434 Mr. D. Thoday. Experimental Researches on [Mar. 1, 



Laurel and Helianthus tuberosus give concordant results for the photosynthetic 

 value of a given intensity of light. In other words, " the same amount of CO2 

 is reduced in both plants by the same intensity of light."* 



Brown and Escombe's screen, then, permitted in full sunlight much more 

 light to pass than was required for the highest rate of assimilation which 

 they observed,! and their low values are not to be explained as due to the 

 limiting of assimilation directly by the supply of energy. 



Light, however, influences the rate of assimilation in nature indirectly 

 through the stomata, and in this respect to reduce the intensity of the 

 illumination is to run the risk of putting the leaf at a disadvantage. 



DarwinJ has shown how sensitive stomata are to changes in the intensity of 

 the light. In one experiment he shaded a leaf of Ciicurhita, growing in bright 

 diffused light, with thick netting, and found that in less than an hour the 

 horn hygroscope gave a reading only one-half of that which it had given 

 before, while on an unshaded control leaf the reading remained practically 

 constant. He also observed that on a dark wet day in August as well as on 

 dark winter days many leaves gave zero readings with the horn hygroscope. 



I have made one assimilation experiment with H. anmius on a dull rainy day 

 in August, 1909. The few hygroscope observations which could be made during 

 the experiment were in agreement with these results which Darwin obtained 

 with greenhouse plants, the readings being low, though not zero. The values 

 obtained for the rate of assimilation were also very low, lying between 1 and 

 2 milligrammes ; but exposure to rain had made the stamped rectangles 

 very faint and the results are therefore not fully trustworthy. 



Although this evidence deals with light of low intensity, the fact that the 

 width of the stomatal openings and the rate of assimilation are very small 

 suggests that some direct relation to the intensity of light may exist through 

 a considerable range of intensity. Assuming that such is the explanation of 

 Brown and Escombe's low results it follows tliat even bright diffuse light, to 

 which their leaves were exposed, is not sufficient to bring about full opening 

 of the stomata. 



Relation of the Stomata to Illumination. — What are the conditions necessary 

 to induce the stomata to open widely is a question which cannot fully be 

 answered, but there are some further facts which may bear on it. 



* Blackmail and Matthaei, loc. cit., j). 445. 



+ Calculating from their Expt. .3 with Tropa'ohim majun, after introducing an appro- 

 priate correction for respiration (from lilacknian and Matthaei, loc. cit., p. 44G), the full 

 illumination possible under this screen was sufhcient for the formation of 10 milligraninies 

 dry substance per liour. Their highest rate of assimilation was equivalent to about 

 5".') milligrammes. 



I Loc. cit., \>\h 573—4. 



