STRUCTURE.] ANATOMY OF THE LEAF EPIDERMIS. 255 



result. (Organogr. p. 270,) Cylindrical leaves are, how- 

 ever, sometimes produced otherwise, as is proved by Brasavola 

 venosa, whose leaf is just intermediate between the flat fleshy 

 leaf of Br. glauca and the absolutely terete species. This 

 shews that in the latter the leaf owes its peculiar appearance 

 in part to a general thickening of its parenchym, and in part 

 to the edges turning inwards, meeting and growing together. 

 And this is probably also the origin of the terete leaves found 

 in other Orchids, such as species of Vanda, Luisia, and Den- 

 drobium, as is indicated by the channel which usually runs 

 along their upper side. 



The cellular tissue of which the rest of the leaf is composed 

 is parenchym, which Link then calls diachyma, or that im- 

 mediately beneath the two surfaces cortex, and the interme- 

 diate substance diploe. De Candolle calls these two, taken 

 together, the mesophyllum. The whole is protected, in leaves 

 exposed to air, by a homogeneous cuticle of indurated organic 

 mucus (p. 1.), and a coating of epidermis, furnished with 

 stomates ; but in submersed leaves the parenchym is naked, 

 no epidermis overlying it. 



The general nature of the parenchymatous part of leaves 

 has been explained, both by Link and others, and figured by 

 Mohl, firstly in 1828 (Uber die Poren des Pflanzenzellgewebes, 

 tab. i. fig. 4, &c.), and afterwards in his elaborate inquiry 

 into the anatomy of Palms. A very complete account is that 

 of Adolphe Brongniart, in 1830 (Annales des Sc. vol. xxi. 

 p. 420.), of which much of what follows is an abstract. 



The epidermis is a layer of vesicles adhering firmly to each 

 other, and sometimes but slightly to the subjacent tissue, 

 from which they are entirely different in form and nature : 

 in form, for their cellules are depressed, and, in consequence 

 of the variety of outline that they present, arranged in meshes 

 either regular or irregular; and in nature, because these 

 bladders are perfectly transparent, colourless, and probably 

 filled with either air or rarefied fluid, for the manner in 

 which light passes through them proves that they do not 

 contain dense fluid. They scarcely ever contain any organic 

 particles, and are probably but little permeable either to 

 fluids or gaseous matters; while, on the other hand, the 



