68

GARREAU'S EXPERIMENTS.

which a plant grows. Submerged in water, perspiration is necessarily arrested; in an ordinary atmosphere, it will be in proportion to the quantity of elastic vapour the atmosphere may contain; and it is probable, although there are no experiments upon the subject, that it is increased in proportion to the rarefaction of the air.

Among the experiments of M. Garreau to which allusion has been already made, was one on the relative amount of perspiration by the two surfaces of ordinary foliage. Leaves growing on healthy plants were selected, and a circular portion inclosed between two closely fitting glass receivers, so arranged that the leaf formed the division between the two glasses-the upper surface was in the one glass, whilst the under surface of the leaf was in the other. The quantity of moisture given off was ascertained by placing in each glass a weighed portion of dry chloride of calcium, which, being very greedy of moisture, would absorb all the vapour as fast as the surface of the leaves gave it out. The result of this experiment was that the lower surface of leaves gives off, from an equal quantity, three times as much as the upper surface; sometimes the proportion was as high as five to one; and the ratio was independent of the position of the leaf itself. This exhalation of water has some connection with the number and size of the stomates, but is by no means wholly dependent on it, as there is evidently a large quantity of water given off independently of them. The evaporation is most abundant along the course of the nerves, and in those parts of the epidermis, on which there is the least quantity of oily matter. Hence it is apparent why carefully washing with soap and water proves so beneficial (see p. 58). The operation increases greatly the power of evaporation.

All such experiments teach us that under ordinary circumstances the growth of a plant causes the formation and development of certain substances, which in time fill up its pores, check perspiration, and consequently interfere with the nourishment and further growth of the plant. But, on the one hand, it is possible that in hot weather these matters may be useful in checking extreme perspiration, and in diminishing for the time the powers of the plant to absorb too much food from the air, or to part with water and oxygen too rapidly. On the other hand, the effect of rain must be to wash away a portion of these deposits, and so to favour the perspiration and consequent growth of the plant. Moreover, as the more heat a plant is exposed to, the more it perspires, and the faster it grows, the greater will the tendency be to fill up its pores; so it follows that when plants are exposed to great heat in a close house, and are not washed or syringed, they are placed in an unnatural condition, where the care of the gardener defeats, to some extent, the object which he has in view.

FORCE OF SUCTION BY LEAVES.

69

Since a plant does not perspire at night, and since its absorbing points, the roots, remain during that period in contact with the same humid medium as during the day, they will attract fluid into the system of the plant during the night, and, consequently, the weight of the individual will be increased, as Hales found to be the case. In like manner, if plants in the shade are abundantly supplied with moisture at the roots, they also will gain more than they can lose; and, as this will be a constant action, the result must necessarily be to render all their parts soft and watery.

It is evident, from what has been stated, that leaves must derive the food they digest from the earth through the medium of the roots, and from the air; and that they, while alive, maintain a kind of perpetual sucking action upon the stem, which is communicated to the spongelets. That this must be of a very powerful nature is apparent from the fact, that the smallest leaf at the extremity of the branch of a lofty tree must assist in setting in action the absorbing power of roots, at a distance equal, perhaps, to three thousand times its own length. If this reciprocal action is not maintained without interruption, and if anything occurs to check it during the period of vegetation, the plant will suffer in proportion to the amount of interruption. For example, if the roots are placed in a warmer medium than the branches, and are thus induced to absorb fluid faster than the slower action of the leaves can consume it, the superfluous sap will burst through the stem and distend its tissue till the excitability is impaired or destroyed. Or if, on the other hand, a branch is caused to grow in a warm medium, while the roots remain in a very cold medium, the former will consume the liquid sap faster than the latter can supply it, and the consequence will be, that the leaves will die, or the fruit will fall off, or the flowers be unable to set their fruit, from want of a constant and sufficient supply of food; or the fruit will shrivel, or, as it is said, will "shank." Not that it is necessary for the temperature of the earth and air to be equal, for this does not happen in nature; but it is requisite that they should have some near relation to each other.

It is generally, however, believed, that leaves absorb fluid

70

FORMATION OF SECRETIONS.

from the air. Their stomates appear well adapted for that purpose, by their position in most abundance on the under side of leaves; and the possibility of recovering drooping or sickly plants, by syringing their epidermis copiously, seems to render this fact almost certain. It is, however, imagined by some, that leaves have no power of absorbing water, even in an elastic state; and that the renovation of plants by syringing is merely owing to a diminution of perspiration, which is improbable.

It is to the action of leaves,-to the decomposition of their carbonic acid, and of their water; to the separation of the aqueous particles of the sap from the solid parts that were dissolved in it; to the deposition thus effected of various earthy and other substances, either introduced into plants, as silex and metallic salts, or formed there, as the vegetable alkaloids; to the extrication of nitrogen; and, probably, to other causes as yet unknown,-that the formation of the peculiar secretions of plants, of whatever kind, is owing. And this is brought about principally, if not exclusively, by the agency of light. Their green colour becomes intense, in proportion to their exposure to light within certain limits, and feeble, in proportion to their removal from it; till, in total and continued darkness, they are entirely destitute of green secretion, and become blanched or etiolated. The same result attends all their other secretions; timber, gum, sugar, acids, starch, oil, resins, odours, flavours, and all the numberless narcotic, acrid, aromatic, pungent, astringent, and other principles derived from the vegetable kingdom, are equally influenced, as to quantity and quality, by the amount of light to which the plants producing them have been exposed.

It is evident that the possibility of the downward distribution described in the previous paragraph rests upon the certainty that elaborated sap descends and is dispersed through the system. That this occurs is so certain, that it would have been needless to maintain it by further proof if some modern naturalists had not ventured to call it in question. To deny it is tantamount to questioning the existence of wood, or its formation as it appears to our eyes when the bark is stripped from a young growing shoot; as in a Lilac for example. In such a case new wood can be demonstrated to descend from each leaf downwards, till it is lost among the multitude of descending currents.

Upon this has been built the theory that wood grows downwards from leaves-which is now known to be erroneous; but although wood does not descend from leaves, it is certain that the organised matter out of which wood is formed does descend. If, indeed, the descent of elaborated sap is denied, it becomes impossible to explain why, when a ring of bark is removed from a branch, the new growth takes place principally on the upper edge of the wound and very slightly on the lower; and why gum, prepared by the leaves of a Potato, is afterwards found in the tubers, in the final form of starch. It is impossible for those who have any practical acquaintance with living plants not to agree with Prof. Mohl (The Vegetable cell. p. 71, English edition), that a denial of a descending current of sap in bark is quite incomprehensible. Certainly, as he says, it is no improvement upon the theory which men attempt to cast aside to say that increased growth above an annular wound is explained by artificial interruption of the upward current of crude sap, in consequence of which the fluids contained in the upper part of the plant must soon become greatly concentrated and potential for development. "When we can succeed in fattening an animal by depriving it of a portion of its accustomed food this explanation may be received as satisfactory."

This explains why there is usually more wood on the south side of a tree than on the north; and why depriving trees of their branches (and leaves) is invariably attended by a diminution of the quantity of timber.

Upon this curious subject some of the best observations are those of Van Hall, as recorded in one of the reports of the Ray Society. This gentleman remarked that the growth of trees in thickness only commences after the leaves are capable of fulfilling their functions; this was proved by all his measurements. The influence of the leaves upon the increase of trunks in thickness exhibited itself most distinctly in the Italian Poplar. On one of these trees being deprived of almost all its branches, in the month of March, the increase in thickness was proportionably slight during the months of June and July. The growth of a Lime-tree, on the other hand, in which the side branches, also those lower down on the trunk, as well above as beneath the point of measurement, had, for the greater part, been purposely left, was considerable, and increasing every year. An experiment was made with two equal sized Oaks, situated under the same circumstances; all the lateral branches were taken from one and left on the other; the result was, that the increase of thickness, in the tree which had not been pruned, was much more considerable than in the one which had been pruned. But the fact is familiar to all intelligent woodmen.

It may be regarded as an axiom in horticulture that the health of

12

72

IMPORTANCE OF LEAVES.

other parts of a plant is in proportion to the health of leaves. There is no real exception, and the neglect of it is the fruitful parent of failures. Nature has given plants leaves not merely to decorate them or to shade us, but as a part of a wondrous system of life quite as perfect as that of the animal kingdom. It would be of no use for a plant to suck food out of the earth by its roots, unless there was some place provided in which such food, consisting principally of water and mucilage, could be digested, and so converted into the matter which maintains the health of the individual. The stem cannot do this; firstly, because it is a mere channel through which fluids pass; and secondly, because many plants have no visible stem, as in the instance of the Primrose; and yet in all such cases the plant feeds and must digest its food. It is to the leaves that this important office is assigned, and to enable them to execute it God has formed them with wisdom no less infinite than has been displayed in the creation of man. The leaves have veins through which their fluids pass, and cells in which they are held while digesting, myriads of little caverns through whose sides respiration is maintained, a skin to guard them from the air, and pores for carrying off perspiration. A leaf is, in fact, both a stomach and lungs; and to destroy it, is to do the same injury to a plant as would be effected in an animal by the destruction of the parts to which those names are given. Of this we may be certain, that neither taste, perfume, colour, size, nor any other property, can be given to a plant except through the assistance of the leaves; and that the more numerous these are, the larger, and the more luxuriant, so, within certain limits, will be all that a plant is capable of forming. Strip the leaves off a tree, and no more wood will appear until the leaves are restored; feed its roots in the hope of thus compensating for the loss of its leaves, and the stem will be filled indeed with watery matter, but the latter will collect in the interior until it forces its way through the bark, and runs down in putrid streams, as happens to the Mulberry-tree when it is incessantly stripped for silk-worms, and as occurs to trees whose leaves are continually destroyed by a noxious atmosphere. Strip the ripening Grapes of their green garments, and no colour or sweetness will be collected in their berries. Rob the Potato of its foliage, and you will seek in vain for nourishment in its tubers; and so of all things else. On the other hand, leave the Mulberry, the Vine, and the Potato uninjured, to the genial influence of the sun and the air, and the dews of heaven, and wood is formed in the one case, sugar and colour in the other-and flour, the staff of life, in the last: and these products will all be in exact proportion to the health and abundance of the foliage. Why then mow off the leaves of Strawberry plants in the autumn as some do ?-the only effect of which must be to rob the plants of the materials out of which the fruit of the succeeding year is to be produced, and to destroy the natural protection afforded during winter by