their leaves are formed; because the leaves gradually empty the tubes, put an end to their distension, and prevent its recurrence so long as they remain in an active state.

The excessive loss of sap in the above-mentioned cases would not have taken place if the roots had been wounded in the summer or autumn. It is probable, moreover, that the bleeding was increased by the unusual coldness of the spring in which these instances occurred. Hales himself was aware that sap falls back at night in consequence of the contraction of the tubes by cold; Mr. Knight observed the same fact: and it has more recently been proved experimentally by M. Biot. It may therefore be supposed that excessive cases of bleeding occur, because, in addition to the natural contraction of the tubes of the wood, their mechanical contraction by unusual cold has to be taken into account.

CHAPTER V.

ACTION OF LEAVES.

OF

THEIR NATURE, STRUCTURE, VEINS, EPIDERMIS, STOMATES.—EFFECT LIGHT.-DIGESTION OR DECOMPOSITION OF CARBONIC ACID.-INSENSIBLE PERSPIRATION.-FORMATION OF SECRETIONS.-FALL OF THE LEAF.— FORMATION OF BUDS BY LEAVES.

A LEAF is an appendage of the stem of a plant, having one or more leaf-buds in its axil. In those cases where no buds are visible in the axil, they are, nevertheless, present, although latent, and may be brought into development by favourable circumstances. As this is a universal property of leaves, to which there is no known exception, it follows that all the modifications of leaves, such as scales, hooks, tendrils, &c., and even the floral organs, hereafter to be described, may have the same property.

Buds are, however, formed with difficulty by such modified leaves as brown scales or mere membranous expansions, even although the latter are capable of assuming the usual condition of leaves when anything occurs to increase their force of development. The more green, the more succulent, the more perfectly organized a leaf is, the greater is its power of forming axillary buds; and vice versa. That the power of forming buds really exists among leaves even when most unlike their usual state is proved by such examples as that at p. 90 of this work, where they are appearing even from among carpellary leaves. In the Author's Elements of Botany, p. 75, a case is figured of buds from the axils of leaves in the state of petals, and he has now before him an example of a Clematis Sieboldi, received from Mr. Wilson, Gardener to the Earl of Burlington, in which six buds are formed in the axils of stamens. Such examples are, however, wholly exceptional.

All leaves arrive at their final condition through intermediate states; and if their growth is arrested by any cause, whether

DEVELOPMENT OF LEAVES.

55

constitutional or accidental, then they remain fixed in the state in which the arrest occurred. Owing to this cause, we find them in the form of points, scales, straps, or perfect organs on the same plant.

The history of their development has been explained by M. Trécul better than by any other writer. The following is the substance of his remarks:-The stem terminates in a very delicate cellular tumour (growing point) from the sides of which the leaves are developed. These first present themselves in the shape of still smaller tumours, alternate, opposite, or verticillate. When opposite or verticillate leaves are to be united at the base, a circular elevation precedes them on the axis; when they are not confluent the tumours are isolated; lastly, when alternate leaves are sheathed, the sheath either takes its rise from a circular eminence round the stem, or else the rudimentary tumour which first shows itself, enlarges, and finally embraces the stem. Leaves are developed after four principal types, the centrifugal (from below upwards), the centripetal (from above downwards), the mixed and the parallel. In the CENTRIFUGAL formation all the parts are formed from below upwards, the leaf is pinnate, and furnished with stipules, the petiole (rachis) first makes its appearance; on its sides come the stipules, then the lower pair of leaflets, then the second pair, then the third, fourth, and so on. If the leaf is supra-decompound the primary tumour or rachis in growing throws out secondary petioles, and these latter tertiary ones, &c., according to the composition of the leaf at the extremity of which the leaflets form. The development of simple leaves may be explained by that of the Lime-tree. This leaf commences with a rudimentary tumour at the apex of the stem. This tumour lengthens and enlarges, leaving at its base a contraction which represents the petiole. The blade, at first entire, is soon divided from side to side by a sinus. The lower lobe is the first secondary nervure; the upper part is subdivided in the same manner five or six times, in order to form as many nervures of the same sort. About the time that the third or fourth upper lobe makes its appearance, the lower one, which was formed first, having also extended, becomes sinuous at its edges. These sinuosities are the indications of the origin of five or six ramifications of the lower nervure. At this period the leaf is furnished with as many toothings as there are nervures. But in a short time fresh toothings appear between those first formed, these correspond with the development of as many secondary nervures. The nervures which unite transversely with the adjoining nerves are produced at the same time. The hairs which cover the under surface of the leaf are also formed from below upwards. Thus the various kinds of nervures in the leaf of a Lime-tree develope like the different sorts of shoots in the tree that bears them. To leaves developed CENTRIPETALLY belong those of

56

ANATOMY OF LEAVES.

Burnet, Roses, &c. In these plants the terminal leaflet is produced before all the others, the pair of leaflets nearest the apex of the leaf next make their appearance, then the second pair, the third, and so on, from the apex to the base. All digitate and radiate leaves belong to the centripetal mode of formation. In Potentilla reptans, &c., not only do the leaflets grow from the top downwards, but their secondary nervures and toothings appear in the same way. In plants belonging to the MIXED TYPE, the two preceding modes of development are combined. The lobes of the leaves of Acer platanoïdes, &c., and the midribs of those lobes which are digitate, form from above downwards; the lower lobes are produced last, but the secondary nervures and the toothings are developed like those of the Lime-tree. The PARALLEL FORMATION is common to many Endogens. All the nervures are formed in a parallel manner; but in this, as well as in the case of dicotyledonous plants, the sheath is the first that makes its appearance (Carex). The leaf lengthens more especially by the base of the blade, or that of the petiole when it exists (Chamaerops); the sheath, often extremely small, does not increase in growth till a later period; the same holds true with regard to Exogens when they have a sheath. As regards the growth of leaves, which has been confounded with their mode of formation, M. Trécul has shown that all leaves which are furnished with sheaths, or those which are very much protected by having their lower portions enveloped with other organs, grow most by the base; on the other hand, those of which the whole petiole is exposed to the air at a very early period, in consequence of the stem lengthening, grow much more towards the upper part of the petiole (Tropæolum majus, Æsculus, &c.) Nevertheless, there is a short space near the insertion of the petiole in the blade where the increase in length is less than a little lower down.

Considered with respect to its anatomical structure, a leaf is an expansion of the bark, consisting of cellular substance, among which are distributed veins. The former is an expansion of the rind; the latter consist of woody matter arising from the neighbourhood of the pith, and from the liber. As the tissue forming veins has a double origin, it is arranged in two layers, united firmly during life, but separable after death, as may be seen in leaves that have been lying for some time in water. Of these layers, one is superior and arises from the neighbourhood of the pith, the other inferior and arises from the liber; the former maintains a connection between the wood and leaf; the latter establishes a communication with the bark. Since sap, or ascending fluid, rises through the wood, and more especially the alburnum, afterwards descending through the

liber, it follows from what has been stated, that a leaf is an organ of which the upper system of veins is in communication with the ascending, and the lower system with the descending, current of sap.

This statement must be understood to express nothing more than what may be called the typical condition of a leaf, and especially of such as are thin and abundantly furnished with veins. In succulent plants no layers can be distinguished, but the veins are dispersed among pulp; in membranous leaves it is uncertain whether more than one layer is present; finally in some there are three distinct layers, as in Brexia spinosa, which has a middle system of coarsely and irregularly netted veins, and immediately below both the upper and lower skin a layer of much finer and closer oblique veins.

A leaf has moreover a skin, or epidermis, drawn over it. This epidermis is often separable, and is composed of an infinite number of minute cells or cavities, originally filled with fluid, but eventually dry and filled with air. In plants

growing naturally in damp or shady places it is very thin; in others inhabiting hot, dry, exposed situations, it is hard and thick; its texture varies between the two extremes, according to the nature of the species. The epidermis is pierced by numerous invisible pores, called stomates, through which the plant breathes and perspires. Such stomates are generally largest and most abundant in plants which inhabit damp and shady places, and which are able to procure at all times an abundance of liquid food; they are fewest and least active under the opposite conditions. It will be obvious, that, in both these cases, the structure of a leaf is adapted to the peculiar circumstances under which the plant to which it belongs naturally grows. Now as this structure is capable of being ascertained by actual inspection with a microscope, it follows, as a necessary consequence, that the natural habits of an unknown plant may be judged of with some certainty by a microscopical examination of the structure of its epidermis. The rule will evidently be, that plants with a thick epidermis, and only a few small stomates, will be the inhabitants of situations where the air is dry and the supply of liquid food small; while those with a thin epidermis, and a great number