[Chap. IV SEASONAL ASPECTS OF PLANTS 29 



Many other examples of leaf pigmentation may be found in every 

 community. 



Pigments not limited to leaves. Of course these pigments are not 

 limited to the leaves of plants. The presence of chlorophyll in most 

 herbaceous stems and the young stems and twigs of woody species is 

 familiar to all. Likewise, certain parts of flowers are usually green, and 

 the term "green fruits" has come to be a synonym of young fruits in 

 common speech. Many varieties of fruits remain green when ripe, while 

 in others the chlorophyll disintegrates and the associated yellow pig- 

 ments become more evident. In still others anthocyanins partially or 

 wholly mask the yellow. Some seeds, such as those of certain varieties 

 of peas and beans, are green. Chlorophyll is formed in the aerial roots 

 of many orchids and in the roots of many other plants when they are 

 exposed to light. 



The carotinoids seem always to be present wherever chlorophyll 

 occurs. They may also occur in the absence of chlorophyll. Carotinoids 

 are the underlying cause of the yellow and orange colors of the flowers 

 of zinnias, sunflowers, and goldenrods; of the fruits of oranges, lemons, 

 and tomatoes; and of the seeds of corn, peas, and clover. 



The anthocyanins are most conspicuous in red, purple, and blue 

 flowers and fruits; but purple cabbage, potatoes, popcorn, and beets 

 exemplify their common occurrence in other plant parts. Among the 

 fungi (molds, lichens, mushrooms) other pigments may be as brilliant 

 and varied as are those in our common green plants. 



Pigment formation dependent upon both heredity and environment. 

 Among the common plants one may find some species that lack antho- 

 cyanins, while otliers are without chlorophyll and carotinoids. The 

 absence of a pigment from a plant, or a part of a plant, may be due to 

 its heredity, its stage of development, or some condition in the environ- 

 ment of the plant. Chlorophyll is not formed in the cells of toadstools 

 or in "Indian pipe" ( Fig. 19 ) in any environment. Neither is it formed 

 in some parts of variegated leaves under any circumstances. The other 

 pigments may also be absent from certain plants regardless of the con- 

 ditions under which they develop. The absence of pigments in these 

 plants must be due to the hereditary constitution of the plant. 



Environment and chlorophyll. Any condition that is necessary for the 

 maintenance of the plant is indirectly essential to the formation of plant 

 pigments. There are, however, certain conditions that are more directly 

 related to it. In most plants light seems to be necessary for the making 



