55 
THE FOLIAGE— AND ITS FUNCTIONS. 
The phenomena which are referable to the leaves of all plants, whether perennial 
— including trees, shrubs, and herbaceous subjects — or annuals, whose course termi- 
nates with the first production of seeds, include some of the most profound mysteries 
of philosophy. 
Yet of late years, we might conclude from the style in which these mysteries are 
treated, that the secrets of Nature had been laid open, that the functions of the 
leaves had been actually determined ; and, henceforth, that a lecturer or writer had 
nothing to do but to expound, in set and orderly terms, the results of a few experi- 
ments which have been deemed conclusive. 
The modesty of doubt may yet, however, be tolerated ; and therefore, as it is the 
objects of these articles to excite inquiry, we shall not hesitate to present to notice 
the leading facts which constitute the basis of modern theory, and thus enable the 
reflecting to draw their own inferences. 
Dogmatism we entirely disclaim, therefore it becomes a duty to appeal to the 
highest authority on all points of consequence. There are persons who have at 
command every refinement of microscopic machinery, and who add to this great 
privilege an adroitness of manifestation which is almost miraculous. The following 
description of the structure of a leaf, though general, and, of course, subject to 
exceptions, must have been derived from some such power of minute inspection. 
" A leaf is an expansion of the bark, from whose axil a leaf-bud is developed. It 
is usually thin, and traversed with one or more veins, composed of woody and vascular 
tissue ; sometimes it is fleshy, cylindrical, or nearly so. Its veins form a double 
stratum, of which the upper is in connexion with the alburnum, and the lower with 
the liber of the branch on which it grows. When leaves have been macerated long- 
enough it is easy to separate the two strata. The veins are held together by a green 
or coloured parenchyma, which is inclosed in an epidermis, pierced by stomates or 
openings, supposed to be for the purpose of respiration." 
A leaf is frequently found in autumn lying on or among vegetable matter, which 
has so communicated decay, (Eremacausis of Liebig,) to its parenchyma as to lay 
bare the veins, and expose their every ramification ; if, according to the foregoing 
authority, the upper stratum is connected with the alburnum or new sap-wood of the 
plants, then we may safely conclude that its tissue is woody, and consists mainly of 
lignin — the base of woody fibre. By parity of reasoning, the lower stratum must be 
similar to the liber or yearling bark, which though ligneous is more spongy than the 
true wood. The positions and insertions of these supporting skeletons or bones of 
the leaf, seem to point to the fact that they are not tubes of conduction, though it 
cannot be doubted that they are so far pervious as to admit a supply of fluid for 
their own nutriment and growth. 
