Substances Made from Foods 53 



In the chloroplasts all three are present at the same time, so 

 that we cannot distinguish them under the microscope. As the 

 three are soluble in alcohol, the presence of the yellow and orange 

 pigment does not become apparent when the coloring matter is 

 extracted from leaves by means of alcohol. The chlorophyll 

 within a leaf is constantly breaking down, and new chlorophyll 

 is being formed constantly in the chloroplast exposed to light. 

 Since it is the chlorophyll in the chloroplast that effects the union 

 of carbon dioxide and water in photosynthesis, it is scarcely an 

 exaggeration to say that chlorophyll is the most important pig- 

 ment in the world. 



Conditions affecting the development of chlorophyll. Chlo- 

 rophyll is produced only in the presence of light, but the yellow 

 and orange pigments are developed in the dark as well as in the 

 light. When we lay a board on grass or shut out the hght to 

 blanch the leaves of celery, the green color disappears, exposing 

 the yellow or orange. Likewise seedhngs grown in the dark and 

 the inner leaves of head lettuce show a yellow but not a green hue. 

 These facts make it clear that the yellow pigments do not require 

 light to develop, while the green pigment does. 



There are a number of conditions besides absence of hght that 

 result in the partial, or complete, disappearance of the green pig- 

 ment, but these affect various plants quite differently. Low 

 temperature, drought, injuries, and diseases of various kinds may 

 interfere with the nutrition of the leaf ; even a sHght decrease 

 in hght may do so. All these factors tend to affect the green 

 pigment more than the yellow and orange. Although these 

 same influences — low temperature, drought, reduced light, 

 injuries, and diseases — may be effective at other seasons, they 

 become generally operative in late summer and autumn. Hence 

 it is at this time of the year that the green pigment disappears 

 from the leaves of most deciduous plants and unmasks the yellow 

 pigments in the chloroplasts. There is every gradation in the 

 readiness with which the green pigment disappears from the 



