CHAP. II. 
LEAVES. 
127 
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 vessels, to which is added 
cinenchymatous tissue. In submersed leaves spiral vessels 
are often wanting, the veins consisting of nothing but woody 
tissue. 
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 
succulent, or from the non-developement of it when they 
become membranous, or from the total suppression of it, and 
even of the veins also in great part, as in those which are 
called ramentaceous, such as the primordial leaves of the genus 
Pinus. Occasionally, the veins only are formed, the paren- 
chyma being deficient, as in Ranunculus aquatilis, the very 
curious Hydrogeton or Ouvirandra, and various species of 
Podostemaceae. 
It has already been seen that a leaf may consist of two 
distinct parts ; the petiole, or stalk, and the lamina, or blade : 
both of these demand separate consideration. These are, how- 
ever, not necessarily present ; the petiole may exist without 
the lamina, as in leafless Acacias, or the lamina without the 
petiole, in all sessile leaves. 
The BLADE lamina (or limbus, as it is called by some) is 
subject to many diversities of figure and division ; most com- 
monly it forms an approach to oval, being longer than broad. 
It is described by two opposite arcs, whose points of inter- 
section are the apex and base. 
That extremity of the blade which is next the stem is 
called its hase ; the opposite extremity, its apea: ; and the line 
representing its two edges, the margin or circumscription. 
If the blade consists of one piece only, the leaf is said to 
be simple, whatever may be the depth of its divisions : thus, 
the entire blade of the Box tree, the serrated blade of the 
Apple leaf, the toothed blade of Coltsfoot, the runcinate 
