Jan., 1920] Resistance of Leaves to Transpiration 63 



Whenever the record sheet was taken from the drum of the 

 chronograph, which turned at the rate of 12 millimeters per 

 hour, the hour intervals were marked. The rate of water loss 

 per hour was found by dividing the time interval (in millimeters) 

 by the distance between shot marks. A table containing rate of 

 water loss for all time intervals of 1 to 12 millimeters and 

 distances between shot from 12 to 48 millimeters was made, 

 and to obtain the hourly rate it was only necessary to measure 

 the distance between shot marks and refer it to the table. The 

 hourly evaporation rates are reduced to the rate of the Standard 

 Cup by multiplying the chronographic rate by the coefficient 

 of the cups. Tables I and II, on page 60, give these 

 coefficients. The radio-evaporation is the difference between 

 the Standard Cup and the black cups. The hourly transpiration 

 rates are reduced to the rate of water loss from 100 square 

 centimeters of leaf area, considering one surface of the leaf, by 

 dividing the hourly chronographic rate by the leaf area. 



All these calculations were aided very much by the use of 

 an engineer's slide rule, an adding machine, and a planimeter. 



EXPERIMENTS. 



The object of these experiments was to determine the 

 relation of the hairs on the mullein leaves to the resistance of 

 the leaf to water loss. Two series of experiments were per- 

 formed. In the first series a comparison of the transpiration 

 rates of mullein and tobacco was made. Tobacco {Nicotiana sp.) 

 was chosen as a suitable plant with which to compare mullein, 

 because it is so nearly like mullein in every respect except the 

 hairy covering. The thickness of the leaves, the arrangement of 

 the intercellular structures as well as the general shape and appear- 

 ance of the plant are as much alike as two plants of different 

 families could possibly be. In the second series of experiments 

 the transpiration rates of three different mullein plants were 

 compared. At first, the normal plants were exposed to similar 

 conditions and their comparative rates were determined; later, 

 the hairs were removed from the lower side of one plant and 

 from the upper side of another, while the third plant was 

 left with hairs on both surfaces, and the rates under different 

 conditions were compared. The removal of the hairs caused 

 no injury to the leaf surfaces as the leaves kept on growing 

 as healthily as ever and a microscopic examination of the 



