542 



ECOLOGY 



the foliage is greater than the diffuse light on the darker side (fig. 

 776 B). 



Petioles. During development leaves assume a position transverse 

 to the local incident light, and this position is kept through life. The 

 assumption of a favorable position where foliage is dense usually is 

 due, especially in dicotyls, to the power of elongation and curvature 



FIG. 779. A horizontal branch of the Norway maple (Acer platanoides), illustrating 

 differential petiole elongation ; the palmately veined leaves are arranged in one plane facing 

 the light, each leaf being well placed for light reception, even though the phyllotaxy is 

 decussate ; every fourth leaf (a, a') issues from the under side of the stem and develops a 

 long petiole, while the other member of each of these pairs (b, b') develops on the upper 

 side and has a short petiole; each member of the intervening pairs (c, c', d, d') issues 

 from the side of the stem and has a petiole of intermediate length; note that the leaves 

 become progressively smaller and the petioles progressively shorter toward the stem tip. 

 From KEENER. 



possessed by the growing petioles. On vertical maple branches 

 the petioles develop equally, but on horizontal branches every fourth 

 leaf originates on the under side and develops a long petiole, while the 

 opposite leaf, originating above, has a short petiole (fig. 779). The 

 leaf of Tropaeolum shows even greater plasticity, having a petiole at- 

 tached to the center of the blade and capable of almost unlimited elonga- 

 tion and degree of curvature. In some rosette plants (fig. 1036) and 

 in plants with simple erect stems, the petioles are progressively shorter 



