BOOKS AND CURRENT LITERATURE 217 



humidity, but in neither of these papers does the author appear to con- 

 sider the importance of this consideration, wliich has l)een so clearly- 

 emphasized by Renner.- The external condition determining the rate 

 of water loss from leaves is of course (aside from the intensity of imping- 

 ing radiant energy) not the relative humidity of the air at all, but its 

 evaporating power as has been repeatedly emphasized in the atmometrie 

 studies of the reviewer.'' Only when the air currents and the radiant 

 energy influx are maintained constant can relative humidity determine 

 the rate of transpiration from a given leaf in a given condition. 



If the rectilinear graph here discussed be produced downward and to 

 the left it furnishes, where it intersects the axis of ordinates, an hypo- 

 thetical value for the transpiration rate corresponding to 100 per cent 

 of humidity, and this value, as has long been known from other sources, 

 is alwaj^s greater than zero. In other words, the line in question passes 

 the y axis above the origin. By continuing the line still farther it finally 

 cuts the horizontal axis at a point where the hypothetical humidity is 

 about 105 per cent. This means, as Darwin well points out, that the 

 vapor tension of the leaf is somewhat higher than that of a saturated 

 atmosphere. Quantitatively, the vapor tension of the leaf is about 

 what it should be if the leaf temperature were 0.8°C. higher than that 

 of the surrounding air. Darwin concludes that such a higher degree of 

 temperature may be supposed to be actually produced in the leaves of 

 his experiments, through respiration, but it does not seem to the reviewer 

 that such a supposition is at all necessary, for the vapor tension of the 

 leaf as an evaporating object is determined by the temperature of the 

 evaporating surface films rather than by the temperature of the entire 

 leaf. Of course the incisions in the leaf may be expected to increase the 

 rate of liberation of heat, by respiration, in their immediate neighbor- 

 hood at least. But, in any case, is it not possible for heat set free by 

 chemical processes or received by radiation to pass directly into the 

 latent form, without any corresponding rise in the temperature of the 

 leaf? What amounts to the same thing, may not the increased evapora- 

 tion brought about by chemical processes cool the evaporating surfaces 

 sufficiently so that a higher temperature may not be produced although 

 kinetic energy is being liberated in the tissues? It seems safe to con- 

 clude that the coated and incised leaves give off water vapor as though 



= Renner, O., Beitriige zur Physik der Transpiration. Flora 100: 541-547. 

 1910. Renner, O., Zur Physik der Transpiration II. Ber. Deutsch. Bot. Ges. 30: 

 572-5. ' 1912. 



* Livingston, B. E., The resistance offered by leaves to transpirational water 

 loss. Plant World 16: 1-35. 1913. 



