STRUCTURE AND TERMINOLOGY OF LEAVES. 29 
spring (¢.g., poplars and willows), and which drop off as soon 
as these leaf-buds are ready to open; so also are bracts, 
sepals, and most probably petals, stamens, and carpels; and 
so, lastly, are the little cotyledons of the embryonal plant. 
All may be termed leaves, and are modifications of a common 
leaf-type, and yet when we apply the term leaf without further 
explanation we of course mean the foliage-leaf, the ordinary 
green leaf of the plant. Still, it is to be borne in mind that 
the terms which are used in describing the forms of these may 
equally well be applied to all other leaf-like organs. The 
functions of these other kinds of leaves are already known to 
-you in some slight degree, but will be more fully referred to 
further on. 
It is usual to say that all leaves are simple which have a 
single blade on each petiole, however much cut into this 
blade may be; while those are compound which have three 
or more blades. Of simple leaves, those which are cut into 
from their margin towards their midrib (Pl. II., fig. 2) ave 
said to be pinnately lobed or pinnatifid if the divisions 
extend about half-way to the midrib, as in the oak; and 
pinnati-sect or -partite when cut very deeply down, as in 
some poppies. Those which are cut down towards the base 
(Pl. IL., fig. 6) are said to be palmately lobed or palmatifid, 
as in maple; and palmati-sect or -partite if deeply cut, as 
in some buttercups and in Geraniwm dissectum (Pl. L., fig. 4). 
Compound leaves are called pinnate (Pl. II., fig. 3) when the 
leaflets are arranged in pairs along the rachis (or common 
petiole), and if the subdivision is carried further, the leaflets 
being pinnately arranged on secondary rachises, they are 
bipinnate (Pl. Il., fig. 4), or even 3-pinnate. Similarly, if 
several leaflets spring from one point at the apex of the 
petiole (Pl. I1., fig. 7), they are called digitate. Digitate 
leaves are 3-foliolate, 5-, 7-, or more foliolate, according to 
the number of leaflets. 
This classification is convenient enough for ordinary pur- 
poses, but an examination of the figures in Plate II. will 
show that it is purely artificial. There are only two main 
plans or types of leaf-structure among the plants commonly 
met with, and these are governed by the venation or arrange- 
ment of the vascular framework of the leaf. Thus, in the plate 
figs. 1-4 represent a series of leaves in which there is a single 
midrib, which is the direct continuation of the petiole, and from 
which veins and veinlets are given off laterally. In each case 
the same form of skeleton or framework is represented, but the 
extent to which the green soft cellular tissue is developed is 
shown in four different ways by dotted outlines. Fig. 1 ig 
that of a semple entire leaf, fig. 2 is simple and pinnatifid, fig. 
3 is compound and unequally pinnate, while fig. 4 is com- 
