122 
ORGANOGRAPHY. 
BOOK I. 
the point of separation from the stem) do so after two different 
systems : they may either constantly preserve the same plane, 
when common flat leaves are formed ; or they may expand in 
any direction, when cylindrical or swollen or triangular leaves 
are the result. (Organogr. p. 270.) 
The cellular tissue of which the rest of the leaf is composed 
is parenchyma, which Link then calls diachyma, or that im- 
mediately beneath the two surfaces cortex^ and the intermediate 
substance diploe, De Candolle calls these two, taken to- 
gether, 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 parenchyma 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 PJlanzenzellgewebeSy 
tab. i. fig. 4, &c.), and afterwards in his elaborate enquiry 
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 con- 
tain 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 vesi- 
cles of the subjacent parenchyma are filled with the green 
substance which determines the colour of the leaf. The epi- 
dermis is not always formed of a single layer of vesicles, but 
in some cases consists of two, or even three. No trace is 
discoverable of vessels either terminating in or beneath the 
cuticle; Brongniart states this most explicitly, and my own 
