32 



THE SYNTHESIS OF CARBOHYDRATES 



constant temperature and varying illumination ; this shows 

 that temperature was the limiting factor. The low value in 

 the first observation was due to the low light intensity. In 

 observation 5 in which the illumination was about the same as 

 in 4, the temperature was raised from 177 to 30-5° C, with 

 the result that the assimilation was about doubled : but since 

 at this higher temperature an assimilation of at least -0289 

 gram is possible, it follows that here the illumination was the 

 limiting factor. 



An important generalization arrived at by F. F. Blackman 

 and his collaborators is that equal intensities of light incident 

 on equal areas of leaf produce the same amount of assimilation, 



provided that light is the limiting factor and that the tempera- 

 ture does not involve the so-called time factor ; agreement 

 within 5 per cent, was found to obtain in such diverse in- 

 stances as Helianthus, Primus, Bomarea, Aponogeton, Elodea 

 and Fontinalis. Hence the conclusion is reached that " leaves 

 in general have the same coefficient of economy in the photo- 

 synthetic * economy." 



The amount of light required by a leaf is a specific value 

 for a given temperature ; thus in the examples studied by 

 Blackman and Matthaei, Helianthus and Prunus have at low 

 temperatures similar assimilatory maxima which diverge at 

 higher temperatures. At 29-5° C. Helianthus can assimilate 



* The word " photosynthetic " is here used in its literal sense, that 

 phase of carbon assimilation in which a chemical change is brought about 

 by light alone. This phase is followed by a purely chemical phase, known 

 as the dark phase, which is independent of light but is accelerated by an 

 increase in temperature. 



