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, e.specially in dicotyls, to the power of elongation and curvature 



Fig. 779. — A horizontal branch of the Norway maple (Acer plalanoides), 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 phyUotaxy 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 (6, 6') develops on the upper 

 side and has a short petiole; each member of the intervening pairs (c, (/, (f, 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 Tropaeoliim 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 



