Fig. 125.—Structure of the epidermis of a leaf. resents, on the contrary, from space to space, small openings formed by the separation of two cells. These openings being elastic, and capable of expansion oT circumstances, are intended tocompression, according to exterior exhale the gaseous and vapoury products of perspiration in the plant, and also to absorb the gas and moisture of the atmosphere, the function of the cellular tissue of the epidermis being to transmit fluids in all directions. They bear the name of stomates, from the Greek word oropa; A stomates of Cycas “mouth.” y, under the microscope is seen i2

Fig. 126. The stomates are most abundant on the lower surface of leaves. Their number varies much according to the plant;and the smaller they are, the more numerous. In the Pink they present four thousand in the space of a square inch; the Iris twelve thousand, and the Lilac a hundred and twenty thousand. The epidermis which covers and protects the parenchyma of the leaf is itself covered with an extremely delicate pellucid membrane, whose structure is, the discovery of which we owe to M. Adrienalmost inappreciable,

Brongniart. It is termed the cuticle. It adheres closely to the epidermis, moulds itself exactly on this membrane, and even upon its hairs, to which it forms a sheath, much as the glove does to the finger, covering the epidermis, but presenting minute openings of great delicacy corresponding at all points with the stomates.

In the parenchyma of the leaves of the greater number of vegetables, two distinct regions, an upper and a lower, may be observed (Fig. 127). In the upper regions, one, two, or three rows of oblong cells may be traced perpendicularly to the surface of the leaf, and closely pressing one against the other, spread out sometimes in such a manner as to leave between them many air-cells, which are generally found to correspond with a stomates. The lower bed is composed of irregular cellules, often branching out and touching each other only at the extreme points of their branchiæ, and leaving them numerous air-cells, which communicate one with another, forming a sort of spongeous tissue. Among these cells, many are situated immediately upon the epidermis of the lower surface, which is riddled with a much greater number of stomates than is the epidermis of the upper surface. It is these stomates with which the air-cells correspond.

Fig. 127.—Transverse Section of a Leaf, showing the Structure of the Parenchyma.

These parenchymentous cellules, whose walls are always very thin, are filled with globules of the green vegetable fluid to which young plants and leaves owe their colour. This chlorophylle is found in great quantities and much denser in the upper zone, than in the lower and more spongy parenchyma. The aggregate of cells thus coloured by the chlorophylle give to the vegetable leaf that green and uniform tint which is its characteristic.

We have noted these peculiarities of organisation, and above all the existence of these intercellular meatus or air-cells,—cavities which communicate freely between them, and which are thrown in connection with the ambient medium by means of the stomatic

openings, so admirably organised for carrying out the vital phenomena of which it is the seat, phenomena which we shall study more minutely by-and-by. We find in the nervure of the leaves, vessels of divers orders bound together by cellules of various forms, which constitute the fibrous part of this bundle. The structure of the nervure, whic - is most complex, becomes more simplified in proportion as it 18 divided and diminished in its dimensions.

If from plants with aerial leaves we pass to plants with leaves which float upon water, or whose natural position is to be submerged, we shall see the structure of these organs modifying themselves according to the medium in which nature has placed them. These curious modifications have been carefully studied in our own day by M. Adrien Brongniart.

The leaves of the Water Lily (NWymphea), which float upon the: water, present, it is true, both epidermis and parenchyma, differing very slightly from those with aerial leaves, but the lower epidermis in contact with the water has no stomates. The submerged leaves of the Pond Weed are generally very thin and quite destitute of any epidermis, and consequently of stomates also. They are channelled with air-cells, which have no communication with each other, but are formed of polyhedrous or many-sided cellules, compressed and gorged with green fluid. These air-cells have 2° analogy with those of aerial leaves; they can only be considered as reservoirs of air furnished by the plant itself, and calculated, 2° doubt, to reduce its weight. They are floating apparatus, which seem to play a part analogous to the natatory vessels of the fish-

The Lanunculus aquaticus present at once, as we have seen, the aerial leaves which float on: the surface of the water, and the much-divided leaves which are submerged. The aerial leaves, furnished with an epidermis provided with stomates, present a parenchyma oe whose structure scarcely differs from that of the aerial leaves already described. Aquatic leaves have no epidermis properly 5°

called, but only isk } t stlul i agai st the other, tituting tl I nine y uniformly dense, hollowed here and there with isolated aeriform cavities, as in Fig. 128. ial: 2 sa + without of leaves our study conclude cannot _ We

words upon the stipula. These stipules are organs of only secondary importance, which accompany the leaves in certain plants. They are organised like leaves, but they are not true leaves. At an early period in the growth of certain — plants, two small tumours appear, one at either side of the base of the leaf- 2

come stipula, a blade. They appear later than the leaf, at whose base they are developed, but at some period of their existence they almost invariably enclose Fig. 128.—Cells of Aquatic Plants. the leaf, being of quicker growth than the leaves. But their form and functions are altogether different, being small foliaceous organs with membraneous appendages, whose points of insertion are very various. In the Tulip-tree, Fig. 129, we observe two stipules,

p: oe

Fig. 129,—Stipula of Tulip-tree.: Fig. 130.—Branch of the Rose-tree,

placed the one on the right and the other on the left of the point of insertion of the leaf-stalk. In the Rose-bush, Fig. 130, the two — Stipules are attached to the petiole of the leaf. In the Hop-plant,

Fig. 181, the two stipules are on the same side of the stem, and belong to two different leaves, being so mingled together as to appear to form two double stipules. In the Buckwheat, Fig. 132, we see only a stipula placed between the leaf and the stem. The stipula appears, as we havesaid, after the leaf which accompanies it. Nevertheless, it often grows much . more rapidly than the leaf, ~ and in the bud they often completely cover these organs. They are probably intended in this case to shelter the young leaves. They are also very fugitive. Sometimes the stipules have neither this rapid development nor this fugitive character. They are then called persistent. It is probable that in their persistent character they are useful to the plant, either in sheltering its more delicate organs and feeding the buds, or in replac-ing the leaves when they are prematurely shed. Inthe grasses ( Gram-imee) there is a little

| membrane which ap-

{ pears to be a continuation of the inner lining of the petiole or sheath

| beyond the origin of

F/ the lamina of the leaf. y is bi ’

Fig. 132.—Buckwheat (Fagopyrum). - Lge bm example of ligula we give a vegetable furnished with the ligula leaf, the Milium multiftorum, Fig. 133, which represents a branch

Fig. 131.—The Hop-plant.

of this plant. The ligula is represented by the letters L.g, at the base and internal face of the leaf, which is sheathed like all the grasses. Here the leaf-stalk or petiole has its origin in one of the nodes of the stem; while at L.g, the apex of the sheathing petiole, there is a small membrane which appears to be a continuation of the inner liningof the petiole beyond the origin of the lamina of the leaf. This is the collar, or ligula.

Fig. 133,.—Ligula of Millet-grass.

OF THE LIFE OF PLANTS. PHENOMENA

EXHALATION, RESPIRATION, CIRCULATION.

Havine acquired some knowledge of the external arrangements and of the internal structure of the roots, stem, and leaves, we may now consider the essential phenomena in the life of plants, namely, vegetable physiology, in some of its most important points.

Vegetation presents in exhalation, respiration, and circulation of the sap in the interior of the tissues, three functions only secondary in their importance to reproduction.

EXHALATION. ,

Exhalation in plants is performed by the leaves and branches. Plants exhale water or vapour by their leaves; it is retarded by the presence on the surface of the leaves of a coating of wa* which gives them their greyish-blue glossy appearance, and tt varies according as the epidermis is thick or thin.

The medium in which the plant is placed also greatly influences the function of exhalation. If the air is very dry, the exhalatio? is abundant and rapid. It is less active where the air is cha ‘ with humidity; it increases as the temperature rises; it is dimim-ished during the night.

It is not alone by the stomates of the leaves, but also through means of the epidermic membrane itself, that exhalation takes place. The result of the perfect equilibrium which exists between the absorption of the roots and the foliaceous exhalation, is proof of a normal state of healthy vigour in the plant. If exhalation exceeds its absorbing powers the plant must fade.

It must be added, however, that at the same time that leaves transpire, they also reciprocate and absorb water by their own surface. is function of absorption does not appear to bear any proportion to the number of stomates, but seems to be more or less considerable according to the quantity of wax which covers their surface, absorption being most active where this coating is thin.

REsPIRATION.

If we place an entire plant or a leafy branch in a balloon filled with gas which cannot be renewed, and leave the whole in darkness for some ten or fifteen hours, we may assure ourselves at the expiration of this time that the atmospheric air contained in the balloon is no longer of the same composition as before the experiment. Carbonic acid will be there in greater abundance, and the quantity of oxygen will be less. But if in place of leaving the plant in darkness we expose the apparatus to the influence of the sun’s rays, the phenomena will be reversed; after a few hours the air in the’ balloon will have lost a noticeable quantity of its carbonic acid, and will be enriched in its oxygen.