As far as is yet known, solar light alone has the power of producing any practical effect upon vegetation. That of the moon has, however, been shown to be not without influence. That the moon has a great mechanical effect upon our globe is undisputed. Of this, we need not say that the perpetually alternate ebbing and flowing of the tide affords the most evident proof. But, whilst the effects of the moon are admitted to be extremely powerful in this respect, the influence of her light, except as regards illumination, has been often considered by scientific men as inappreciable; and the proverbs to the contrary, current among the unlearned, have been accordingly estimated as popular errors. It has, however, been at last demonstrated that the moon's rays are very far from powerless. We learn from a note by M. Zantedeschi (Comptes Rendus, October, 1852), that these rays do affect vegetation. This philosopher states that, "the influence, physical, chemical, and physiological of the moon's light, which has hitherto been the object of so much research and speculation amongst scientific and agricultural writers, has been recently investigated by him in consequence of his having had occasion to give a historical summary of the works on the subject. In the course of his inquiries he found it necessary to clear many doubtful points, in doing which his attention was forcibly arrested by the movements exercised in mere moonlight, under certain circumstances, by the organs of plants; and this led him to make the whole subject a serious and profound study. His observations were commenced in 1847, in the Botanic Garden at Venice; they were continued in 1848 in the Botanic Garden at Florence, and at Padua in 1850, 1851, and 1852. In the whole series of his experiments M. Zantedeschi always remarked certain motions in plants having a delicate organization as soon as they were brought under the influence of the lunar rays. In those experiments the rays were always diffused, being neither concentrated by lens nor mirror. Such movements could not be obtained by the action of heat, in whatever way thermal influences were applied. It was in vain to elevate or depress the temperature: in the absence of moonlight the phenomena in question could not be elicited.

The plants on which M. Zantedeschi principally experimented were Mimosa ciliata, Mimosa pudica, and Desmodium gyrans. He always took great care to determine exactly the position of the leafstalks and leaflets of the plants after they had been exposed to the open air, and before they were directly illuminated by the lunar rays. He thus avoided any causes of error which might have arisen from the imperceptible motion of the air, or from a slight change of temperature; and he satisfied himself fully that the effects observed did result entirely from the action of the rays of light from the moon. Without entering into minute details, it is sufficient to say that the results were ascertained when the temperature of the air was 70° Fahr.; and when Saussure's hygrometer indicated a medium state of humidity. Under such conditions, the leafstalks of Mimosa ciliata were raised half a centimetre, or about four-tenths of an inch; those of the Mimosa pudica were raised one inch and two-tenths; whilst the leaflets of Desmodium gyrans exhibited distinct vibrations. It was thus demonstrated that moonlight has the power, per se, of awakening the Sensitive Plant, and consequently that it possesses an influence of some kind on vegetation. It is true that the influence was very feeble, compared with that of the sun; but the action, such as it is, is left beyond further question. This being so, the question remains; what is the practical value of the fact? It will immediately occur to the reader that possibly the screens which are drawn down over hothouses at night, to prevent loss of heat by radiation, may produce some unappreciated injury by cutting off the rays of the moon, which Nature intended to fall upon plants as much as the rays of the sun.

Even artificial light is not wholly powerless. De Candolle succeeded in making Crocuses expand by lamp-light, and Dr. Winn, of Truro, has suggested that the oxy-hydrogen lamp may be made subservient to horticulture in the long dark days of winter. It does not, however, appear that this hypothesis rests upon any experimental basis.

The mere fact of plants absorbing water from the earth would render it probable that they have some means of parting with a portion of it by their surface; but that they do perspire

is susceptible of direct proof, and is by no means a mere matter of inference. We do not indeed see vapour flying off from the surface of plants; neither do we from that of animals, except when the air is so cold as to condense the vapour; yet we know that in both cases perspiration is perpetually going on, and it would appear that in plants it takes place more abundantly than in animals. If a plant covered with leaves is placed under a glass vessel, and exposed to the sun, the sides of the vessel are speedily covered with dew, produced by the condensation of the insensible perspiration of the plant. If the branch of a plant is placed in a bottle of water, and the neck of the bottle is luted to the branch, so that no evaporation can take place, nevertheless the water will disappear; and this can only happen from its having been abstracted by the branch which lost it again by insensible perspiration. Hales, an excellent observer, devised many experiments connected with this subject;* among others the following, which he relates thus:-" August 13. In the very dry year 1723, I dug down two and a half feet deep to the root of a thriving baking Peartree, and laying bare a root half an inch in diameter (Fig. XIV.), I cut off the end of the root at i, and put the remaining stump (in) into the glass tube d r, which was an inch in diameter, and eight inches long, cementing it fast at r; the lower part of the tube d z was eighteen inches long, and a quarter of an inch diameter in bore. . . . Then I turned the lower end of the tube (2) uppermost, and filled it full of water, and then immediately immersed the small end ≈ into the cistern of mercury at the bottom, taking away my finger which stopped up the end of the tube z. . . . The root imbibed the water with so much vigour, that in six minutes' time the mercury was raised up the tube d z as high as z, namely, eight inches. . . . The next morning at eight o'clock the mercury was fallen to two inches in height, and two inches of the end of the root i were yet immersed in water. As the root imbibed the water, innumerable air bubbles issued out at i, which occupied the upper part of the tube at r as the water left it." On another occasion

2

* See Vegetable Staticks, London, 1727.

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he planted a sunflower three and a half feet high in a garden pot, which he covered with thin milled lead, cementing all the joints so that no vapour could escape except through the sides

of the pot and through the plant itself; but providing an aperture, capable of being stopped, through which the earth in the pot could be watered. After fifteen days, viz., from July 3 to August 8, he found, upon making all necessary allowances for waste, that this sunflower plant, three feet and a half high,

with a surface of 5616 square inches above the ground, had perspired as follows:—

and that when the dew was copious, or there was rain during the night, the plant and pot were increased in weight two or three ounces. Other persons have instituted other experiments of a similar nature, the result of all which is, that the insensible perspiration of plants is very considerable. Hales says his sunflower perspired seventeen times more than a man. There is, however, this important peculiarity in vegetable perspiration, that it takes place only or principally in sunlight. The last experiment shows that, while the sunflower was losing from twenty or thirty ounces of water daily during the day, it lost only three ounces during the night without dew, and that there was no loss whatever if a slight dew were present. Here it is probable that the small amount which was lost at night was parted with by the sides of the garden pot, and that the plant itself lost nothing; for it is in evidence that the perspiration of plants is in proportion to the quantity of sunlight that strikes them, and that in darkness they perspire little or not at all. It is no doubt true, that in a dry atmosphere plants will lose their water day and night; but it is equally certain that under such circumstances they will lose very much more by day than by night. They will, however, lose much more by day in a dry atmosphere in a given time, than they will in an atmosphere abounding in moisture.

Although perspiration thus appears to be principally excited by the solar rays, and to be in a given plant in proportion to their intensity, yet we are not authorised in concluding that perspiration is not increased or diminished by the medium in

M. De Candolle distinguishes between exhalaison, or perspiration, which is a vital action, and deperdition or evaporation, which is merely physical. But the latter is too small in amount to be worth taking into account for practical purposes.