ADJUSTMENT TO LIGHT 



79 



98. The production of chlorophyll. The primary response of 

 the plant to light is the production of chlorophyll. This response 

 does not occur in plants, such as the bacteria, mushrooms, broom- 

 rape, etc., in which the power to make chlorophyll has been lost 

 in consequence of parasitic or saprophytic habits. On the other 

 hand, a few plants, conifers, ferns, and cacti, are able to make 

 chlorophyll in darkness, though it probably cannot be formed 

 continuously under such conditions. The rule is that plants with 

 plastids produce chlorophyll only in the light. Conversely, their 

 chlorophyll disappears in darkness. The light intensity necessary 

 for the production of chlorophyll varies in different species. This 

 pigment is formed in shaded habitats with light values as low as 

 .001, even though very few flowering plants can function at all 

 under such conditions. For the majority of plants, sunshine 

 presents the best conditions for the formation of chlorophyll. If 

 the sunlight becomes too concentrated, it tends to decompose the 

 chlorophyll more rapidly than it can be built up. Although this 

 rarely occurs in nature, chlorophyll undergoes constant decom- 

 position by light, and it persists only because it is built up at the 

 same time. 



99. The nature of chlorophyll. Chlorophyll is a complex 

 pigment, or a mixture of several pigments. It is produced by 



Fig. 22. Absorption spectrum of chlorophyll removed from the leaf by 

 solution in alcohol. The letters B-G indicate the position of Fraun- 

 hofer's lines. The groups of dark lines from the red to the violet end 

 indicate the seven absorption bands of chlorophyll, i.e., those parts 

 of the spectrum in which the light is absorbed by the chlorophyll. 

 (After Sachs.) 



specialized bits of protoplasm, the plastids. When the necessary 

 conditions are met, i.e., the presence of light, water, carbon dioxide, 

 and a trace of iron salts, the plastids of active tissues are always 

 green, i.e., they are chloroplasts. The latter lose their color in 

 the dark, or, ordinarily, when carbon dioxide is absent. The 

 pigment is broken down in ripening fruits and in flowers, and also 



