LEAVES 



607 



It would seem that the chief determining factors of leaf size and pro- 

 portion are those that control the water supply. High transpiration 

 whether caused by dry air, wind, light, or high temperature, and low 



absorption, whether caused by 

 dry soil, soil salts, soil toxins, 

 low oxygen pressure, or low tem- 

 perature, are undoubtedly the 

 dominating influences in deter- 

 mining the small size and the 

 great thickness of the xerophytic 

 leaf. Furthermore, a combina- 

 tion of factors is more effective 

 than any single factor, both as 

 to the rapidity of development 

 and as to the degree of xero- 

 phytism attained; an "alpine" 

 leaf, for example, is produced 

 most quickly by exposing a plant 

 to intense sunlight, high tem- 

 perature, and dry air by day, 



FIG. 873. Leaf variation in the hare- 

 bell (Campanula rotundifolia); the apex of 

 the shoot has been cut off, and lateral buds 

 have developed at b and b'; note that the 

 first leaves of the axillary rosettes are short 

 and roundish, as in seedlings and basal 

 rosettes; such an occurrence sometimes is 

 called a reversion to a juvenile stage. From 

 FAMILLER. 



875 



FIGS. 874, 875. Leaf variation in the 

 arbor vitae (Thuja occidental is); 874, a 

 seedling, showing the cotyledons (c) and 

 the awl shaped " juvenile " leaves (j) ; 

 875, an older plant, showing " juvenile " 

 leaves (f) and the first lateral branches 

 bearing scale-shaped "adult" leaves (a). 



and to low temperature by night. The large size and the thinness 

 of the mesophytic leaf, on the other hand, are due to the less 

 extreme influence of the factors (transpiration, heat, light, etc.) pro- 

 ducing the xerophytic leaf, supplemented by the optimum amount of 



