PHOTOSYNTHETIC PIGMENTS. 267 



up the extract with benzol (Kraus's method) ; the lighter benzol solution 

 floats as a deep-green stratum above the yellow alcoholic layer. Accord- 

 ing to Schunck and Marclilewski, chlorophyll proper is frequently accom- 

 panied by a second green substance, which they call allorchloropJiyll ; 

 the yellow pigments are known as xanthopliylls. 



The spectrum of the crude extract is characterised by six absorption- 

 bands, of which four, belonging to chlorophyll proper, are located in 

 the " red " half of the spectrum, while the other two, situated in the 

 more highly refrangible or " blue " region, pertain to the ocanthophylls. 

 Very soon after the discovery of these absorption-bands attempts were 

 made to bring them into relation with the photosynthetic dissociation of 

 carbon-dioxide, which, as had long been known, only takes place under 

 the influence of light. 139 Lommel, Jamin and Becquerel have all 

 assumed, on theoretical grounds, that it is precisely those radiations 

 which are absorbed by a solution of chlorophyll as shown by its spec- 

 trum, that are also responsible for the photosynthetic decomposition 

 of carbon-dioxide. Timiriazeff and Engelmann have tried to obtain 

 experimental confirmation of this conclusion. As a matter of fact, 

 the graphs corresponding respectively to the intensities of [optical] 

 absorption and to the rates of photosynthesis in different parts of the 

 spectrum do on the whole follow parallel courses ; this statement 

 applies more particularly to the " red " half of the spectrum, for which 

 the photosynthetic maximum falls between the Frauenhofer lines 15 

 and C, or, in other words, just in the position of the most intense 

 absorption-band (No. 1) of chlorophyll-extract. In the other half 

 of the spectrum the curve of absorption rises steadily, whereas the 

 curve of photosynthesis reaches a secondary maximum near the Frauen- 

 hofer line F, falling off again beyond this point. The discrepancy 

 is probably due to the fact that the total energy of solar radiation 

 diminishes somewhat rapidly in this region of the spectrum. 



These views in favour of a close correspondence between [optical] 

 absorption and photosynthetic activity have received strong confirma- 

 tion from the discovery of the phenomenon of complementary chromatic 

 adaptation by Engelmann and Gaidukov. Engelmann found, in the 

 case of seaweeds growing at different depths, that the rays of light 

 which are most effective in respect of photosynthesis are invariably 

 those that are complementary to the coloration of the respective plants, 

 the reason being that it is, of course, precisely these complementary 

 rays that are absorbed by the pigments concerned. Even at a com- 

 paratively slight depth below the surface of the ocean green and 

 blue-green light predominates ; in these circumstances the Flokideae 

 derive an undoubted advantage from their red coloration. It has since 

 been shown by Gaidukov that cultures of certain species of Oscillatoria 



