CHLOROPHYLL AND ENERGY 201 



different plants and that, even for sun plants, it often has an 

 inhibitory effect. See Fig. II, 4. 



As might be deduced from the absorption spectrum of 

 chlorophyll, it is not only the quantity of Hght which is 

 important, but also its quality or wave-length. Experiments 

 have shown that the maximum of efficacy for photosynthesis 

 is found in the red band. Although the secondary maximum 

 in the blue has been challenged, Engelmann demonstrated the 

 existence of the two maxima by a spectacular experiment using 



SOOmjx 700 600 500 400 



Fig. II, 4. Proportion of the absorption, in the different 



regions of the spectrum, of chlorophyll-a ( ) and 



chlorophyll-Z? (- ), according to Zscheile 



motile bacteria with green algae which liberate oxygen in 

 proportion to their photosynthetic activity. He illuminated a 

 filament of the algae by a spectrum in the presence of bacteria 

 which were very avid of oxygen. The bacteria grouped them- 

 selves along the filament, principally round the spot illuminated 

 by the red Hght. In the region of the blue the grouping was 

 smaller but it still exceeded the average density on the rest 

 of the algae. See Fig. II, 5. 



The Influence of Carbon Dioxide 



One cubic metre of air contains 589 mg. of carbon dioxide, 

 which, if it were all transformed into glucosides, could give 



