26 



BURTON EDWARD LIVINGSTON 



much more in agreement than they are in disagreement, and 

 the student of such series of observations as have been presented 

 above finds himself unable logically to ascribe greater reUability 

 to either of the two methods. Of course neither is to be consid- 

 ered as quantitatively accurate, yet in the present state of our 

 knowledge of the dynamics of plant transpiration, it is clear 

 that both methods promise to be of great value for the future 

 development of physiology and physiological ecology. 



Experivient III. In this experiment is considered merely the 

 comparison of the indices of transpiring power for two widely 

 different forms at the same time. At 10 hours 30 minutes Sep- 

 tember 17, 1911, potted plants of Zea mats L. (about 40 cm. in 

 height) and of Martynia louisiana Mill, (about 6 leaves each), 

 which had been grown in the open at the Desert Laboratory, 

 were subjected to the cobalt chloride test. The average time 

 periods required for the color change (4 to 6 tests in each case) are 

 summarized below, together with the indices of transpiring power 

 derived therefrom. 





LEAF SURFACE 



TIME 



INDEX 





■ 



upper 



seconds 

 40 



0.68 



Martynia ■ 





lower 



36 



0.76 







entire 





0.72 (average) 







upper 



430 



0.064 



Zea ■ 





lower 



446 



0.061 







entire 





0.063 (average) 



Water test 



(free surface) 



27 





It thus appears that in both plants the transpiring powers of the 

 two foUar surfaces are about the same, but that the index for 

 Martynia is over ten times that for Zea. It is also worthy of 

 note that, according to these figures, the transpiring power of 

 Martynia leaves should be about 72 per cent as great as the water- 

 supplying power of our standard water surface. 



Experiment IV. These tests bring out the difference between 

 the indices for different leaves of the same plant, at the same time. 

 Potted plants of Phaseolus vulgaris L. (the Mexican bean) were 



