148 THE BIOLOGY OF FLOWERING PLANTS 



similar to those of the closely related sunflower. The leaves 

 were allowed to assimilate in an atmosphere containing 

 2 to 3 per cent, carbon dioxide, an amount which never 

 limited assimilation, and their assimilation under the 

 natural changes of illumination and temperature was 

 determined. From former work the maximum assimilation 

 at diflferent temperatures was known — that is the assimilation 

 with excess of light and carbon dioxide, and with temperature 

 limiting. Thus for the cherry laurel the maximum assimi- 

 lation at 20° and 25° C. is 0*0085 ^^^ 0'0ii5 grm. of carbon 

 dioxide per 50 sq, cm. leaf surface per hour. 



By comparing the rate of assimilation actually observed 

 in natural conditions with these figures it could be deter- 

 mined whether, in any given case, the light or the tempera- 

 ture was limiting the rate of assimilation ; for if the observed 

 figures fall below the maximum value for a given tempera- 

 ture, obviously the light is limiting, while if they are approxi- 

 mately the same, then the temperature is limiting. Some 

 of Blackman's results are given in Table XXIX. 



TABLE XXIX 



Assimilation by Cherry Laurel in Natural Illumination 



The second and third readings approach the maximum, 

 and in these temperature may be limiting. The light was 

 not full sun, but in the middle of the day it is at its strongest 

 and, even when clouded occasionally, apparently allows of 



