BOOKS AND CURRENT LITERATURE 



Influence of Humidity and Illumination on Transpiration. — 

 Sir Francis Darwin^ contributes two papers to the current volume of the 

 Proceedings of the Royal Society, in which he emphasizes the importance 

 of the relations holding between the transpiration rate from leaves and 

 the relative humidity and light conditions of the surroundings. To 

 avoid the influence of stomatal movement the stomatal surfaces of the 

 leaves are thoroughly coated with cocoa-butter or vaseline, and knife 

 incisions are then made through the leaf lamina, between the main 

 lateral veins and parallel to these. These incisions of course put the 

 external air into direct connection with the internal atmosphere of the 

 leaf, in a manner comparable to that in which open stomata effect the 

 same connection. Since the incisions do not open and close, as do the 

 stomatal pores, with changes in the environmental complex, leaves 

 so treated may be subjected to various kinds of conditions without 

 danger that stomatal movements may seriously influence the rate of 

 water loss. Thus, the most important varying internal condition is 

 rendered approximately constant and the direct effects of external condi- 

 tions are unmasked. 



In the first paper the author presents the results of some studies of the 

 relation of relative humidity to the magnitude of the transpiration rate 

 and finds, in most cases, that this relation is actually a linear one, as was 

 to be inferred from the physics of the conditions involved. In some cases 

 there was a lag apparent, so that the experimental graph becomes a 

 curve instead of an oblique straight line, but the simple linear relation 

 appears to be usual, at least in the cases recorded. Darwin does not 

 discuss the nature of this relation more in detail , but it is clear from his 

 graphs that the straight line representing the relation in question does 

 not vary far from the position giving it an angle of 45 degrees with the 

 horizontal. The temperature varied but little during the experiments 

 and is not seriously discussed. It is apparent from the general consist- 

 ency of the results that the air currents and the form of the vapor blanket 

 about the leaves must have been sensibly uniform in the different tests. 

 If this had not been the case it would not be possible to bring out a defi- 

 nite relation between rate of transpirational water loss and relative 



1 Darwin, F., On a method of studying transpiration. Proc. Roy. Soc. London 

 B 87: 269-80. 1914. Darwin, F., The effect of light on the transpiration of 

 leaves. Proc. Roy. Soc. London B 87: 281-99. 1914. 



216 



