. Popular science monthly. e is admirably suited ; but in a cx-own of leaves, such as those of the sycamore, space would be wasted, and itis better that they should expand at once assoon as their stalks have borne them freefrom those within. Moreover, the spreadinglobes leave a triangular space (Fig. 11) withthe insertion of the stalk at the apex, which-(-■i^ seems as if expressly designed to leave room^ for the pointed end of the leaf within. Hence we see how beautifully the wholeform of these leaves is adapted to the modeof growth of the trees themselves and theFig- 14. arrangement of their
. Popular science monthly. e is admirably suited ; but in a cx-own of leaves, such as those of the sycamore, space would be wasted, and itis better that they should expand at once assoon as their stalks have borne them freefrom those within. Moreover, the spreadinglobes leave a triangular space (Fig. 11) withthe insertion of the stalk at the apex, which-(-■i^ seems as if expressly designed to leave room^ for the pointed end of the leaf within. Hence we see how beautifully the wholeform of these leaves is adapted to the modeof growth of the trees themselves and theFig- 14. arrangement of their buds. Before we proceed to consider the next series of species to which Iwish to direct attention, it will be necessary for me to say a few wordson the microscopical structure of the leaf. Although so thin, the leafconsists of several layers of cells. Speaking roughly, and as a generalrule, we may say that on each side is a thin membrane, or epidermis,underneath which on the upper side are one or more layers of elongated. OJSr LEAVES. 353 cells known from their form as pallisade-eells, beneath which is aparenchymatous tissue of more or less loose texture. The leaf isstrengthened by ribs of woody tissue. From this general type thereare, of course, numerous variations. For instance, some water-plantshave no epidermis. If the surface of the leaf be examined with a tolerably high power,small opaque spots will be observed, resembling a sort of button-hole,with a thick rim or border composed of two more or less curvedcells, the concavities being turned inward. When dry, they arenearly straight, and lie side by side ; but when moistened they swell,become somewhat curved, and gape open. It is difficult to realize the immense number of these orifices or stomata which a single bush or tree must possess when we rememberthat there are sometimes many thousand stomata to a square inch ofsurface. In a large proportion of herbs the two sides of the leaf areunder conditions so nearly similar
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectscience, bookyear1872
