THE STEM AND THE LEAF 



57 



54. Leaf positions on horizontal stems, and overshadowing. 



If a rapidly growing plant, as a sunflower, is bent over so as to 

 lie flat on the ground, its younger leaves soon readjust them- 

 selves to the new position. Horizontal branches of trees and 

 shrubs are very different from vertical shoots as regards the 

 position of the leaf blades with reference to the stem. Though 

 the opposite or the spiral arrangement of the leaf origins is the 

 same, a twisting of the stem, or a lengthening, or twisting, or 



FIG. 42. Rosettes of evening primrose 



Two species are shown : CEnothera rhombifolia on the left, and (E. biennis, a very 

 widely distributed species, on the right. Photograph by W. J. G. Land 



other change of position of the leafstalks usually occurs. This 

 comes about in such a manner as to put the leaves in a favor- 

 able position to receive the sunlight (Fig. 40). Prostrate stems, 

 like those of pumpkins, squashes, cucumbers, poison ivy, 

 English ivy, and a host of others, when lying on the ground, 

 arrange their leaves much as do horizontal branches of trees. 

 Trees that have fallen in such a way as to leave the roots in the 

 soil may have one or more branches assume the form of a tree. 



