ASSIMILATION IN NATURE 149 



maximum assimilation at a temperature of 23° C. At 

 2 p.m. the bright sun again permits a high rate of assimi- 

 lation, but the temperature is also high, and the possible 

 maximum is not reached. Towards the evening, even 

 although a good deal of sun is available, the natural decline 

 in light intensity never allows the possible maximum to be 

 reached, and light is always limiting. In this experiment 

 the leaf faced south throughout the day, and the change in 

 the incidence of the sun's rays materially affected the rate 

 of assimilation. In a companion experiment, under very 

 similar weather conditions, the leaf was continuously moved 

 so as to lie at right angles to the sun's rays, and here it was 

 found that the maximum was reached, except under very 

 heavy clouds, and that the temperature, ranging from 20° to 

 25° C, was limiting from 9 a.m. to mid-day. This shows 

 how the position of the leaf affects the play of the external 

 factors. It was also showrr that in the sun at noon at the 

 summer soltice the maximum assimilation at a temperature 

 at 29*5° C. is obtained with 0*36 of the available Hght for 

 the cherry laurel, and with 0'69 for Helianthus, the 

 assimilation of the latter being about twice as vigorous. 



These experiments were under natural conditions, 

 except as regards carbon dioxide supply, which was super- 

 normal. Thoday's work (19 10) supplements them in this 

 respect. His values are calculated from the increase in dry 

 weight of the leaves of the sunflower during assimilation 

 under completely natural conditions, with the exception 

 that he used cut leaves ; this enabled him to control the 

 water supply and maintain the leaves in a condition of 

 turgor, by supplying water under pressure. The sunflower 

 wilts very readily when the leaves are attached to the plant. 



Thoday reached the important conclusion that in bright 

 sun the maximum assimilation by sunflower leaves is 17 

 mg. increase in dry weight per 100 sq. cm. per hour ; 

 this is exactly the value obtained by Sachs, so that it may 

 be taken as accurate. It is equivalent to an assimila- 

 tion of 27*5 mg. of carbon dioxide. He points out that, 

 for the artichoke, Blackman found the maximum rate of 



