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



of the leaves themselves is several decrees higher still. Where the 

 thermometer recorded temperatures as low as 20° (after the canvas screen 

 was first put up, in Expt. 5), or 21-22° (as in the cloudy intervals during 

 Expt. 3), the leaf temperature was probably slightly higher than this ; 

 although, considering the exceptionally rapid transpiration of this leaf, it is 

 questionable whether it reached the minimal temperature. Thus at times 

 the rate of assimilation may have been less than the average : if so, it must 

 also at times have been more than the average. 



Still Higher Rate of Assimilation Possible. — This suggestion is made 

 probable by observations which were made with the horn hygroscope. 

 These show considerable variations during an experiment lasting several 

 hours. On the whole, the readings increase gradually towards noon, reaching 

 a maximum about that time. They then remain more or less constant for 

 one or two hours, and afterwards gradually fall. 



Although the exact interpretation of the individual readings is open to 

 doubt, yet as a whole they clearly support the general conclusion that the 

 condition of the stomata is not uniform over so many hours. If, as we have 

 assumed to be generally the case, the factor which limits assimilation is, 

 throughout, the supply of carbon dioxide, it follows that the rate of assimila- 

 tion must vary during the experiment. 



The rate of increase may, then, be even higher than 18 milligrammes per 

 hour. The maximum rate cannot be accurately estimated, but we may safely 

 conclude that it is above 20 milligrammes. 



This being so, a minimal temperature as high as 25° C. may sometimes be 

 required for the leaf to make full use of the supply of carbon dioxide which 

 its widely opened stomata make possible. 



The need for such high temperatures suggests that, even in ordinary air, 

 temperature may be more often than has been suspected the factor limiting 

 assimilation. Such a state of things might occur, for instance, on a bright 

 cold day, unless the low temperature prevent the stomata from opening to 

 their fullest extent notwithstanding bright illumination, and thus limit the 

 rate of assimilation indirectly. 



T/ic Waste of Photosynthetic Radiation. — Blackman and Matthaci* have 

 calculated the proportion of the energy of sunlight, available for assimilation, 

 which is actually used by leaves under various external conditions, natural and 

 experimental. 



For llclianthvs avnuus tliey estimated the waste of energy in ordinary air 



and Mattliaoi {loc. cit., 1905) and by A. M. Smith ("On tho Internal Temperature of 

 Leaves in Tropical Insolation," ' Ann. Eoy. Bot. Gard., Peradeniya,' 1909, vol. 4). 

 * Zoc. ciL, p. 455. 



