ANATOMY OF LEAVES. 307 



eral, as Mr. Francis Bauer has demonstrated in the 

 genus Htemanthus, and others. 



The slits or apertures already noticed as existing 

 on one or both surfaces of all leaves, were first de- 

 scribed by Grew as orifices ; and the observations of 

 Hcdwig and of Decandolle have confirmed this opin- 

 ion, and under a good microscope it is easy to per- 

 ceive that they are real pores. In the leaves of trees, 

 and of some other plants, they are observed on the in- 

 ferior disk only ; but in others, particularly in the 

 Grasses, the Lilies, and the Palms, they occupy both 

 surfaces. They exist also in the lower tribes of 

 plants, as may be perceived in Marchantia, and a few 

 of the Mosses. Plants which have no leaves, as the 

 Cactus tribe, and many of the Rushes, and some of 

 those, also, which have leaves, as the Grasses, have 

 pores on the stem ; but, in general, they are confined 

 to the leaves. The leaves of aquatic plants, however, 

 which are constantly under water, are destitute of 

 pores ; the upper disk only of leaves which float on 

 the surface of water, possess them ; and when a land 

 plant is made to grow under water, the new leaves, 

 evolved under the water, have no pores, although those 

 which they have succeeded, or the aerial leaves, were 

 furnished with them. Even in plants which are part- 

 ly immersed and partly submersed, as Ranunculus 

 aquatilis, the leaves growing under water are desti- 

 tute of pores, while those which float above are pro- 

 vided with them. 



These foliar apertures vary very considerably in 

 form, size, number, and position, in different leaves. 

 They are commonly oblong, but in some instances cir- 

 cular, and in the Agave tribe and a few other fami- 

 lies of plants, they are quadrilateral. In almost all 

 leaves they are surrounded by a border, in which the 

 vessels forming the cuticular meshes appear to ter- 



