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character in their organisation to show which of their faces is destined to become the upper surface, and which the' under. For the purpose of ascertaining whether there existed any natural but invisible predisposition in the two faces to undergo the changes which subsequently become so apparent, and by means of which their respective functions are performed, or whether the tendency is given by some cause posterior to their first creation, the following experiments were instituted: Five bulbs were sown upon powdered sandstone, and it was found that the face which touched the sandstone produced roots, and the opposite face formed stomates. It was, however, possible that the five bulbs might have all accidentally fallen upon the face which was predisposed to emit roots; other experiments of the same kind were therefore tried, first with eighty, and afterwards with hundreds of little bulbs, – and the result was the same as with the five. This proved that either face was originally adapted for producing either roots or stomates, and that the tendency was determined merely by the position in which the surfaces were placed.
The next point to ascertain was, whether the tendency once given could be afterwards altered. Some little bulbs, that had been growing for twenty-four hours only, had emitted roots ; they were turned, so that the upper surface touched the soil, and the under was exposed to light. In twenty-four hours more the two faces had both produced roots : that which had originally been the under surface went on pushing out new roots; that which had originally been the upper surface had also produced roots : but in a few days the sides of the young plants began to rise from the soil, became erect, turned over, and finally recovered in this way their original position, and the face which had originally been the uppermost immediately became covered with stomates. It, therefore, appears that, the impulse once given, the predisposition to assume particular appearances or functions is absolutely fixed, and will not change in the ordinary course of nature. This is a fact of high interest for those who are occupied with researches into the causes of what is called vegetable metamorphosis.
The parenchyma is, if casually examined, or even if viewed in slices of too great thickness, apparently composed of heaps
of small green bladders, arranged with little order or regularity ; but if very thin slices are taken and viewed with a high magnifying power, it will be seen that nothing can be more perfect than the plan upon which the whole structure is contrived, and that, instead of disorder, the most wise order pervades the whole. Upon this subject I extract the words of Adolphe Brongniart:– “There exists beneath the upper cuticle two or three layers of oblong blunt vesicles, placed perpendicular to the surface of the leaf, and generally much less in diameter than the bladders of the cuticle ; so that they are easily seen through it. These vesicles, which appear specially destined to give solidity to the parenchyma of the leaf, have no other intervals than the little spaces that result from the contact of this sort of cylinder : nevertheless, in plants that have stomates on the upper surface of their leaves, as is the case in most herbaceous plants, and in such as float on the surface of water, there exists here and there among the vesicles some large spaces, through which the stomates communicate with the interior of the leaf.
“This parenchyma is entirely different from what is found beneath the cuticle of the lower side. There, instead of consisting of regular cylindrical vesicles, it is composed of irregular ones, often having two or three branches, which unite with the limbs of the vesicles next them, and so form a reticulated parenchyma; the spaces between whose vesicles are much larger than the vesicles themselves.
“ It is this reticulated tissue, with large spaces in it (to which the name of cavernous or spongy parenchyma might not improperly be applied), that, in most cases, occupies at least half the thickness of the leaves between the veins. The arrangement of the vesicles is very obvious if the lower cuticle of certain leaves be lifted up with the layer of parenchyma that is applied against it; it may then be seen that these anastomosing vesicles form a net with large meshes — a sort of grating-inside the cuticle. It must not, however, be supposed that this structure, which I have remarked in several ferns, and in a great many dicotyledonous plants, is without excep tion. In many monocotyledonous and succulent plants we have some remarkable modifications of this structure. Thus, in the Lily, and several plants of the same family, the vesicles of parenchyma that are in contact with the lower cuticle are lengthened out, sinuous, and toothed, as it were, at the sides : these projections join those of the contiguous vesicle; and a number of cavities is the consequence, which render this sort of parenchyma permeable to air. An analogous arrangement exists in the lower parenchyma of Galega. In the Iris, there is scarcely any space between the oblong and polyhedral vesicles which form the parenchyma; but it is remarked, that the subjacent parenchyma is wanting at every point where the cuticle is pierced by a stomate. In such succulent plants as I have examined, the spaces between the cellules of parenchyma are very small; but, nevertheless, here and there, there are often larger cavities, which either correspond directly with the stomates, or are in communication with them. The same thing happens in plants with floating leaves, where the stomates placed on the upper surface correspond with the layer of the cylindrical and parallel vesicles; in such case there are, here and there, between these vesicles, empty spaces which almost always correspond to the points where the stomates exist, and which permit the air to penetrate between the vesicles as far as the middle of the parenchyma of the leaf.”
Thus much Adolphe Brongniart; who adds, that in submersed leaves there is no cuticle, but the whole consists of solid parenchyma alone, in which there are no other cavities than such as are necessary to float the leaves. The observations of Mohl, Meyen, and myself generally confirm this; but, at the same time, numerous cases exist in which the texture of the leaf has been found to be nearly the same throughout; in fact, the only circumstance which is found to be uniform in respect to the internal anatomy of leaves is, that their parenchyma is cavernous, and that the air cavities are uniformly in communication with the stomates.
Dutrochet states in addition (Ann. des Sc., xxv. 245.) that the interior of a leaf is divided completely by a number of partitions covered by the ribs and principal veins, so that the air cavities have not actually a free communication in every direction through the parenchyma; but are, to a certain extent, cut off from each other. This is conformable to what Mirbel has described in Marchantia, where the leafy expansions are separated by partitions into chambers, between which, he is of opinion, there is no other communication than what results from the permeability of the tissue.
The veins being elongations of the medullary sheath, necessarily consist of woody tissue and vessels, to which is added cinenchymatous tissue. In submersed leaves spiral vessels are often wanting, the veins consisting of nothing but woody tissue.
Such are the general anatomical characters of leaves; but it must be borne in mind, that, in different species, they undergo a variety of remarkable modifications. These arise either from the addition of parenchyma, when leaves become succulent, or from the non-developement of it when they become membranous, or from the total suppression of it, and even of the veins also in great part, as in those which are called ramentaceous, such as the primordial leaves of the genus Pinus. Occasionally, the veins only are formed, the parenchyma being deficient, as in Ranunculus aquatilis, the very curious Hydrogeton or Ouvirandra, and various species of Podostemaceæ.
It has already been seen that a leaf may consist of two distinct parts; the petiole, or stalk, and the lamina, or blade : both of these demand separate consideration. These are, however, not necessarily present; the petiole may exist without the lamina, as in leafless Acacias, or the lamina without the petiole, in all sessile leaves.
The BLADE lamina (or limbus, as it is called by some) is subject to many diversities of figure and division ; most commonly it forms an approach to oval, being longer than broad. It is described by two opposite arcs, whose points of intersection are the apex and base.
That extremity of the blade which is next the stem is called its base ; the opposite extremity, its apex ; and the line representing its two edges, the margin or circumscription.
If the blade consists of one piece only, the leaf is said to be simple, whatever may be the depth of its divisions : thus, the entire blade of the Box tree, the serrated blade of the Apple leaf, the toothed blade of Coltsfoot, the runcinate 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 number, as in the many varieties of pinnated leaves. Another notion of a compound leaf consists in its divisions being articulated 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 distinction 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, however, 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 spontaneously 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