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



widely open, showed an average net rate of increase of nearly 17 milligram me s 

 per hour in their dry weight per square decimetre. Thus the result which 

 Sachs obtained in a similar experiment with detached leaves, 16'5 milligrammes, 

 is entirely confirmed. 



When a moderate allowance is made for assimilation of the CO2 produced 

 in respiration, the photosynthetic products reach a total of about 18 milli- 

 grammes per hour. 



21. The rate of production is not uniform throughout an experiment of 

 seven or eight hours' duration ; it will, therefore, at times exceed the average 

 rate and may reach more than 20 milligrammes per hour. 



2c. When the stomata allow enough CO2 for this assimilation to diffuse iuto 

 the leaf, the internal leaf-temperature will probably be the limiting factor 

 unless it exceeds 23° to 25° G. All the available evidence points to the con- 

 clusion that on a bright summer's day the leaf-temperature is much higher 

 than this ; but the necessity for such high temperatures suggests the 

 possibility that temperature limits the rate of assimilation more frequently 

 than has usually been supposed. In this connection it would be of increased 

 interest to know exactly how the stomata react to variations in leaf- 

 temperature and illumination. 



2d. The establishing of this high rate of open-air assimilation has the 

 effect of decreasing the estimate of the proportion of available energy which 

 is wasted by leaves of H. annuus on a bright sunny day. On such a day, so 

 long as they are able to keep their stomata widely open, these leaves utilise 

 about 30 per cent, of the photosynthetic radiation intercepted l)y them ; 

 so that the waste is about 70 per cent., and ])ossibly at times still less. 



2e. When leaves of H. annuus lose their turgidity, their rate of assimilation 

 is diminished. Some diminution occurs even when but a sliglit degree of 

 limpness is perceptible to the eye or to the touch ; when the leaves are very 

 flaccid, the horn hygrosco])e shows that their stomata are very little open, and 

 they increase in dry weight very little, if at all. 



2f. That I5rown and Escombe observed no higher rates of assimilation in 

 H. anmius than corresponded to an increase of about 5*5 milligrammes per 

 hour is owing prol)ably to several causes. In bright sunlight the tempera- 

 ture inside tlieir glass leaf-cases rose unnaturally, if not injuriously, high, and 

 the results of sucli experiments liave little bearing on the (piestiou of the 

 maximum rate of assiinilatioii ])()SHihle under natural conditions. In their 

 experiments in moderate illumination, althougli tlie supply of radiant eiiei-gy 

 was in excess of the amount used in assimilation, it is probable tiiat the 

 stomata were not fully open. 



The response of tlic stomata to diderciit intensities of light seems to offer 



