RESISTANCE BY LEAVES TO WATER LOSS 



5 



A METHOD FOR COMPARING TRAXSPIRATIOXAL RETARDATION 



The tendency of water to evaporate from a free surface depends 

 upon two sets of conditions, one internal, resulting in the vapor 

 tension of the surface, and the other external, the evaporating 

 power of the supernatant air. The vapor tension is conditioned 

 by several factors, such as the temperature of the surface, the 

 amount of dissolved material in the water, etc. Now transpira- 

 tional retardation in plants may be compared to the retardation 

 of evaporation from a solution, caused by the presence of solutes. 

 This suggests that the problem of comparing transpirational 

 retardations involves the finding of some sort of hygrometric 

 device, which may indicate the relative rates of evaporation from 

 the leaves tested in terms of the rate of evaporation from a stand- 

 ard evaporating surface, as of free water, at the same tempera- 

 ture and under the same general conditions. 



After preliminary experimentation, it became apparent that 

 a modification of the well known cobalt paper test devised by 

 Stahl^ gave promise of meeting the conditions of our problem. 

 It will be remembered that cobalt chloride has the property of 

 appearing blue when dry and red when h5-drated. Taking advan- 

 tage of this fact, Stahl prepared an hygrometric paper which is 

 blue when dry and which alters its color to white or pale rose- 

 color when moist. The paper loses all of its blue color when it 

 has absorbed a certain amount of water, and it can be prepared 

 so as to be very uniform and very sensitive. Stahl's object was 

 to compare the transpiring power of different leaves and of the 

 two surfaces of the same leaf, no attempt being made, however, to 

 standardize these findings to any physical surface. When small 

 pieces of the cobalt paper were appressed to a leaf surface, held 

 in position and protected from the external ah- by means of glass 

 or mica plates, the time required for loss of blue color was taken 

 as an index of the transpiring power of the leaf in question. 



It was early found in the present studies that the paper used 

 must be comparatively thin, in order to give a quick response. 



^ Stahl, E., Einige Versuche iiber Transpiration und Assimilation. Bot. Zeitg. 

 52: 117-146, 1894. 



