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The Effect of Light on the Transpiration of Leaves.* 

 By Sir Francis Paewin, F.K.S. 



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



The -method employed is essentially that described in my paper! " On a 

 Method of Studying Transpiration," where it was applied to the investigation 

 of the relation between the relative humidity of the air and the loss of water 

 by leaves. The stomata of the plants used were closed by vaseline or cocoa-fat 

 rubbed in, and the leaves were then incised to allow of transpiration. No 

 attempt was made to subject the plants to light of known intensity. My 

 object was to compare the transpiration occurring in a dark room with that 

 in a north light at a laboratory wiudow. The rates of transpiration were 

 estimated either by weighing or by means of a potometer, and the general 

 plan was to subject the plant to alternate bight and dark periods of something 

 like an hour.^ The psychrometric condition of the laboratory air and that of 

 the dark room was estimated by the wet and dry bulb thermometer, and the 

 transpiration rates corrected for any differences, in the manner described in 

 the paper above referred to. 



The first experiment was made by a plan which has some merits, but was 

 afterwards replaced by the simpler method of moving the apparatus from the 

 window to dark room and back again to the light. 



Experiment 1. — December 9, 1909. P. lanrocerasus. 



Branch fitted to potometer December 8 and the lower surfaces of the leaves 

 greased; leaves cut about 10 a.m., December 9. 



Placed under a bell-jar through which a current of laboratory air is drawn 



* It is not easy to find any recorded experiments on the transpiration of leaves in 

 light and darkness, in which the action of the stomata is absolutely excluded. In 

 Bonnier and Mangin's experiments on the transpiration of fungi this is ipso facto the 

 case (see 'Ann. Sc. Nat.,' 1884, vol. 17, p. 298). The average of the experiments on 

 Trarroetes s>taveolens is : — LD = 119,100. For Polyporus versicolor the corresponding 

 fraction is 127/100. The symbol L D stands for the relation between the transpiration 

 in light and darkness. 



t ' Eoy. Soc. Proc.,' this vol., p. 269. 



X A few weighing experiments were, however, made on the effect of the natural 

 darkening occurring at night. The average of eight experiments gave the proportion 

 between transpiration in the day (L) and in the evening (D), as L/D = 129 100. 

 Four experiments made with the potometer under similar conditions gave 

 day (L)/evening (D) = 112/100. This subject, including the effect of continuous dark- 

 ness, requires fresh investigation. 



