598 ECOLOGY 



those forms with dissected water leaves, features that have been sug- 

 gested as advantageous are the filtration of light rays, an easy yielding 

 to currents, and a relative increase of absorptive area. If such water 

 leaves as those of Sagittaria are due to poor nutrition, their form 

 scarcely would be imagined to have advantageous significance. As to 

 air leaves, so far as they are expanded (as in Sagittaria), they favor 

 increased synthesis, while so far as they are small and compact (as in 

 Proserpinaca) , they are suited for low transpiration. The dying of air 

 leaves in water or of water leaves in air is due, more probably, to lack of 

 fitness in leaf structure than in leaf form. 



Form variations in land plants. — Variation in leaf size and propor- 

 tion. — The most universal and best understood of form variations are 

 not those involving notable changes in shape, but those in which the 

 changes are chiefly of size and proportion. As previously shown, 

 xerophytic or sun leaves differ from mesoph3rtic or shade leaves in that 

 they are considerably smaller and thicker. Not only do these differences 

 characterize xerophytic species on the one hand and mesophytic species 

 on the other, but there are also many plastic species (as Tilia americana, 

 Rhus Toxicodendron) in which such differences are induced readily by 

 growing the plants in different environments. When Tropaeolum is 

 grown in dry air and dry soil, the leaves are much smaller and thicker 

 than in moist air and moist soil, those in the latter not infrequently 

 being five times as large as those in the former. Smaller leaves develop 



FlGS. 867, 868. — Diagrammatic cross sections of harebell leaves {Campanula ro- 

 lundifolia): 867, a mesophytic leaf from a moist shaded habitat; 868, a xerophytic leaf 

 from a dry sunny habitat; note that the xerophytic leaf is much the narrower and 

 thicker; v, veins; considerably magnified. 



when the soil is dry and the air moist than when the soil is moist and 

 the air dry, appearing to indicate that diminished absorption outweighs 

 increased transpiration as a size-reducing factor. !Many species behave 

 in a similar though less striking manner (figs. 867, 868). Even such 

 succulent plants as Sedum and Sempervivum may develop thin, ex- 

 panded leaves in moist air. 



