RESISTANCE BY LEAVES TO WATER LOSS 29 



Ruellia tuberosa var. occidenlalis 







TIME 



INDEX 









teconds 









upper leaf surface 



949 



0.020 



Hour 16 



1 



lower leaf surface 



145 



0 124 







water test, saturated paper 



18 









upper leaf surface 



958 



0.038 



Hour 23 



1 



lower leaf surface 



1101 



0.033 







water test, saturated paper 



36 





While the index of transpiring power at the dayhght observa- 

 tion is about six times as great for the lower leaf surface as for 

 the upper, the two leaf surfaces exhibit practically no difference 

 in transpiring power at the time of the night observation. At the 

 same time it is to be noted that this alteration is not entirely pro- 

 duced by change in the lower surface; the upper surface reveals 

 a much higher index at night than in the day. The average index, 

 representing the entire leaf surface, has a magnitude about twice 

 as great for the daylight observation as for that in the night. 



Experiment VI. A flowering plant of Nicotiana trigonophylla 

 Dunal., in the open ground and unshaded, near the streamway 

 of the Canada del Oro, north of Tucson, was tested during 

 the 12th hour, September 1, 1912. The summary of results is 

 given below. 



Nicotiana trigonophylla 



LEAF SURFACE 



TIME 



INDEX 





seconds 





Upper 



270 



0.052 



Lower 



149 



0.094 



Entire 





0.073 (average) 



Water test (saturated paper) 



14 





In this case the upper leaf surface appears capable of suppljdng 

 water to the surroundings at a rate only about one-half as great 

 as that for the lower surface. 



Experiment VII. Leaves of the grass Andropogon halapense 

 Brot. (Johnson grass) were tested during the fourth hour, August 

 31, 1912. The plant was growing in the experimental grounds of 

 the Desert Laborator3^ The results follow. 



