584 Physiologie. 



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

 subjected. These lacunae in our knowledge depend on the fact that 

 in leaves transpiration is largely dependent upon the behaviour öf 

 the stomata, and since the aperture of the stomata depends on 

 external conditions no distinction can be drawn between the dimi- 

 nution in evaporation due to increased air humidity and the dimi- 

 nution in transpiration-rate due to stomatal closure. It is impossible 

 to learn anything accurately concerning transpiration until the 

 varying aperture of the Stoma is excluded from the problem. The 

 author's method was to block the stomata with cocoa butter or 

 Vaseline and then to place the interceilular Spaces of the leaves of 

 Prunus laurocerasus in communication with the air by means of 

 incisions, as was done by Stahl who showed that greased leaves 

 pierced with holes form starch in the tissues around these. The 

 author found that when the leaf is cut into Strips the transpiration 

 rises rapidly, and calculations of the surface exposed by incision 

 as compared with the total area of the stomata showed that when 

 these two areas were practically equal the transpiration rate was 

 also roughly the same in incised leaves and intact leaves with the 

 stomata open. Other experiments with this method showed that when 

 plotted on squared paper the points representing the transpiration 

 rate for different degrees of relative humidity are roughly in a 

 straight line, showing that a definite relation exists between transpi- 

 ration and relative humidity; this conclusion, which is a physical 

 necessity, has apparently not hitherto been proved definitely or 

 represented diagrammatically. In some cases, however, the line of 

 dots (the transpiration curve) is not straight, the change in rate of 

 transpiration lagging behind the change in humidit3^ for reasons 

 not yet clear. Another fact brought out is that transpiration is not 

 zero in saturated air, but at about 5 per cent, above Saturation. The 

 degree of supersaturation at which transpiration is nil gives a 

 measure of the internal leaf temperature wich can distil off water 

 in saturated air; thus at air temperature 16^* C, vapour pressure is 

 13,51, and on adding 5 per cent, to this we get 14.2 which is the 

 vapour pressure corresponding to 16.8. There seems no improbabi- 

 lity in leaf respiration producing a temperature öf roughly 1° C. 

 above that of the atmosphere. F. Cavers. 



Darwin, F., The Effect of Light on the Transpiration. (Proc. 

 Roy. Soc. Lond. LXXXVII. p. 281—299. 1914.) 



The object of the experiments here described was to compare 

 the transpiration occurring in a dark room with that in a north 

 light at a window; the method employed was essentially that pre- 

 viously described by the author, in which the stomata were closed 

 with cocoa-fat or Vaseline and the interceilular Spaces then put in 

 communication with the atmosphere by means of incisions, and thus 

 the evaporating surface remains constant under varying external 

 conditions. The results varied greatly, from cases in which transpi- 

 ration was equal in dififused dajdight and in darkness to cases in 

 which the transpiration in light was double that in darkness. The 

 average ratio for transpiration in light and in darkness was 136 : 100 

 in Hedeva helix, and 132:100 in Prunus laurocerasus; the latter 

 plant however reacted more to illumination in early summer than 

 in spring, but there is no evidence as to whether the increased 

 permeability to water is a periodic effect, or connected with the 



