ON LEAVES. 



353 



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 

 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 pe- 

 culiarity 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 sur- 

 faces 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 in- 

 terfering with one another. In foliage arranged like that, for instance, 

 vol. xxvii. 23 



Fig. 15. 



