NATURE OF PLANTS 31 



or the water beech (Carpinus) and the beech, noting how the 

 leaves of one fit when placed upon the branch of the other. This 

 subject of leaf form and arrangement may be summed up by the 

 statement that these features are devices for exposing the 

 maximum leaf surface without one leaf interfering with another. 

 This is in accord with the fact that light is practically of no value 

 in photosynthesis after passing through two leaves. 



Another interesting feature about the leaf is its relation to the 



i/B 



FIG. 18. FIG. 19. 



FIG. 1 8. Leaves of red oak: A, sunned leaf. B, shaded leaf. 

 FIG. 19. Compound leaves: A, red ash. B, horse chestnut. 



intensities of light. Some plants demand the full intensities of 

 sunlight while others can tolerate only a small fraction of it. 

 Some lichens will grow in a light only 1/156 part of the full in- 

 tensity while many plants, as the grasses, will endure the strongest 

 illumination. Beech, maple and spruce are tolerant of shade and 

 oak, hickory and chestnut are intolerant. This relation of plants 

 to light is an important factor in controlling their associations 

 and we frequently speak of plants as sun loving, partially so, 

 shade, and deep shade plants. 



The coloration of leaves is often of significance. In addition 

 to the green color, of which we now have some understanding, 



