﻿532 



BOTANICAL GAZETTE 



corresponding time required for the same change when the paper is applied 

 to the plant surface, he has termed the "index of transpiring power." 



Bakke,s in making use of this method, has shown that the experimental 

 error incurred in determining the time of color change is not large, and further 

 that it may be possible so to standardize the hygrometric paper that the 

 constant use of the standard surface may be dispensed with by making a cor- 



experiments, however, he continues to use the standard surface. Testing the 

 leaves of 43 different species growing under irrigated conditions at the Uni- 

 versity of Arizona, indices were obtained ranging from 0.9288 for Dahlia 

 variabilis, 0.8473 for Verbena hybrida, and 0.7893 for Medicago saliva, to 

 o. 2258 for Olea europea, o. 1676 for Nerium oleander, 0.0603 f° r Chenopodium 

 incanum, and 0.0025 for Atriplex elegans. These data would seem to afford 

 a most satisfactory basis for ecological classification, and he makes the tentative 

 proposition of including under mesophytes plants with diurnal foliar indices, 



below 0.30, with the possibility of an intermediate critical point at the index 

 o. 50; the desirability and designation of the groups thus formed to be deter- 

 mined by further study. Bakke also shows that the method may well be 

 applied to the determination of the daily march of foliar transpiring power, and 

 gives some data upon the relation of position upon the plant and age of leaves 

 to their transpiring power, making his paper a good example of the use of this 



More recently, in studying the vegetation of the Santa Catalina Moun- 

 tains, Shreve 6 has measured the transpiring power of some 20 species and has 

 found that (1) while the indices of the transpiring power are similar within 

 various groups of life forms, they are not so to the extent that they may be 

 predicted from an examination of the foliage of the plant; (2) the indicas ere 

 higher in plants which grow in flood plains with good moisture supply than in 

 individuals of the same species upon arid slopes; and (3) the transpiring power 



for the day is reached at an earlier hour at lower than at higher elevations, while 

 for some species at some 6000 feet, no check in the rate is manifested, and the 

 maximum transpiring power and maximum evaporation are simultaneous. 



Some of these results were to be expected from the former results of Mrs. 

 Shreve/ obtained in a very exact study of the actual amount of water lost 

 by the desert shrub Parkinsonia microphylla, using both potted plants and 



s Bakke, A. L., Studies on the transpiring power of plants as indicated by the 

 ►metric paper. Jour. Ecology 2:145-173- «9*4- 

 tiring power of plants as influenced by differences of 



