same day and night. That if fresh Mushrooms are placed in an atmosphere of pure oxygen, a great part of that gas disappears at the end of a few hours. One portion of the oxygen which is absorbed combines with the carbon of the plant to form carbonic acid; whilst another part appears to be fixed in the vegetable, and to be replaced, at least in part, by nitrogen disengaged by the Mushroom. That when fresh Mushrooms remain some hours in an atmosphere of nitrogen, they modify very slightly the nature of that gas. The sole effect produced is confined to the disengagement of a small quantity of carbonic acid, and sometimes to the absorption of a very small quantity of nitrogen."

But, although the experiments of phytochemists have led to these general conclusions, it is not at all probable that the respiratory functions of plants are limited to the decomposition and recomposition of carbonic acid. It has already (page 360.) been stated that Messrs. Edwards and Colin have proved that water is decomposed in the act of germination, the hydrogen being fixed and the oxygen set free; and there can be little doubt that this phenomenon occurs in plants during other periods, perhaps all periods, of their vigorous growth.

Théodore de Saussure found that germinating seeds absorb nitrogen. It has been shown by M. Boussingault, that plants abstract from the air a quantity of this gas, which they fix in their tissue. But under what circumstances, or in what state, this element is fixed in plants is unknown at present. Nitrogen may enter directly into plants if their green parts are fit to fix it; it may pass into plants with the aerated water absorbed by the roots; and it may be possible, says M. Boussingault, that, as some suppose, there exist in the atmosphere very small quantities of ammoniacal vapour. M. Payen has also ascertained that this gas exists in abundance in plants. He finds it most plentiful in nascent organs, in those in the act of first developement, and in cambium; but he meets with it in wood generally. If a large quantity of water is passed through a stick of elder wood recently cut, the wood loses all its azotised matter, which is carried off by the water: this and some other observations satisfy him that wood gene

rally contains a fluid charged with nitrogen; and he thence infers that the substances employed to prevent the decomposition of wood do so by acting upon the azotised matter, which they coagulate and render insoluble in water. (Comptes rendus, vi. 102. 132., vii. 889.) The accuracy of these statements, although opposed to the opinions formerly entertained by vegetable chemists, seems generally admitted. The cause of the importance of nitrogen to vegetation having been so long overlooked, is well explained by the committee which reported to the Institute upon M. Boussingault's observations, (Comptes rendus, vi. 130.) "One has been involuntarily led to suppose that nitrogen takes no part in the phenomena of vegetation, because we know that in its gaseous state it enters into combination with much difficulty. Sufficient attention has not been paid to the facility with which, on the other hand, dissolved nitrogen forms energetic combinations, nor to the pasturage of cattle on high mountains, whence there is annually abstracted, in the form of fat or milk, so much nitrogen, which nevertheless can scarcely reach such situations except by the atmosphere."

The result of the foregoing phenomena is, the formation of numerous principles peculiar to the vegetable kingdom, and the deposition of others which are foreign to plants, but which have been introduced into their system in the current of the sap. Thus are produced the silex of the Grass tribe; the sugar of the Cane, and of various fruits; the starch of Corn, Potatoes, and other farinaceous plants; the gum of the Cherry; the tannin of the Oak; and all those multitudes of alkaline, oily, resinous, and other principles, of which the modern chemist has ascertained the existence. These, belonging to the province of Chemistry rather than of Botany, need not be recapitulated here. It will be more useful to make some general observations upon the practical application of the physical laws we have been examining.

It is, however, desirable to explain that the old ideas of certain secretions of inorganic matter being formed by the vital forces of plants are altogether disproved. It has, in particular, been asserted that silex is formed by the vis formatrix of vegetation but Dr. Daubeny has shown, by well conducted

experiments, that siliceous matter disappears in Corn, in proportion as care is taken to deprive the medium in which Corn grows of all access to siliceous solutions. He states that, upon a general review of his experiments, the results indicate decisively a connexion between the quantity of earthy matter contained in a plant, and the readiness with which the plant is supplied with such matter from without; since, even if we confine ourselves to the examination of the parts above ground, where there can be no suspicion of any foreign admixture, it is found, as stated in the tables of experiments, that the largest amount of calcareous earth (for instance) was obtained from straw grown in Carrara marble; and so on. See, for some curious experiments on this subject, the Transactions of the Linnæan Society, vol. xvii. p. 262.

While, however, both experiment and theory disprove the formation of foreign matters by plants, it seems certain that silica and other earthy matters become, when they enter the tissue of a plant, organisable products, occupying in some cases a definite and invariable position in the structure, as in Grasses. There are some good observations upon this subject in Taylor's Magazine, vol. lxvii. p. 414., by the Rev. J. B. Reade, who states that the skeletons of vegetable tissue remain after all the carbonaceous matter is removed; and that both lime and potash enter as elements into the basis of vegetable tissue. He says he can prove that vessels are actually composed of silica, by showing that, if the latter is removed, no trace of vessel remains.

It is not, however, a necessary inference from these data, that earthy matter is indispensable to vegetable organisation, partly because Dr. Daubeny's experiments above referred to prove the contrary; and, secondly, because Göppert has shown that mineral matter artificially introduced into plants will take entire possession of them, destroying or displacing the vegetable matter, without altering their tissue or their structure. (Comptes rendus, iii. 656.) Moreover, the old experiments of growing plants in pure water, where they are cut off from access to foreign matter, have been repeated by M. Boussingault and M. Colin (Comptes rendus, vii. 889. 949.); the former found that Peas, fed with nothing but air and

water, flowered and ripened seeds; the latter obtained the same result with Beans, Onions, and Polygonum tinctorium.

As light*, if not strictly a vis creatrix, is the great agent by which the decomposition, recomposition, and assimilation of the juices of plants take place; and as it must be obvious that the intensity of the action of vegetable secretions, or their abundance, will depend upon the degree of their elaboration; it follows that these must be in direct proportion to the quantity of light they have been exposed to. As has been observed by the author of the article Botany, in the Library of Useful Knowledge, "We see in practice that the more plants are exposed to light when growing naturally, the deeper is their green, the more robust their appearance, and the greater the abundance of their odours or resins; and we know that all the products to which these appearances are owing are highly carbonised. On the contrary, the less a plant is exposed to sunlight, the paler are its colours, the laxer its tissue, the fainter its smell, and the less its flavour. Hence it is that the most odoriferous herbs are found in greatest perfection in places or countries in which the sunlight is the strongest; as sweet herbs in Barbary and Palestine, Tobacco in Persia, and Hemp in the bright plains of extra-tropical Asia. The Peach, the Vine, and the Melon, also, no where acquire such a flavour as under the brilliant sun of Cashmere, Persia, Italy, and Spain.

"This is not, however, a mere question of luxury, as odour or flavour may be considered. The fixing of carbon by the action of light contributes in an eminent degree to the quality of timber, a point of no small importance to all countries.

"It is in a great degree to the carbon incorporated with the tissue, either in its own proper form, or as resinous or astringent matter, that the different quality in the timber of the same species of tree is principally owing. Isolated Oak trees, fully exposed to the influence of light, form a tougher and a more durable timber than the same species growing in dense forests; in the former case its tissue is solidified by the greater

* For some highly interesting experiments upon the effect of light passing through coloured media, in determining the appearance of the lower plants and animals, see Morren's Essais sur l'Hétérogénie dominante; Liège, 1838.

quantity of carbon fixed in the system during its growth. Thus we have every reason to believe that the brittle Wainscot Oak of the Black Forest is produced by the very same species as produces the tough and solid naval timber of Great Britain. Starch, again, in which carbon forms so large a proportion, and which, in the Potato, the Cassava, Corn, and other plants, ministers so largely to the nutriment of man, depends for its abundance essentially upon the presence of light. For this reason, Potatoes grown in darkness are, as we say, watery, in consequence of no starch being developed in them; and the quantity of nutritious or amylaceous matter they contain is in direct proportion to the quantity of light to which they are exposed. For this reason, when orchard ground is under-cropped with Potatoes, the quality of their tubers is never good; because the quantity of light intercepted by the leaves and branches of the orchard trees, prevents the formation of carbon by the action of the sun's rays upon the carbonic acid of the Potato plant. Mr. Knight has turned his knowledge of this unquestionable fact to great account, in his application of the principles of vegetable physiology to horticultural purposes."

That the intensity of light does in fact vary most materially in different climates, is a matter of inference from the difference of temperature. But it never has been actually measured, to my knowledge, by any one except Herschel, who, in a communication made to the Athenæum newspaper of April 25. 1835, speaks of an instrument called an actinometer, which he finds extremely sure and uniform in its indications. This instrument gives the force of sunshine at the Cape of Good Hope as 48.75°, while ordinary good sunshine in England is only from 25° to 30°.

The principal part of the secretions of plants is deposited in some permanent station in their system; as in the roots of perennials, and the bark and heartwood of trees and shrubs. It appears, however, that they have, besides this, the power of getting rid of superfluous or deleterious matter in a material form. In the Limnocharis Plumieri there is a large pore terminating the veins of the apex of the leaf, from which water is constantly distilled. The pitchers of Nepenthes, which are only a particular kind of leaves, secrete water enough to fill