90 



MORPHOLOGY OF LEAVES. 



within the leaf, and thus transpiration be much facilitated. 

 When closed, this interchange will be interrupted or impeded. 

 The mechanism of stomata is somewhat 

 recondite, and will be illustrated in the 

 anatomical and physiological volume of 

 this series. 



172. It is only when leaves assume 

 a vertical or edgewise position that the 

 stomata are in equal numbers on both 

 faces of a leaf. Ordinarily, the y occupy 

 or most abound on the lower face, which 

 is turned away from the sun ; but in certain coniferous trees the 

 reverse of this is true. In the Water Lilies (Nymphsea, Nuphar), 



and other leaves which float 

 upon the water, the stomata all 

 belong to the upper surface. 

 Leaves which live under water, 

 where there can be no evapora- 

 tion, are destitute, not only of 

 stomata, but usually of a distinct epidermis also. The number 

 of the stomata varies from 800 to about 170,000 on the square 

 inch of surface in different leaves. In the Apple, there are said 

 to be about 24,000 to the square inch (which is under the average 

 number, as given in a table of 36 species by Lindley) ; so that 

 each leaf of that tree would present about 100,000 of these 

 orifices. The leaf of Dragon Arum is said to have 8,000 

 stomata to a square inch of the upper surface, and twice that 

 number in the same space of the lower. That of the Coltsfoot 

 has 12,000 stomata to a square inch of the lower epidermis, 

 and only 1,200 in the upper. That of the White Lily has 

 from 20,000 to 60,000 to the square inch on the lower sur- 

 face, and perhaps 3,000 on the upper; and they are so re- 

 markably large that they may be discerned Ly a simple lens of 

 an inch focus. 



172. Venation, the veining of leaves, &c., relates to the mode 

 in which the woody tissue, in the form of ribs, veins, &c., is 

 distributed in the cellular. There are two principal modes, the 

 parallel-veined and the reticulated or netted-veined. The former 

 is especially characteristic of plants with endogenous stem and 

 monocotyledonous embryo, and also of gymnospermous trees, 



FIG. 138. A highly magnified piece of the epidermis of tlie Garden Balsam, with 

 three stomata (after Brongniart). 



FIG. 139. Magnified view of the 10,000th part of a square inch of the epidermis of 

 the lower surface of the leaf of the White Lily, with its stomata. 140. A single stoma, 

 more magnified. 1 11. Another stoma, widely open. 



