ILLUMINATION 27 



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 

 grams is possible, it follows that in this instance the illumina- 

 tion was the limiting factor. 



An important generalization arrived at by F. F. Blackmail 

 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 instances 

 as Helianthus, Prunus, Bomarea, Aponogeton, Elod:a 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 

 twice as much carbon dioxide as can Prunus, but in so doing 

 it requires twice the amount of illumination. The difference 

 in the two leaves lies in their having different coefficients of 

 acceleration of their assimilatory activity with increased tem- 

 perature. 



With regard to the use made by the plant of specific parts 

 of the spectrum, it has hitherto been generally accepted that 

 those wave lengths associated with the prominent bands in the 

 red of the absorption spectrum of chlorophyll were the only 

 _^ ones concerned with carbon assimilation. The important 

 investigations of Ursprung f show that radiant energy of any 

 wave length to, and including the greater part of, the ultra- 

 violet is capable of inducing starch formation in green leaves. 



* The term photosynthetic is used in its narrow sense to refer only to those 

 radiations specific to this process. 



t Ursprung: " Ber. deut. hot. Gesells.," 1917, 35, 44; 1918, 36, 73. 86, in, 



122. 



