INFLUENCE OF THE DIFFERENT RAYS OF THE SPECTRUM 349 



assimilating carbon dioxide, although the absorption of energy still continues 

 (cf. Sects. 52, 53). These possibilities have been long neglected, and it is 

 evident that the amount of chlorophyll present docs not afford any direct 

 measure of the activity with which carbon dioxide may be assimilated, and 

 that no definite relationship need necessarily exist between absorption and 

 assimilation l . As a matter of fact, only a small percentage of the radiant 

 energy is converted into work, and each region of the spectrum contains 

 much more energy than is required for the most active assimilation of 

 carbonic acid (cf. En., Fig. 52). It is even possible that colourless proto- 

 plasts may be found to exist which are capable of active photosynthetic 

 assimilation. Ostwald has shown that iodide of silver affords an example 

 of a substance upon which the greatest photo-chemical effect is produced 

 by rays (at G) which have but feeble optical intensity. 



Hence it is doubtful whether a general approximate coincidence exists 

 between absorption and assimilation, as Engelmann found to be the case 

 in the plants examined by him, and indeed very marked aberrations were 

 frequently observed. Such observations are, however, of the utmost 

 importance, and if used with proper caution afford important material for 

 clearing up the mystery surrounding the process of photosynthesis. The 

 displacement of the assimilatory curve caused by the presence of phyco- 

 erythrin in Florideae is of the utmost interest (Ass. Red., Fig. 51), for 

 since it appears that a chromatophore tinged only with phycoerythrin 

 cannot assimilate, it follows that this pigment can only act indirectly as 

 a sensitizer, enabling the chlorophyll to make better use of certain rays of 

 light, a function which is the more remarkable since the chlorophyll and 

 phycoerythrin do not appear to enter into physical or chemical union with 

 one another 2 (Sects. 52, 53). 



Under certain cultural conditions no phycoerythrin is formed, and thus it 

 may be possible to determine experimentally its precise function ; and since 

 in the presence of such sensitizing pigments a small percentage of chlorophyll 

 may suffice for active photosynthesis, it is possible that a trace of chlorophyll 

 may be present in the purple bacteria, for the displacement of the maximal 

 point to the infra-red is not greater in their case than the similar displacement 

 towards the green which can be observed in the case of the red algae 3 . The 



1 This applies to Lommel's theoretical conclusion (Ann. d. Chem. u. Phys., 1871, Bd. CXLIV, 

 p. 581) that those rays must be most active which are most markedly absorbed, and which contain 

 the greatest amount of energy. Even before this it was attempted to establish a direct relation 

 between the absorption by chlorophyll and the assimilatory activity of the different rays. Thus 

 Dumas (Essai de statique chim. d'etres organises, 1824, p. 24) supposed that the blue and more 

 refrangible rays exercised the maximal assimilatory effect , for they also are very markedly absorbed 

 by chlorophyll. 



-' Cf. Abs. Red, Fig. 51, constructed from the absorption curves of chlorophyll and phycoerythrin 

 given by Reinke (Bot. Zeitung, 1886, Taf. n, Figs, n, 12). 



3 [As a matter of fact, Ewart has recently shown that apparently normal chlorophyll may be 

 extracted from purple bacteria Sect. ?2\] 



