38 

 . Leaf form inoluoLing shape and cross section due raainly to 

 light and need for protection but also to a necesf ity for plasticity 

 as in compound leaves, the "best examples of which are found in the 

 water. Si^e in contrast to form seems to be largely a matter 

 of food supply. Sometimes form may also be modified by food-supply. 

 In determining direction and araangement the light relations come in. 



Leaf color . 



Certain rays are absorbed and certain colors reflected. 

 Green commonest color in plant as to leaves ; due to chlor. which 

 works only in sunlight, though plastids are present. Decolorization 

 of green leaves due to "blanching in the darkness ; to disease ; 

 Exper. show iron is necessary to presence of chlor. Chlorosis a 

 and Etiolation or bleaching, both diseases. Partial covering of 

 plants as by sand dunes "produces decoloration ahove. In bixllrush 

 ^ones of color were shown. 



In spring, leaf grows so rapidly that development of chlor. 



is not apt to keep up with the growth of the leaf, hence the 



brilliant color of fresh spring foliage. Wliite and yellow colors 



come from disease ; the color being due to the degeneracy of the 



plants. White and yellow are purely pathiological conditions. 



Explanation of green color . 



One theory is tliat the green color has no ecological meaning,. 



the other that it has. It may have "been evolved by the evolution 



of nature. More rays absorbed at the ends of spectn^m than in 

 the middle. At the red end ther'e is the greatest amount of proto- 



