THE PRINCIPLES OP BOTANY. 91 
Will be seen to be uneven, or crenated, at the margin, while 
its heart-like shape has caused it to be described as cordate. 
Our next example, that of the common English ivy, is 
seen to be more complicated in outline, being divided into 
lobes, and as some leaves have a determinate number of lobes. 
Eig. 7.—Common ivy. 
Fig. 8.—Poppy. 
as o-lobed, 5-lobed, 7-lobed, and the like, the numbers of the 
lobes thus become of importance in plant discrimination. 
The annexed cut of a poppy leaf shows still deeper divisions 
down even to the midrib. This is a pinnatifid form of leaf, 
which differs from the pinnate leaves of the rose and the ash, 
inasmuch as the latter divisions or leaflets are still more 
determinate in their structure, and, indeed, are portions ar¬ 
ticulated to the mid-rib, and thus the ash leaf, in frost, will 
sometimes fall to bits leaflet by leaflet, while this highly com¬ 
plicated structure can after all be shown to be the result of 
morphological change, as the leaf of the Fraooinus Tietero- 
pliyllus —a mere variety of the common ash, F. excelsior —has 
a large flat simply lanceolate leaf. 
Now as leaves are the more prominent parts of plants, it 
follows that they are constantly appealed to in plant discri¬ 
mination, and hence it becomes important for the student to 
become acquainted with the simple terms by which their 
forms are distinguished, and, no less so, the manner in which 
these organs may be situate on the plant. For example, the 
laurel leaf is attached by a short petiole (leaf-stalk), while 
that of the ivy is comparatively long. In most leaves the 
petiole is broad and thick, while in the poplar the leaf-stalk 
is much flattened at right angles to the pagina or blade, and 
when, then, we are next invited to 
“ Come where the aspens quiver,” 
if not too much preoccupied, it may not be amiss to mark the 
beautiful mechanical principles upon which the ceaseless 
motion of the aspen and other poplars depend. 
Leaves, again, are placed on stems in different ways accord- 
