Leaves in Relation to Light 



6s 



PF. 5. Cooper 



Fig. 35. Leaf mosaics formed by maple leaves {Acer macro phy II urn and Acer circinatum), 

 Olympic Mountains, Washington. The light affects the growth of the petioles and branches 

 in such a manner that the leaves are fitted together side by side. 



and tulip trees the leaves are less sensitive to light, and the re- 

 sult is that their leaves assume a great variety of positions. If 

 leaves are extremely sensitive to hght, the blades may turn 

 toward the sun in the early morning and follow it throughout 

 the day, always keeping the broad face of the leaf to the light. 

 The leaves of the common mallow move in this way. 



Leaf mosaics. The leaves of many plants, hke the Boston ivy, 

 sugar maple, and beech, are so arranged that if we look at them 

 from the direction from which they receive the most hght they 

 seem to be fitted together like the stones in a mosaic. In this 

 way each leaf is exposed to the most possible hght. 



Many small herbs, like the dandelion, moth mullein, common 

 plantain, and evening primrose, form rosettes of leaves near the 

 soil. An examination of these rosettes will show that each leaf 

 is arranged so that it fihs a space in the circle. Further exam- 

 ination will show that the leaves that would otherwise be more 

 or less shaded have changed their positions and occupy the 

 spaces between the leaves above them. 



