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 

 (P- 551). but many 

 plants (as Lepidium) 

 have their compound 

 leaves below, 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 more light 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. 



