156 
Transactions of the 
TMonthly Microscopical 
L Journal, March 1, 1869.; 
sides and anastomosis of their vessels, have thus assumed a compo- 
site character, and formed a lobed leaf of one single blade. 
It is by the recognition of this principle of repetition of similar 
structure, which is very palpable when once noticed, that the course 
of the veins in lobed leaves can be interpreted; for when once 
examined from this aspect, their ramifications from their origin to 
their final distribution can to a very considerable extent be pre- 
dicted ; and the reason why a vein should travel in one direction 
rather than in another, as if by a process of selection, becomes 
intelhgible. 
Leaves are divided into sim^ple and compound. A simple leaf 
has one blade on one stalk ; a compound several on one stalk, when 
they are called leaflets. 
It is proposed in this paper to show that, of the four or five 
kinds of simple entire leaves which are distinguished by their 
venation, and classified in botanical works, there are two which are 
of more frequent occurrence than all the rest, and which for reasons 
to be specified demand especial attention. These are the true 
Netted Leaf and the Feather-veined Leaf. These two leaves should 
be well and attentively considered, and every vein in them thoroughly 
made out; because, while they are sufficiently interesting in their 
entire state — in which state, and as isolated from the rest, they have 
usually been treated of in the works on Botany — yet that they 
become especially so when studied in their relation to other leaves, 
entering, as they will be ^hown to do, into a multiplicity of combi- 
nations in which they may be distinctly recognized ; thus affording 
evidence of the elementary forms out of which by their repetition 
thousands of other leaves are constructed. 
It is then proposed to consider how these two simple leaves 
enter into the composition of almost all compound, metamorphosed, 
and simple lobed leaves. That when they do so they are arranged 
upon a petiole or leaf-stalk in one of two ways : first, in a ray on 
the summit of the stalk (radiating, palmate) ; secondly, in rows on 
the sides of an elongated axis (pinnate, feather- veined). That when 
the leaflets are thus arranged in compound leaves, they are always 
separate, not united at their edges ; that there is a series of com- 
pound leaves, however, in which two or more of the leaflets are 
apparently joined at their edges : these are called metamorphosed 
leaves. Such leaves are important, showing, as they do, the possi- 
bility of union of the leaflets, and also the modification in the 
venation which takes place in those parts where the union is 
effected. That there is still another series called lohed leaves, in 
which the union of all the leaflets can be shown to have taken 
place, giving rise to forms which are precisely similar to compound 
leaves in the character of their leaflets, and to metamorphosed leaves 
in their mode of union. These are called simple lobed leaves, and 
