CHAP. II. 
LEAVES. 
103 
are very small ; but, nevertheless, here and there, there are 
often larger cavities, which either correspond directly with 
the stomates, or are in communication with them. The same 
thing happens in plants with floating leaves, where the stomates 
placed on the upper surface correspond with the layer of the 
cylindrical and parallel vesicles ; in such case there are, here 
and there, between these vesicles, empty spaces which almost 
always correspond to the points where the stomates exist, and 
which permit the air to penetrate between the vesicles as far 
as the middle of the parenchyma of the leaf." 
Thus much Adolphe Brongniart; who adds, that in sub- 
mersed leaves there is no cuticle, but the whole consists of 
solid parenchyma alone, in which there are no other cavities 
than such as are necessary to float the leaves. The obser- 
vations of Mohl and Meyen generally confirm this ; but, at 
the same time, the latter mentions several cases in which the 
texture of the leaf has been found to be nearly the same 
throughout. 
Dutrochet states in addition (A7in. des Sc, xxv. 245.) that 
the interior of a leaf is divided completely by a number of par- 
titions, covered by the ribs and principal veins, so that the air 
cavities have not actually a free communication in every 
direction through the parenchyma ; but are, to a certain 
extent, cut off from each other. This is conformable to what 
Mirbel has described in Marchantia, where the leafy expansions 
are separated by partitions into chambers, between which, he 
is of opinion, there is no other communication than what 
results from the permeability of the tissue. 
The veins, being elongations of the medullary sheath, neces- 
sarily consist of woody tissue and spiral vessels, to which are 
sometimes added annular ducts. In submersed leaves spiral 
vessels are often wanting, the veins consisting of nothing but 
woody tissue. In these veins Schultz finds what he calls 
vessels of the latex, or of the nutritive fluid; concerning the 
probable nature of which see p. 31. 
Such are the general anatomical characters of leaves ; but 
it must be borne in mind, that, in different species, they 
undergo a variety of remarkable modifications. These arise 
either from the addition of parenchyma when leaves become 
H 4 
