LEAVES 



541 



hour by hour and day by day results in great diversity of light direction; 

 only in tropical regions are such organs strictly transverse to the dominant 

 incident light. The fact that they are about equally horizontal at high 

 and at low latitudes shows that some factor other than light determines 

 their position; in- 

 deed, rosette leaves 

 are more nearly hori- 

 zontal in winter than 

 in summer in spite of 

 the slanting rays in 

 the former season. 

 However, liverworts 

 like Marchantia or 

 Fegatella clearly are 

 diaphototropic, and 

 can be induced to 

 develop even vertical 

 thalli if the incident 

 light is horizontal. 



Leaves on erect 

 stems and their 

 branches. -- Leaf 

 orientation. - - Most 

 leaves are not hori- 

 zontal or even trans- 

 verse to the prevail- 

 ing incident light of 

 the region where they 

 grow. In trees and 

 in treelike herbs and 

 shrubs the leavescom- 

 monly face outwards 

 and upwards on all 

 sides (fig. 778), each 



leaf being transverse to those rays that are dominant for that par- 

 ticular leaf; leaves on the north side of a tree face north, where the 

 sun is in the south, because more light is available from the former 

 direction. In a plant by a window or at the edge of a forest, all of the 

 leaves may face in one direction, because the direct light that penetrates 



FIG. 778. A cineraria plant (Senecio cruentus), illus- 

 trating a conical habit that is due to a decrease in the 

 length and to a change in the orientation of the petioles from 

 the base to the apex, the directional variations resulting in 

 the assumption by each blade of a position such that it 

 receives the maximum available light. Photograph by 

 FULLER. 



