IgI2| LIVINGSTON & BROWN—TRANSPIRATION 315 
rapidly at first and more slowly later, the water films gradually 
retreating into the pores of the cellulose and not only decreasing 
the extent of the exposed evaporating surfaces, but also greatly 
increasing the surface tension of the latter. When the surface 
tension of a liquid film is increased, its vapor tension is correspond- 
ingly decreased,’ so that incipient drying of these exposed cell walls 
should be accompanied by a marked fall in the rate of vaporization 
of water therefrom. This should mean nothing less than a measur- 
able retardation in the relative rate of water loss, a retardation 
due to an excessive evaporation rate 
A physical parallel of this phenomenon may readily be arranged 
by mounting one of two similar paper disks as in the Piche atmome- 
ter, so that it will be constantly supplied with water, and its water 
content will remain constant during the progress of evaporation, 
while the second disk is similarly mounted on an empty tube, so 
that its moisture content will fall with water loss. An experiment 
of this sort, carried out by weighings in the laboratory, showed that 
a fall in the moisture content of the paper of 6 per cent produced a 
corresponding decrease in the rate of evaporation of 5 per cent. 
Similarly, a fall of 17 per cent in water content was accompanied 
by an evaporation rate 8 per cent lower than when the paper was 
Saturated. The effect becomes more and more pronounced as the 
paper dries out; when only 54 per cent of the original moisture was 
present the rate of water loss had diminished to 77 per cent of that 
from the saturated disk. 
7 Patten, H. E., On the relation of surface action to electrochemistry. Trans. 
Am. Electrochem. Soc. 19:359-380. 1911. Also, Freunpticu, H., Kapillarchemie. 
Leipzig. 1909. p. 4 
Drxon has Breed the only experimental evidence of this with which we are 
acquainted. . H., Transpiration and the ascent of sap. Prog. Rei. Bot. 
3:1-66. 1909. p. re 
NER has pointed out this same thing: “Denn wenn die Wasserzufuhr a 
aie — hae? Membranen trockener werden und damit die Transpiration wake 
mnken, .; .. RENNER, O., Beitrige zur Physik der Transpiration. Flora 1 
451-547. Igo. p. 516; see also 
€ same author has contributed a most excellent analysis of the water relations 
of stem and leaf, many points of which have a bearing upon the present question, but 
we are unable to do more here than merely to mention the paper: RENNER, O., E 
mentelle Beitrige zur Kenntnis der Wasserbewegung. Flora 103:171-247. 1911. 
