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CHAP. IV.

ACTION OF LEAVES.

Their Nature, Structure, Veins, Epidermis, Stomates.-Effect of Light.-Digestion or Decomposition of Carbonic Acid.— Insensible Perspiration. - Formation of Secretions. — Fall of the Leaf. Formation of Buds by Leaves.

59. 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 developement 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, have the same property.

60. 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 connexion between the wood and leaf; the latter establishes a communication with the bark. As sap, or ascending fluid,. rises through the wood, and principally 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.

61. A leaf has moreover a skin, or epidermis, drawn all over it. This epidermis is often separable, and is composed of an infinite number of minute 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 very hard and thick; and 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 considerable 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 extremely small; while those with a thin epidermis, and a great number of large stomates, will belong to a climate damp and humid; and intermediate degrees of structure will indicate intermediate degrees of atmospherical and terrestrial conditions. It is, however, to be observed, that the relative size of stomates is often a more important mark in investigations of this nature than their number; those organs being in many plants extremely numerous, but small and apparently capable of action in a very limited degree; while in others, where they are much less numerous, they are large and obviously very active organs. Thus the number of stomates in a square inch of the epidermis of Crinum amabile is estimated at 40,000, and in that of Mesembryanthemum at 70,000, and of an Aloe at 45,000; the first inhabiting the damp ditches of India, the last two natives of the dry rocks of the Cape of Good

Hope but the stomates of Crinum amabile are among the largest that are known, and those of Mesembryanthemum and Aloe are among the smallest, so that the 70,000 of the former are not equal to 10,000 of the Crinum. Again, the Yucca aloifolia has four times as many stomates as a species of Cotyledon in my collection, but those of the latter are about the 7 of an inch in their longer diameter, large and active, while the stomates of the Yucca are not more than 2500 of an inch long in the aperture, and comparatively inert. The Yucca, therefore, with its numerous stomates, has weaker powers of perspiration and respiration than the Cotyledon.

62. A leaf, then, is an appendage of the stem of a plant, consisting of an expansion of the cellular rind, into which veins are introduced, and enclosed in a skin through which respiration and perspiration take place. It is in reality a natural contrivance for exposing a large surface to the influence of external agents, by whose assistance the crude sap contained in the stem is altered and rendered suitable to the particular wants of the species, and for returning into the general circulation the fluids in their matured condition. In a word, the leaf of a plant is its lungs and stomach, traversed by a system of veins.

63. As the leaf is an extension of the rind of a stem, its epidermis is also an extension of the skin of the same part; and hence it is that in

plants which produce no true leaves, such as the Stapelia, the office of the leaf is performed by the rind and epidermis of the bark.

6. The functions of respiration, perspiration, and digestion, which are the particular offices of leaves, are essential to the health of a plant; its healthiness being in proportion to the degree in which these functions are duly performed. Consequently, whatever tends to impede the free action of leaves, tends also to diminish the healthiness of a plant.

65. These functions are performed by means of the vital forces of vegetation, which we cannot estimate or comprehend, assisted by the influence of an external agent, the nature of whose action may be understood from its effects. That agent is solar light.

66. It is the property of solar light, when striking upon the leaf of a plant, to cause: 1. A decomposition of carbonic acid; 2. An extrication of nitrogen; and, 3., Insensible perspiration. By their vital forces plants appear to decompose water, independently of the action of light.

67. Carbonic acid is originally introduced into the interior of a plant, either dissolved in the water it imbibes by its roots, or by attraction from the atmosphere, or by the combination of the oxygen obtained by a decomposition of water or otherwise, with the carbon in its interior. When a leaf is exposed to the direct influence of the

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