Lecture VI. 45 



Enough has been said to show you that the stages by which 

 carbohydrates are built up from carbon dioxide and water by 

 plants are uncertain. The process is usually called carbon as- 

 similation, or, if it is desired to emphasise the fact that the 

 synthesis is dependent upon light, it is spoken of as photo- 

 synthesis. 



An essential and probably the most important agent in photo- 

 synthesis is the chloroplast. As we have seen, this chloroplast is 

 a mass of protoplasm with which is associated a green pigment. 

 This pigment is called chlorophyll. It is insoluble in water but 

 it is readily soluble in many organic solvents, e.g. alcohol, acetone, 

 chloroform, etc. A solution of it has very striking properties. 

 It has a magnificent grass green colour with a red fluorescence. 

 When viewed in thin layers by transmitted light it has a brilliant 

 green colour. If a glass vessel containing some of the solution 

 is placed in front of a black background and viewed with reflected 

 light it appears blood-red. A thick layer of a concentrated solu- 

 tion viewed by a strong transmitted beam shows no green, but 

 has the deep red colour of port wine. 



Dilute solutions examined spectroscopically show that the pig- 

 ment has broad absorption bands in the blue and violet, and four 

 less extensive ones in the red, orange, yellow and green. Absorp- 

 tion in the red is the most complete while in the green it is least 

 marked. The band in the red is very sharply defined on both 

 sides and the solution allows the deepest red rays as well as much 

 of the orange red to pass untouched. The other bands have 

 indistinct margins which extend as the thickness of the solution is 

 increased. This peculiar absorption will supply you with the 

 explanation of the striking fact that a thin layer 'of chlorophyll 

 solution in transmitted light looks green, while a thick layer of 

 the same solution under similar conditions looks red. In thin 

 layers the absorption stops most of the orange, yellow, blue and 

 violet, while most of the green and the deep red are transmitted. 

 The latter is so feebly luminous that the solution appears pure 

 green. When, however, a thick layer is used the feeble absorp- 

 tion in the green becomes effective and practically all the visible 

 spectrum is obliterated save the deep red. This latter now be- 

 comes visible when the more luminous part of the spectrum is 

 cut off. 



The fluorescence is caused by the absorption in the blue and 

 violet. The light absorbed by these bands reappears as red light 

 which escapes from the solution. With this fluorescence, how- 

 ever, we need not concern ourselves, since it is only shown by 



