Leaves in Relation to Light 



41 



only in the morning and late afternoon that the flat sides of 

 the leaves are perpendicular to the sun's rays. This response 

 to the Ught also places most of the leaves in a vertical north- 

 and-south plane and suggests the name " compass plant." 

 When grown in partial shade, the leaves of these same plants 

 are horizontal. Hence it is clear that the position of their 

 leaves in sunny situations is the result of hght conditions. 



How the blade attains its position with reference to the light. 

 The attainment of its position by the leaf blade is partly ac- 

 comphshed, as has been noted, by the bending and twisting 

 of the plant stem during its development. To a much greater 

 extent the blade owes its position to the bending, twisting, and 

 elongating of the petiole. In- 

 deed, this is the particular ad- 

 vantage of the petiole. Its 

 length and direction of growth 

 are for the most part deter- 

 mined by the way in which the 

 Hght falls on the blade during 

 the period of development. An 

 examination of a branch of a 

 maple will disclose how the 

 lengthening and bending of the 

 petioles help to fit each leaf 

 into a position where it will 

 receive the hght. 



Vertical leaves. In a number 

 of common plants, including 



the iris, cat-tail, calamus, and ^^ __^ _ ,_. 



many grasses, the leaves are ^^^ ^g j^^_ ^^^^^ i^^^ ^^^ j^ 



vertical because they are held vertical position by the sheathing bases. 



