64 Science of Plant Life 



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 formed in the chloroplasts to 

 take its place. 



Conditions affecting the development of the pigments. 

 Chlorophyll is produced only in the presence of light, but the 

 yellow and orange pigments develop in the dark as well as in 

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

 light to blanch the leaves of celery, the green color gives way 

 to yellow or orange. Likewise seedlings grown in the dark 

 and the inner leaves of head lettuce show a yellow but not a 

 green hue ; and when the light is cut off from a green leaf, 

 the green pigment disappears, leaving the yellow pigments 

 visible.^ These facts make it clear that the yellow pigments 

 do not require Hght to develop, while the green pigment does. 



There are. a number of conditions besides absence of light 

 that result in the partial or complete disappearance of the 

 green pigment, but these affect various plants quite differ- 

 ently. Low temperature, drought, and injuries and diseases 

 of various kinds may interfere with the nutrition of the leaf ; 

 even a slight decrease in light 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 ef- 

 fective at other seasons, they become generally operative in 

 late summer and autumn. Hence it is at this time of the year 

 1 A number of evergreens are exceptions to this rule. 



