128 
ORGANOGRAPHY. 
BOOK I. 
blade of Taraxacum, the pinnatifid blade of Hawthorn (which 
is often divided almost to its very midrib), are all considered 
to belong to the class of simple leaves. But if the petiole 
branches out, separating the cellular tissue into more than one 
distinct portion, each forming a perfect blade by itself, such a 
leaf is often said to be compound^ whether the divisions be two, 
as in the conjugate leaf of Zygophyllum, or indefinite in num- 
ber, as in the many varieties of pinnated leaves. Another 
notion of a compound leaf consists in its divisions being arti- 
culated with the petiole, by which it is better distinguished 
from the simple leaf than by the number of its divisions. 
Thus, the pinnated leaf of a Zamia, and the pedate leaf of an 
Arum, both in this sense belong to the class of simple leaves ; 
while the solitary blade of the Orange, the common Barberry, 
&c. are referable to the class of compound leaves. This dis- 
tinction is said to be of some importance to the student of 
natural affinities ; for, while division of whatever degree may 
be expected to occur in different species of the same genus 
or order (provided there is no articulation), it rarely happens 
that such compound leaves, as are articulated with their petiole, 
are found in the same natural assemblage with those in which 
no articulation exists. Alphonse De Candolle remarks, how- 
ever, that in Gleditschia, whose leaves are mostly articulated, 
we find some leaves with their leaflets united, and therefore 
not articulated with their midrib ; and this, and other similar 
instances, diminishes the value of articulation as the test of a 
compound leaf ; moreover, in such apparently simple leaves 
as those of Zamia, the leaflets are, in fact, articulated with 
their midrib, as is proved by macerating them, when they spon- 
taneously disarticulate. 
In speaking of the surface of a leaf it is customary to make 
use of the word pagina. Thus, the upper surface is called 
pagina superior ; the lower surface, pagina inferior. The upper 
surface is more shining and compact than the under, and less 
generally clothed with hairs ; its veins are sunken ; while those 
of the lower surface are usually prominent. The epidermis 
readily separates from the lower surface, but with difficulty 
from the upper. There are frequently hairs upon the under 
surface while the upper is perfectly smooth ; but there are few 
