LEAVES 



607 



the optimum of synthesis, while in so far as the upper leaves are small and 



thick, they are protected from excessive transpiration. In essentially 



all of the contour categories noted, that of Lepidium with compound 



basal leaves, as well as that of Campanula or Geum with simple basal 



leaves, these differences in size and thickness obtain, and the advan- 



tages follow as cited. Advantages from differences in contour, however, 



are not so obvious. 



It often is assumed 



that each of these 



differences is bene- 



ficial, and that the 



very fact of change, 



whether from with- 



out or from within, is 



prima facie evidence 



of usefulness. But 



attempts to discover 



advantages have met 



with failure. It is 



true that compound 



leaves have been 



thought to be useful 



in the sifting of light 

 / v i , 



* 

 plants (as Lepidium) 



have their compound 

 I r i j 



low, and in 

 no case is there evi- 

 dence that the capacity of leaves to sift light has had much effect upon 

 the survival of species. Probably contour variability in leaves is of no 

 special import in determining the success or failure of plants. 



Asymmetry and anisophylly. In certain plants (as Celtis and Begonia) the 

 leaves exhibit asymmetry, the basal region bulging more on one side than on the 

 other, giving a general oblique effect (fig. 895). It has been shown in a number 

 of instances that leaf asymmetry is due to unequal illumination, the bulged portion 

 having received moreTight in its development, because of its~more favored posi- 

 tion. The smaller portion commonly develops close to the stem and often is 

 shaded by the next leaf. Probably the light influence is indirect (i.e. affecting 

 synthetic activity) rather than direct. By twisting a petiole or by making incisions 

 so as to check the water supply, it is possible to produce in Begonia a symmetrical 



FIG. 895. A horizontal shoot of the hackberry (Celtis 

 occidentalis), showing leaf asymmetry; note that the leaves 

 form a "mosaic," the expanded portion of the base of one 

 blade coming into juxtaposition with the contracted portion 

 of the base of the adjoining blade ; all the leaves are in one 



plane by reason of stem twisting or petiole growth . 



