 It also appears that in all three forms stomatal or some internal foliar 

 responses probably play the most important role in this regulation of 

 water loss, these being aided perhaps by nyctitropic movements in the 

 first two forms mentioned. As far as the limited data at hand can be 



FIG. 16. Curve of relative transpiration for 

 three plants of Boerhavia, August 16-17, 1904. 



