544 



ECOLOGY 



various ivies, fig. 782), which commonly have a predominance of vertical 

 leaves, the prevailing incident light being not far from horizontal. 



Grasslike foliage. — The leaves of most grasses and sedges grow so 

 close together that the assumption of a position transverse to incident 

 light is mechanically impossible. In a meadow, not only the grasses, 

 but many other plants as well,. have leaves more nearly vertical than 

 horizontal (fig. 783), and in swamps the verticality of the foliage organs 



Fig. 782. — A " leaf mosaic" formed by leaves of the Japan ivj' (Psedera Iricuspidala), 

 which is climbing upon a vertical wall ; the leaves occupy most of the available space 

 and yet have a minimum of overlap; though diaphototropic, they face outward and lie 

 in a nearly vertical plane, because the dominant incident light is nearer horizontal than 

 vertical; note the gradations between three-lobed and ternately compound leaves. — 

 Photograph by Land. ' 



among the sedges, rushes, and flags is most striking. Even such horizon- 

 tal leaves as those of the water lilies have upturned edges, where the 

 growth is dense (fig. 777). Leaf verticality or parallelism to the incident 

 light results obviously in minimum lighting for any individual leaf, but 

 there is maximum lighting for the vegetation as a whole, since the more 

 vertical the leaves, the more numerous may they be in any given space 

 and yet have sufficient light to live. Thus the position that seems the 

 worst for the individual leaf appears to be the best (as well as mechani- 

 cally unavoidable) for the meadow or swamp vegetation as a whole, 



