. A textbook of botany for colleges and universities ... Botany. 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
. A textbook of botany for colleges and universities ... Botany. 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 { 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
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
