[Chap. IV SEASONAL ASPECTS OF PLANTS 27 



sider some of the most apparent changes that occur in plant processes 

 and plant organs. 



The autumnal aspect. The summer with its long days, high tempera- 

 tures, and intense light is waning. The eflFects of cooler weather and of 

 the shorter period of daylight upon certain chemical processes in plants 

 soon become evident in the leaves of our deciduous trees and shrubs. 

 The green color of chlorophyll gradually disappears, and the yellow pig- 

 ments with which the green pigment had been associated in the cells of 

 the leaves now become conspicuous. In many leaves there is an addi- 

 tional formation of other pigments which range in color from red to 

 purple and which are called collectively the anthocyanins. As the season 

 advances both the yellow pigments and the anthocyanins break down; 

 and brown substances, especially tannins, increase and modify the leaf 

 color. When the autumn is characterized by bright sunshine and moder- 

 ately cool weather coloration is at its best. When frost occurs early or 

 when the weather is wet and cloudy, the anthocyanins are formed to 

 only a slight extent and the yellow and brown pigments are dominant 

 in the landscape (Plate 2). 



These autumn color changes in the leaves of many deciduous trees 

 contrast sharply with the persistent green color of both the needle-leaved 

 and broad-leaved evergreens. Many herbs likewise remain green through 

 the winter months, at least in parts near the soil. Furthermore, the leaves 

 of some trees and shrubs remain green until after they have fallen to 

 the ground. 



Colors and pigments. The color we ascribe to an object or substance 

 depends upon the kinds of light rays that pass from it to the retinas of 

 our eyes. When a beam of "white" light passes through a clear glass 

 prism the rays of different wave length are separated by refraction 

 and appear as bands of color. Similarly we see a rainbow when the 

 different rays of ordinary daylight are reflected to our eyes in separate 

 bands by drops of water in the atmosphere. On the one side of the bow 

 we perceive the longer waves of light as red, on the other side the short 

 waves as violet. Between these extremes are bands of orange, yellow, 

 green, blue, and indigo, each produced by rays of successively shorter 

 wave lengths that reach our eye. 



The colors of objects are also partly dependent upon the relative sensi- 

 tiveness of our eyes to the different rays of light. They are especially 

 sensitive to the rays we perceive as green and yellow. 



