156

PECULIAR ADAPTATIONS.-WATER-LILY.

it is found that both sides are equally furnished with intercellular passages; and that the number of stomata above these is nearly equal. This is the case, for example, in the common Iris. But there are some instances in which the general plan of structure is completely reversed, the stomata being restricted to the upper surface, and the upper part of the parenchyma being much looser in texture than the lower. This is the case, for example, with the Water-Lily, and other plants whose leaves float on the surface of the water. The thick spongy leaf of the Water-Lily contains a large amount of air-channels, which serve to give it buoyancy; but these are all immediately beneath the upper surface, and communicate with the external air through its numerous stomata; whilst in contact with the lower surface,-which, as it lies upon the water, is cut off from the actions that are usually performed by it,-are two rows of closely-packed cells, corresponding to those generally in contact with the upper surface. In all these instances we observe such a beautiful adaptation of the structure of these wonderfully-organized beings to the circumstances in which they are to live and grow, that the intelligent observer can scarcely feel a doubt of the Wisdom and Omnipotence of the Designing hand which contrived it.

CHAPTER VIII.

OF THE FUNCTIONS OF LEAVES.

251. It is in the leaves, as already stated more than once, that those changes are effected, which convert the crude fluid absorbed by the roots (consisting as it does of little else than water in which is dissolved a very minute proportion of the various matters existing in the surrounding soil) into the proper juice or nutritious sap, -capable not only of supplying to the different parts of the structure the materials necessary for the maintenance of their healthfulness, the repair of injuries, and the production of entirely new parts, -but also of furnishing the ingredients of those several products, which the various tribes of plants may be said almost to create from the elements around them, and which are so valuable to man as articles of diet, as medicines, or as articles of use in his various manufactures. Many of these will have to be considered hereafter under the head of Secretions; but it is interesting to observe here, that, although almost every tribe of plants forms some substance peculiar to itself, some of which are of a highly poisonous character, whilst others are of the mildest and most wholesome nature, they all originate in ascending sap, which is of nearly an uniform character in each tribe.

252. In this process of elaboration, as this conversion has been termed, several distinct changes are involved. The first is the concentration of the fluid by the loss of a considerable proportion of its water, so that the amount of solid matter contained in any quantity of it is much greater than before. This is effected by a process which resembles the perspiration of animals; a large quantity of watery vapour being given off, under favourable

158

LOSS OF FLUID FROM THE LEAVES.

circumstances, from the surface of the leaves as from the pores of the skin of a man.

253. If a glass vessel be placed with its mouth downwards on the surface of a meadow or grass-plot during a sunny afternoon in summer, it will speedily be rendered dim in the interior by the watery vapour which will rise into it, and this will soon accumulate to such a degree as to run down in drops. From an experiment of this kind repeated by Bishop Watson during several successive days, on a meadow which had been cut during a very intense heat of the sun, and after several weeks had been passed without rain. it was calculated by him that an acre of grass land transpires in 24 hours not less than 6400 quarts of water. This is probably an exaggerated statement; as the Bishop does not seem to have been aware how completely transpiration is checked during the night; but it will serve to give an idea of the enormous amount of fluid which must be thus disengaged. Any person walking in a meadow on which the sun is shining powerfully, especially in a hot day in summer, when the grass has not long previously been refreshed by rain, may observe a tremulous motion in distant objects, occasioned by the rising of the watery vapour; exactly resembling that which takes place along the sea-shore, when the sun shines strongly on the pebbles that have been left in a moistened state by the retreating tide.

254. It is necessary, however, to distinguish the evaporation which is the cause of the latter occurrence, from the peculiar function we are now considering; which, as we shall see, is influenced by circumstances that only act during the life of the plant, in such a manner as to prove it to be something of a different character from that which we observe in dead substances. All moist bodies exposed to a tolerably warm and dry atmosphere have a tendency to become dry, the fluid they contain slowly passing off in the form of vapour. The rapidity with which this takes place will depend upon the amount of heat to which they are exposed, and the degree of dryness of the surrounding air. Every one knows that warmth is of great assistance in drying moistened snbstances of any kind; and this results from its promoting the conversion

CONDITIONS OF SIMPLE EVAPORATION.

159

of the water into vapour. It may easily be observed, too, that a damp atmosphere retards the process; and the air sometimes has so large a quantity of vapour suspended in it, that it deposits it as a dew upon dry substances, instead of raising fresh moisture from damp ones.

255. Now the living fabrics of plants are subject, like all other moist substances, to the loss of fluid by evaporation; and this would take place under the conditions just mentioned, from all the parts which have this character, were it not for the protection afforded by the cuticle. This membrane, as formerly stated, covers the whole surface of every plant which is exposed to the air; and, from its dry nature, and the absence of any fluid in its cells, it is not liable to be thus influenced by heat or dryness of the atmosphere, so that it effectually protects from the undue influence of these agents the soft tissues beneath. The difference which results from the presence or absence of this cuticle may be well seen by comparing the long-continued freshness of the leaf of any flowering-plant which is kept in the dark (so that its exhalation, or transpiration of fluid through the stomata, as presently to be explained, is prevented,) and the rapid shrivelling of the frond of a sea-weed, or of any Flowering-Plant that naturally grows beneath the water, when equally exposed to the influence of a warm and dry atmosphere. And, as already noticed, the cuticle is almost invariably found to be the thickest and firmest in plants which frequent very hot and dry situations.

256. Nevertheless the cuticle does not entirely check evaporation; but this takes place from the surface of a dead plant, or any portion of one, as well as from one in the most active vegetation. The shrivelling of Apples long kept, and the loss of weight of Potatoes, are examples of this slow and gradual change. It may be stated, then, that Plants, like other moist soft substances, are liable to part with some portion of their fluid by evaporation, especially when exposed to a warm and dry atmosphere; but that the amount of this loss is far too small to account for the large quantity of vapour, which, as just stated, may be easily ascertained to pass off at certain times from the surface of the living plant.

160

PASSAGE OF VAPOUR THROUGH STOMATA.

257. Now a few simple experiments will show that there is a strong probability that this rapid transpiration takes place through the stomata. If a piece of glass be held near the upper surface of the leaf of a vine actively growing in a hot-house, little effect will be produced upon it; but if it be held near the under surface, the glass will soon be dimmed by the vapour, and in a short time longer this will accumulate so as to form drops. As the upper surface of a vine-leaf is nearly destitute of stomata, whilst the lower is thickly covered with them, the disproportion in these effects is at once explained, if the transpiration really take place through these apertures. Similar experiments on other plants lead to the same general result. Where the stomata are equal in number on the two surfaces, both seem to transpire alike; and when neither possess stomata capable of action, the transpiration is scarcely to be observed. Again, if a plant actively transpiring under the influence of sun-light, be carried into a dark room, its transpiration is immediately and almost entirely checked; and if its stomata be then examined they will be found to have closed. Thus it appears almost unquestionable that the rapid loss of fluid from the whole vegetable surface, but especially from the leaves, which constitutes a most important part of the economy of the living plant, is regulated by the number of stomata which each part contains, and by the degree in which light acts upon them (§ 94.).

258. Still, this kind of transpiration (which, to distinguish it, may be termed exhalation) is not altogether different in its character from the common evaporation first described. It will be recollected that the stomata open into large passages channelled out, as it were, in the fleshy substance of the leaf; and that the walls of these are everywhere composed of a very soft tissue, which is constantly kept moist by the crude sap conveyed so plentifully into the leaves. If, therefore, the atmosphere be admitted into these passages, a very large amount of evaporation must take place from their sides, which resemble, in the want of any protection, the substance of plants habitually living under water; and this evaporation will be the more considerable, as the