6oo 



ECOLOGY 



and shade leaves rather than of xerophytic and mesophytic leaves. While there is no 

 evidence that light is a direct factor, it is of undoubted indirect importance through 

 its influence both upon carbohydrate synthesis and upon transpiration. Increased 

 synthesis implies increased available food for leaf construction, and hence, probably, 

 results in increased size, while increased transpiration, as has been seen, results in 

 decreased size. The simplest situ- 

 ation is in the water, where syn- 

 thesis is unhindered by transpira- 

 tion, and here (as has been noted 

 for Sagittaria and Castalid) the 

 smallest leaves develop where the 

 light intensity is least. In air 

 leaves synthesis, and therefore the 

 food available for leaf construc- 

 tion, increases as the light in- 

 creases to a certain optimum, be- 

 yond which there is a decrease. 

 Thus, intense direct light in con- 

 trast with diffuse light opposes the 

 development of expanded leaves, 

 partly, perhaps, because of less 



871 



872 



FIGS. 871, 872. 871, a young mesophytic individual of the harebell (Campanula 

 rotundifolia\ showing leaf variation ; note the broad basal leaves and the narrower stem 

 leaves; 872, the upper portion of a xerophytic individual of the harebell (Campanula 

 rotundifolm), showing the characteristic linear stem leaves. 



available food, but much more, probably, because of increased transpiration. 

 Maximum leaf size appears to be found in the moist tropics, where transpi- 

 ration is low, and yet where there is sufficient light for the optimum of synthesis. 

 It is believed usually that a moderate increase of carbon dioxid favors an in- 

 crease in leaf size; indeed, the luxuriance of Carboniferous vegetation often has 

 been ascribed to a supposed abundance of this gas. However, a large increase 

 in the percentage of carbon dioxid has been shown to result in decreased leaf 



