RESISTANCE BY LEAVES TO WATER LOSS 



15 



chloride paper gives its color response when a certain definite 

 amount of moisture has been absorbed, and since the moisture 

 absorbed in the leaf tests must necessarih^ emanate from the leaf, 

 it follows that we have, in the indices here presented, approxi- 

 mate measures of the possible rates of water exit from the leaves, 

 these measures being stated in terms of the possible rale of evapo- 

 ration, under the same conditions, from a water surface covered 

 by a millimeter of vapor and air blanket. Thus, when the index 

 of transpiring power of the lower leaf surface of our plant is 

 0.096, this means simply that, at the end of hour 12, 1000 area 

 units of the leaf surface in question could supply moisture to any 

 set of aerial surroundings at the same rate as would 96 area units 

 of a free water surface blanketed by a millimeter layer of air. 



It is to be noted that the range of transpiring power is very 

 much greater for the lower than for the upper leaf surface; for 

 the former the range is from 0.005 to 0.120 or from unity to 24.0, 

 and for the latter it is from 0.018 to 0.081 or from unity to 4.5. 

 Furthermore, it is interesting to observe that the greater range of 

 variation of transpiring power exhibited by the lower leaf surface 

 is not entirely due to a higher maximum than that attained by 

 the upper surface, but also to a markedly lower minimum. At 

 the time of greatest retardation of water loss on both sides of 

 the leaf, the transpiring power of the upper surface is still 3.6 

 times as great as that of the lower. A discussion of the causes of 

 these differences would be premature at present and out of place 

 in this paper. 



Table 3 presents the data of transpiration from the two plants, 

 those of evaporation for the two atmometers, and the relative 

 transpiration ratios derived for each plant as compared to each 

 instrument. Each series of data has been reduced to a compara- 

 ble series, with the datum for hour 1 as unity. That the actual 

 losses from which the reduced rates have been obtained may 

 readily be reverted to, the actual value for hour 1 is inserted in 

 the table, in parentheses, below the item 1.00 (for hour 1) in each 

 series. Thus it is possible to obtain the actual observed rate from 

 plant or instrument, for any given hour. The inserted atmometric 



