VEGETABLE PHYSIOLOGY. — THE LEAVES OF PLANTS. 177 
whole is covered by the cuticle. Leaves generally become 
articulated with the stem at the point whence they diverge, and 
thus they are enabled to separate from it on decaying ; but this 
is not quite universal. 
For the most part, leaves are formed of two distinct portions, 
which are called the petiole , or stalk, and the lamina , or blade, 
both of which demand our attention. 
That part of the blade, or lamina, which is situated nearest 
the stem, is called its base ; the opposite extremity is the apex ; 
and the line of its edges is the margin. If the blade consists of 
one piece only, it is said to be simple; and the same term 
applies also to all leaves which are not articulated with the 
petiole, whatever may be their form, division, or outline : thus, 
the entire blade of the box tree, the serrated blade of the apple, 
the toothed blade of the coltsfoot, the runcinate blade of the 
dandelion, and the pinnatifid blade of the hawthorn, are equally 
to be regarded as simple leaves, because they are not articulated 
with their petioles. On the other hand, if the blade be articu¬ 
lated with its petiole, it is considered as a compound leaf, with 
equal disragard to its figure ; the apparently simple blade of the 
orange tree, and those of the barberry, being no less truly com¬ 
pound than those of the Acacia or Mimosa. 
The ramifications of the petiole among the tissue of the blade 
are called veins , and their distribution the veyiation: the course 
taken by these ramifications determines, in great measure, the 
form of the leaf, and is deserving of consideration. The vein 
which forms a continuation of the petiole is termed the midrib, 
from which others, diverging to the margins, produce what is 
termed venation, or reticulation in the leaves. If similar veins 
to the midrib pass through the leaves in the same direction, 
such leaves have been called nerved leaves. This term has been 
objected to, on the ground that expressions perfectly understood 
as applying to a particular organ in the members of one kingdom 
of nature, if indiscriminately applied to different organs in the 
members of another kingdom, must be destructive of anything 
like accurate ideas in natural history ; and taking the present 
case as an example, persons might be led to suppose that these 
so-called nerves in plants performed functions similar to those 
of animals, while the fact is, that they are merely channels for 
fluids. In place of this, it is proposed to call these veins, which 
extend from the base to the apex of the leaf, ribs; those which 
VOL. v. NO. IX. 
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