SPECTRUM OF CHLOROPHYLL 113 



Photosynthesis. The spectrum of chlorophyll shows 

 definite absorption bands. 1 These absorption bands 

 represent the rays most strongly absorbed by the 

 chlorophyll, and thus the radiant energy available 

 for the work which the chloroplast carries on. That 

 these are the rays actually used in photosynthesis, 

 or at least in one or more of the processes that go to 

 make up photosynthesis, can be shown by a very pretty 

 experiment first devised by the German botanist 

 Engelmann. A thread of green alga living and carry- 

 ing on its work in water is illuminated by a solar spec- 

 trum so that different parts of the thread are illuminated 

 by different coloured rays. A culture of a certain 



Red Orange TelLow Green Blue Violet 



FIG. 9. Diagram illustrating Engelmann's experiment, to show by 

 the aggregation of a motile bacterium (b) highly sensitive to oxygen 

 the regions of maximum evolution of oxygen by a green alga (a a) 

 illuminated by a solar spectrum. The most active rays correspond 

 with the red and orange. 



bacterium (Bacterium termo) which is very sensitive 

 to oxygen, so that its cells move towards any source 



1 When a beam of white light is split up by passing through a prism 

 into a band or spectrum of differently coloured rays, owing to the 

 different angles through which the different rays are bent by the 

 prism, and is allowed to fall on a white surface, the spectrum so formed 

 (solar spectrum) is continuous. But if the beam has passed through 

 a translucent coloured substance, the spectrum formed shows dark 

 bands (absorption bands) corresponding inversely with the colour of 

 the translucent substance, the colour of the substance being caused 

 by the combined effect on the eye of the coloured rays that have, 

 passed through it. The absorption bands are caused by the absence' 

 of the rays that have been stopped or absorbed by the coloured 

 substance. Thus a pure red substance shows absorption in the other 

 colours of the spectrum blue, green, etc., a blue substance in the 

 red, orange, etc., and so on. 



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