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On a Method of Studying Transpiration. 

 By Sir Francis Darwin, F.E.S. 



(Eeceived October 22, — Read December 4, 1913.) 



Transpiration is, perhaps, more directly under the rule of external physical 

 conditions than any other physiological function. Yet proofs of this 

 conclusion are wanting, at any rate in regard to the transpiration of leaves. 



Thus, as far as I know, we have no complete experimental determination 

 of the relation between the loss of water-vapour from leaves and the relative 

 humidity of the air. Nor again have we any complete evidence as to the 

 effect on transpiration of variation in the illumination to which the leaf is 

 subjected. 



These lacuna? in our knowledge depend on the fact that in leaves, tran- 

 spiration is largely dependent on the behaviour of the stomata, being 

 relatively large when they are wide open, and diminishing as they close. 

 And since the aperture of the stomata depends on external condition, it is 

 clear that no distinction can be made between the diminution in evaporation 

 resulting from increased relative humidity of the air, and the diminution 

 in the transpiration-rate due to stomatal closure. In fact it is impossible to 

 learn anything accurately concerning transpiration imtil the varying aperture 

 of the stoma is excluded from the problem. This might possibly be done by 

 estimating the transpiration of leaves of aquatic plants in which the stomata 

 vary but slightly in aperture ; but the experiment would not be easily made 

 in a trustworthy form. 



The method I have actually employed is to block the stomata with a fatty 

 substance,* and then to place the intercellular spaces of the leaves in com- 

 munication with the external air by means of incisions. 



Most of the experiments were made on laurel (P. laurocerasus). The lower 

 surface of the leaf was smeared with melted cocoa-butter or with vaseline 

 rubbed in with the finger, and four to six cuts were made with scissors or a 

 razor, reaching from the periphery to the midrib between the large veins. 

 Other plans were also tried, e.g. pricking the leaf with a needle or making 

 numerous small incisions by stabbing with a scalpel.f 



The method is similar to that of Stahl,j who showed that greased leaves 

 pierced with holes assimilate and form starch in the tissues surrounding 



* Cocoa butter in the earlier experiments, vaseline in all the later ones. 

 + The method was described in a paper read at Section K of the Sheffield meeting of 

 the British Association, 1910 (title alone published). 

 % ' Bot. Zeit.,' 1894. 



