FOLIAGE LEAVES. 385 



749. Adaptation in leaf arrangement. As a result, however, of one 

 arrangement or another we see a beautiful adaptation of the plant parts 

 to environment, or the influence which environment, especially light, has 

 had on the arrangement of the leaves and branches of the plant. Access 

 to light and air are of the greatest importance to green plants, and one 

 cannot fail to be profoundly impressed with the workings of the natural 

 laws in obedience to which the great variety of plants have worked out 

 this adaptation in manifold ways. 



750. Distribution of leaves with reference to the entire plant. In this 

 case, as in the former, we recognize that it is primarily a light relation. 

 As the plant becomes larger and more branched the lower and inner leaves 

 disappear. The trees and shrubs have by far the larger number of leaves 

 on the periphery of the branch system. A comparison of different kinds 

 of trees in this respect shows, however, that there is great variation. Trees 

 with dense foliage (elm, Norway maple, etc.) present numerous leaves 

 on the periphery which admit but little light to the interior where leaves 

 are very few or wanting. The sugar maple and red maple do not cast 

 such a dense shade and there are more leaves in the interior. This is 

 more marked in the silver maple, and still more so in the locust (Gledit- 

 schia tricanthos). 



751. Color of foliage leaves. The great majority of foliage leaves are 

 green in color. This we have learned (Chapter VII) is due to the presence 

 of a green pigment, chlorophyll, in the chloroplastids thickly scattered in 

 the cells of the leaf. We have also learned that in the great majority of 

 cases, the light stimulus is necessary for the production of chlorophyll 

 green. There are many foliage leaves which possess other colors, as red 

 (Rosa rubrifolia), purple (the purple barberry, hazel, beech, birch, etc.), 

 yellow (the golden oak, elder, etc.); while many others have more or less 

 deep tints of pink, red, purple, yellow, when young. All of these leaves, 

 however, possess chlorophyll in addition to red, yellow, purple or other 

 pigment. These other pigments are sometimes developed in great quan- 

 tity in the cell-sap. They obscure the chlorophyll from view, but do not 

 interfere seriously with the action of light and the function of chlorophyll, 

 and perhaps in some cases serve as a screen to protect the protoplast. 



752. Autumn colors. Foliage leaves of many trees display in the autumn 

 gorgeous colors. These colors are principally shades of red or yellow, 

 and sometimes purple. The autumn color is more marked in some trees 

 than in others. In the red maple, the red and scarlet oak, the sourwood, 

 etc., red predominates, though sometimes yellow may be present with 

 the red in a single leaf. Sugar maples, poplars, hickories, etc., are prin- 

 cipally yellow in autumn. The sweet gum has a rich variety of color-red, 

 purple, maroon, yellow; sometimes all these colors are present on the same 

 tree 



