cells known from their form as "pallisade-cells," beneath which is a parenchymatous tissue of more or less loose texture. The leaf is strengthened by ribs of woody tissue. From this general type there are, of course, numerous variations. For instance, some water-plants have 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 curved cells, the concavities being turned inward. When dry, they are nearly 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 remember that there are sometimes many thousand stomata to a square inch of surface. In a large proportion of herbs the two sides of the leaf are under conditions so nearly similar that the stomata are almost equally numerous on the upper and on the lower side. In trees, however, as a general rule, they are found exclusively on the under side of the leaf, which is the most protected; they are thus less exposed to the direct rays of the sun, or to be thoroughly wetted by rain, so that their action is less liable to sudden and violent changes.
There are, however, some exceptions; for instance, in the black poplar the stomata are nearly as numerous on one side of the leaf as on the other. Now, why is this? If we compare the leaves of the
Fig. 15. black and white poplar, we shall be at once struck by the fact that, though these species are so nearly allied, the leaves are very different. In the white poplar (Populus alba), the upper and under sides are very unlike both in color and texture, the under side being thickly clothed with cottony hairs. In the black poplar (P. nigra, Fig. 15), the upper and under surfaces are, which is not frequent, very similar in color and texture. The petioles or leaf-stalks, again, are unlike; those of P. nigra presenting the peculiarity of being much flattened at the end toward the leaf. The effect of the unusual structure of the petiole is that the leaf, instead of being horizontal as in the P. alba and most trees, hangs vertically, and this again explains the similarity of the two surfaces, because the result is that both surfaces are placed under nearly similar conditions as regards light and air. Again, it will be observed that, if we attempt to arrange the leaves of the black poplar on one plane, they generally overlap one another; the extent is larger than can be displayed without their interfering with one another. In foliage arranged like that, for instance,
