half their cavity. But, besides this more subtle fluid, secretions of a grosser quality take place in plants. The honey dew, which is so often attributed to insects, is one instance of the perspiration of a viscid saccharine substance; the manna of the Ash is another; and the gum ladanum that exudes from the Cistus ladaniferus is a third instance of this kind of perspiration. It is, however, by the roots, that the most remarkable secretions are voided.

It has long been known that some plants are incapable of growing, or at least of remaining in a healthy state, in soil in which the same species has previously been cultivated. For instance, a new apple orchard cannot be made to succeed on the site of an old apple orchard, unless some years intervene between the destruction of the one and the planting of the other; in gardens, no quantity of manure will enable one kind of fruit-tree to flourish on a spot from which another tree of the same species has been recently removed; and all farmers practically evince, by the rotation of their crops, their experience of the existence of this law.

Exhaustion of the soil is evidently not the cause of this, for abundant manuring will not supersede the necessity of the usual rotation. The celebrated Duhamel long ago remarked that the Elm parts by its roots with an unctuous dark-coloured substance; and, according to De Candolle, both Humboldt and Plenck suspected that some poisonous matter is secreted by roots; but it is to Macaire, who, at the instance of the first of these three botanists, undertook to enquire experimentally into the subject, that we owe the discovery that the suspicion above alluded to is well founded. He ascertained that all plants part with a kind of fæcal matter by their roots; that the nature of such excretions varies with species or large natural orders: in Cichoraceae and Papaveracea he found that the matter is analogous to opium, and in Leguminosæ to gum; in Gramineæ it consists of alkaline and earthy alkalies and carbonates, and in Euphorbiaceae of an acrid gum-resinous substance. These excretions are evidently thrown off by the roots, on account of their presence in the system being deleterious; it was also found, by experiment, that plants artificially poisoned parted with the poisonous matter by their roots. For

instance, a plant of Mercurialis had its roots divided into two parcels, of which one was immersed in the neck of a bottle filled with a weak solution of acetate of lead, and the other parcel was plunged into the neck of a corresponding bottle filled with pure water. In a few days the pure water had become sensibly impregnated with acetate of lead. This, coupled with the well-known fact that plants, although they generate poisonous secretions, yet cannot absorb them by their roots without death, as, for instance, is the case with Atropa Belladonna, seems to show that the necessity of the rotation of crops is more dependent upon the soil being poisoned than upon its being exhausted.

While oxygen and carbon are thus essential to vegetation. when not administered in excess, almost all other gases are more or less deleterious.

Although nitrogen is, as has already been shown, an important and constant element in vegetation when dissolved or obtained by the decomposition of the atmosphere, yet in a pure gaseous state it seems incapable of affording any support to the developement of plants, as proved by Théodore de Saussure, who found that, five days after immersion in pure nitrogen, the buds of poplars and willows were in a state of decay. But he inclined to ascribe the apparent incapability of leafy plants to absorb nitrogen to the artificial conditions under which the experiments were conducted. And this is probable, considering the nature of modern discoveries with respect to the action of nitrogen in vegetation.

Pure hydrogen seems to act unequally upon vegetation. Saussure found that a plant of Lythrum Salicaria, after five weeks, had caused no alteration in a known volume of hydrogen by which it was surrounded, and had not itself expeperienced any apparent effect. Sir Humphry Davy, however, states that some plants will grow in an atmosphere of hydrogen, while others quickly perish under such treatment.

Drs. Turner and Christison found that so small a quantity as Toooo of sulphurous acid gas, a proportion so minute as to be imperceptible to the smell, was sufficient to destroy the life of leaves in forty-eight hours. The same observers state, in an excellent paper in Brewster's Journal for January,

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1828, the effects of other gases upon plants. I much regret that want of space prevents my giving their experiments in detail: the results, which are as follows, are very important. Hydrochloric, or muriatic, acid gas was found to produce effects not inferior, nay, even superior, to those of the sulphurous acid. It was found that so small a quantity as a fifth of an inch, although diluted with 10,000 parts of air, destroyed the whole vegetation of a plant of considerable size in less than two days. "Nay, we afterwards found that a tenth part of a cubic inch, in 20,000 volumes of air, had nearly the same effects. In twenty-four hours the leaves of a laburnum were all curled in on the edges, dry and discoloured; and, though it was then removed into the air, they gradually shrivelled and died. Like the sulphurous acid, the hydrochloric acid gas acts thus injuriously in a proportion which is not perceptible to the smell. Even a thousandth part of hydrochloric acid gas is not distinctly perceptible; a tenthousandth made no impression on the nostrils whatever, although great care was taken to dry thoroughly the vessels used in making the mixtures.

"Chlorine may be expected to have the effects of hydrochloric acid gas; and so indeed it has, but they appear to be developed more slowly. Two cubic inches, in two hundred parts of air, did not begin to affect a mignonette plant for three hours; half a cubic inch, in a thousand parts of air, did not injure another in twenty-four hours: but when the plants did become affected, the same drooping, bleaching, and desiccation were observed.

"Nitrous acid gas is probably as deleterious as the sulphurous and hydrochloric acid gases. In the proportion of a hundred and eightieth, it attacked the leaves of a mignonette plant in ten minutes; and half a cubic inch, in 700 volumes of air, caused a yellowish green discolouration in an hour, and drooping and withering in the course of twenty-four hours. The leaves were not acid on the surface.

"The effects of sulphuretted hydrogen are quite different from those of the acid gases. The latter attack the leaves at the tips first, and gradually extend their operation towards the leafstalks; when in considerable proportion, their effects

began in a few minutes; and if the quantity was not great, the parts not attacked generally survived, if the plants were removed into the air. The sulphuretted hydrogen acts differently; two cubic inches, in 230 times their volume of air, had no effect in twenty-four hours. Four inches and a half, in eighty volumes of air, caused no injury in twelve hours; but, in twenty-four hours, several of the leaves, without being injured in colour, were hanging down perpendicularly from the leafstalks, and quite flaccid; and, though the plant was then removed into the open air, the stem itself soon began also to droop and bend, and the whole plant speedily fell over and died. When the effects of a large quantity, such as six inches in sixty times their volume, were carefully watched, it was remarked that the drooping began in ten hours, at once from the leafstalks; and the leaves themselves, except that they were flaccid, did not look unhealthy. Not one plant recovered, any of whose leaves had drooped before it was removed into the air.

"The effects of ammonia were precisely similar to those of sulphuretted hydrogen just related, except that after the leaves drooped they became also somewhat shrivelled. The progressive flaccidity of the leaves; the bending of them at their point of junction with the footstalk, and the subsequent bending of the stem; the creeping, as it were, of the languor and exhaustion from leaf to leaf, and then down the stem, were very striking. Two inches of gas, in 230 volumes of air, began to operate in ten hours. A larger quantity and proportion seemed to operate more slowly.

"Cyanogen appears allied to the two last gases in property, but is more energetic. Two cubic inches, diluted with 230 times their volume of air, affected a mignonette plant in five hours; half a cubic inch, in 700 volumes of air, affected another in twelve hours; and a third of a cubic inch, in 1700 volumes of air, affected another in twenty-four hours. The leaves drooped from the stem without losing colour; and removal into the air, after the drooping began, did not save the plants. "Carbonic oxide is also probably of the same class, but its power is much inferior. Four cubic inches and a half, diluted with 100 times their volume of air, had no effect in twenty

four hours on a mignonette plant. Twenty-three cubic inches, with five times their volume of air, appeared to have as little effect in the same time; but the plant began to droop when it was removed from the jar, and could not be revived.

"Olefiant gas, in the quantity of four cubic inches and a half, and in the proportion of a hundredth part of the air, had no effect whatever in twenty-four hours.

"The protoxide of nitrogen, or intoxicating gas, the last we shall mention, is the least injurious of all those we have tried; indeed, it appears hardly to injure vegetation at all. Seventytwo cubic inches were placed with a mignonette plant, in a ar of the capacity of 500 cubic inches, for forty-eight hours; but no perceptible change had taken place at the end of that time."

Göppert has also found that hydrocyanic acid in a gaseous state is fatal to vegetation. Numerous experiments upon the action of this and other substances deadly to plants are to be found in this author's dissertation, De Acidi Hydrocyanici Vi in Plantas: Vratislav. 1827.