CHAP. ]I. LEAVES. 99 



tes; but in submersed leaves the parenchyma is naked, no 

 cuticle overlaying it. 



The general nature of the parenchymatous part of leaves 

 has been very well explained, both by Link and others, and 

 figured by Mohl, firstly in 1828 ( Uher die Poren des Pflanzen- 

 zellgewebes^ tab. i. fig. 4, &c.), and afterwards in his very ela- 

 borate enquiry into the Anatomy of Palms. But the most 

 complete account is that of Adolphe Brongniart, in 1830 

 {Annales des Sc, vol. xxi. p. 420.), of which the principal 

 part of what follows is an abstract. 



The cuticle is a layer of bladders 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 the cellules are depressed, and, in consequence of the variety 

 of outline that they present, form meshes either regular or 

 irregular ; and in nature, because these bladders are perfectly 

 transparent, colourless, and probably filled with air, — 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 matter; while, on the other hand, tlie 

 bladders of the subjacent parenchyma are filled with the green 

 substance that determines the colour of the leaf. The cuticle 

 is not always formed of a single layer of bladders, but in some 

 cases consists of two, or even three. No trace whatever is 

 discoverable of vessels either terminating in or beneath the 

 cuticle ; Brongniart states this most explicitly, and my own 

 observations are entirely in accordance with his : an opinion, 

 therefore, which some botanists have entertained, that spiral 

 vessels terminate in the stomates (D. C. Organogr. p. 272, 

 &c.), must hereafter be abandoned. At the margin of a leaf 

 the cuticle is generally harder than elsewhere, and sometimes 

 becomes so indurated as to assume a flinty texture, as in the 

 Aloe, and many other plants. 



Stomates (p. 39,) are found upon various parts of the cu- 

 ticle : in some plants only on that of the under side of leaves, 

 in others on the upper also ; in floating leaves upon the latter 

 onlv. When leaves are so turned that their margins are 

 directed towards the earth and the heavens, the two faces are 



H 2 



