1910.] 



Vegetable Assimilation and Respiration. 



435 



In my Expt. 5 the leaves were shaded from the full force of the 

 sun's rays by a screen of coarse canvas which allowed only 40 per cent, 

 of the light to pass. The high rate of assimilation was nevertheless main- 

 tained, showing that the stomata were fully open. The photosynthetic value 

 of diffuse light may often be approached, sometimes even exceeded, by that of 

 the light which passes through this screen in full sunshine. For instance, 

 Blackman and Matthaei, in discussing the waste of available energy in nature, 

 take as an example of moderate intensities of illumination on a sunny day a 

 case in which the diffuse light alone allowed of 32 per cent, as much assimi- 

 lation as the whole of the available radiation. Yet Brown and Escombe, 

 exposing the leaf to a diffuse light from a northern sky, observed a rate of 

 assimilation less than one-third of that which was given by the turgid 

 leaves under my canvas screen. Assuming that Brown and Escombe's 

 leaves were also fully turgid, it seems to follow that some quality of the light 

 other than its photosynthetic value induced the stomata to open more widely 

 in my experiment. This factor may possibly be the greater preponderance of 

 heat rays in the direct sunshine, and the consequent raising of the leaf- 

 temperature. 



Brown and Morris' experiment on a dull day gave as the rate of increase 

 of dry weight 9-8 milligrammes. This is a little more than half the rate 

 reached in full sunlight. If the result is not excessive owing to shrinkage 

 errors the diffuse light must have been bright compared with that available 

 during my experiment on a dull day, mentioned above, notwithstanding the 

 fact that the very low result in the latter case is partly to be explained by 

 the blocking of the upper stomata by the rain. Therefore, also, in this 

 experiment of Brown and Morris, the photosynthetic value of the illumina- 

 tion may have been nearly equal to that under my canvas screen ;* if so, 

 some other quality of the radiation must again account for the great difference 

 in the rate of assimilation. 



The possibility of leaf-temperature and intense illumination affecting the 

 stomata has received little attention hitherto. Whether the suggestion 

 here made can be substantiated remains to be seen, but one thing seems clear, 

 that the stomata of H. anmms do not open fully in even bright diffuse 

 light. 



Section III. — Experiments with Catalpa hignonioides. 



Helianthus anmius is probably exceptional, even among herbaceous plants, 

 in its high rate of assimilation. Other rapidly growing plants, such as 



* Cf. high maximum rate of assimilation during dull spell, Blackman and Matthaei, 

 loc. ext., p. 442, Expt. 16. 



