an objection to Mr. Knight's hypothesis, the fact of the upward growth of the radicle of the misseltoe, at least when the seed is lodged on the under side of the supporting branch. But I now find that Mr. Knight obviates the objection by saying, that the misseltoe has no root, and that the part in question gains the bark only by receding from the light, like the stem and tendrils of other parasitical plants.

I am not acquainted with many plants that are strictly parasitical; but I do not find in those with which I am acquainted any peculiar disposition to recede from the light. The dodder, Cuscuta europea, cannot be said to have it, because it twines round a supporting stem from right to left; so that in its very outset, it must rather approach the light than recede from it; and again, in every new spire or gyration, broom-rape, Orobanche major, does not fly the light, for it comes up quite erect and I have seen many plants of the misseltoe, Viscum album, whose growth is wholly to the south of the point at which they issue from the stem, as well as chiefly ascending. Hence if any part of the germ of a parasitical seed is found to recede from the light, it is most likely, because it is of the nature of a radicle, since radicles are known to do so. Besides, the embryo of the seed of the missletoe is just like many other embryos, furnished with cotyledons, enclosing a plumelet, and what we are bound to call a radicle (though perhaps caulescent), unless for some good reason with which I am not yet acquainted; because it is that part of the embryo which first begins to shoot in the process of germination, and in a direction opposite to the plumelet.

In this opinion I am supported by an authority which I am sure Mr. Knight will respect, namely, that of the great and illustrious Gœrtner, who expressly describes the embryo of the misseltoe as being furnished with a somewhat swollen and capitate radicle, that is, separated from the cotyledons by a slender stipe. All indeed that is situated beneath the cotyledons, may, in the opinion of Gartner, be regarded as a radicle in every embryo whatever;* whereas, with regard to the misseltoe, Mr. Knight's opinion implies that all below the cotyledons is a stem. But will Mr. Knight allow me to cut off the point of it to see whether it will insinuate itself into the bark then? If it is wholly a stem, it ought still to do so. But if it refuses, then it is plain that there was something in the point more than a mere stem. This experiment must be made and succeed before Mr. Knight can establish his position; that is, a graft of the misseltoe must succeed by being bound to the outside of the bark of some stock. If it be said that it would be unfair to cut off the point because it may contain something fitted to make it unite with the supporter, then I contend that this something is the very radicle in question.

De Fruct. et Sem. Introd.

It is true that some botanists have regarded parasitical plants as being altogether destitute of roots, applying to them the term arrhiza, and, perhaps, Linnæus may be squeezed into the number; because in his distribution of the parts of the plant, he describes only a parasitical stem, and says nothing of a parasitical root; though Linnæus's authority will not, perhaps, be regarded as of much weight in this case, when it is recollected that he elsewhere † represents the stems of all trees and shrubs as being merely roots above ground. But the most scientific definitions or descriptions of the root, amongst which I include those of Malpighi and Du Hamel, as well as that of our worthy and learned President, Sir J. E. Smith, ‡ do evidently include parasitical plants; because they represent that part of the plant by which it attaches itself to the substance on which it grows or feeds, as being the root. Besides, there are some parasitical plants that have even conspicuous roots, as any one who has ever seen a mature and complete plant of Orobanche major will acknowledge; and although systematic botanists do describe some plants of the class cryptogamia as being destitute of roots, because they have no visible or conspicuous root appearing as a distinct organ, yet the phytologist knows that this is not absolutely correct.

We may regard the embryo of the misseltoe, therefore, as being furnished with a radicle, though not very conspicuous; and it need not be thought strange if it grows occasionally upwards. We find that roots in general possess a capacity of accommodating themselves to circumstances in the direction which they affect, independent of, and even in opposition to, gravitation. The roots of trees, which are planted in a bottom, near to sloping banks, will extend not merely in a horizontal direction, but will follow the direction of the ascent. An ash tree which is so situated, and is now within my view, has roots at the distance of five yards from the trunk elevated at least three feet above the level of the collar. If a piece of the root of the horse-radish, Cochlearia armoracia, is planted at the depth of 15 inches, it will send up root shoots erect to the surface of the soil; § and if it is planted at the surface of the soil, it will no doubt send down root shoots to the same, or to a greater depth. There are even some stems, or at least fronds, that seem to be wholly indifferent to the direction in which they grow. Many of the lichens which grow upwards when situated on the upper side of a branch, are very well content to grow downwards when situated on the under side; or to grow horizontally when attached to the surface of an upright trunk. The lichen prunastri may be quoted as an example. Further, if gravitation were the sole cause giving direction to the root, there would be no such thing as a root's selecting the best soil, which roots are

* Phil. Bot. sect. 82.

Introd, to Bot. 102.

+ Ibid. sect. 80.

Mawe's Gardener's Dictionary.

well known to do. For then it would have no choice but to descend, unless prevented by an obstacle that could not be surmounted; which might stop it or turn it to the one side, but could not surely make it grow upwards, or ascend a bank; for that would be like making a river to run up a hill.

In short, the more we examine the subject, the more we feel the want of a principle "inherent in vegetable life" to determine the direction of the plant. We see that such a principle must be the cause of many of the other phenomena of vegetation, and why not also of the phenomenon in question. To what but to the operation of such a principle are we to ascribe the movements of Hedysarum gyrans; the irritability of the Mimosa; the spiral ascent of the twining stem, as being directed to the right or to the left respectively, and never otherwise; the phenomenon of the sleep of plants; and, perhaps, of the Horologium Flore? and how shall we account without it for the adaptation of the vegetable structure to the wants of the species, as exemplified in the hollow stems of the grasses, interrupted with knots; and the hollow but knotless scape of the onion inflated in the middle; together with the growth and maturation of the leaves, flowers, and fruit, which are formed complete in all their parts, and arranged in the most appropriate order, long before their ultimate evolution, and totally independent of gravitation, or of the position in which art or accident may happen to have placed them, or of any other cause that is merely either chemical or mechanical? But if gravitation is really the agent that gives direction to the root and stem of plants, then, I presume, there will be no absurdity in inquiring, whether the upright growth of the horns of the stag, and the twisted and spiral growth of the horns of the ram, are not the effect of gravitation also; or whether the teeth of the upper jaw of a man do not grow downwards, and the teeth of the under jaw upwards, by virtue of gravitation.

I am ready to acknowledge Mr. Knight's great merit in the introduction of several important horticultural improvements, as well as in the discovery, or rather in the more complete establishment of certain important phytological facts; but I do not think that he has been equally successful in the establishment of the several hypotheses which he has advanced for the purpose of explaining the phenomena of vegetation. Perhaps my opinion may be singular, but it has not been formed without much examination, especially on the subject of the present hypothesis, which, I think, I have proved to be not only contradicted by the result of Mr. Knight's own experiments, as well as by a multiplicity of well-known facts; but even indebted for its plausibility to a misapprehension of facts. I am, Sir,

Your most obedient humble servant,

P. KEITH.

ARTICLE III.

On the Phenomena of Sanguification, and on the Blood in general. By W. Prout, M.D.

(Continued from p. 25.)

Sanguification.-The chyle from the thoracic duct proceeds into the sanguiferous system, mixes with the general mass of circulating fluids, and almost immediately passes through the lungs, where it is exposed to the air, and appears to undergo the final process, and to be converted into blood. This process is termed respiration; the phenomena of which we shall briefly consider under the following heads of inquiry.

First, Whether the phenomena of respiration be the same in kind in all animals.

Secondly, Whether any other gas can be substituted for oxygen in respiration.

Thirdly, Whether the phenomena of respiration be the same in degree in different classes of animals compared with one another, or in different animals of the same class.

Fourthly, Whether the phenomena be liable to any differences in degree in the same animal at different times.

Fifthly, Whether the blood as a whole, or in part only, be concerned in the production of these phenomena.

First, With respect to all the more perfect animals which have organs of respiration, &c. similar to man, it need only be stated generally, that precisely the same appearances take place. In the inferior animals, some variations occur which it will be proper to notice. Fishes, for example, have no lungs, and do not breathe air; it was, however, an early discovery, which has been confirmed by all succeeding experimentalists, that these animals cannot live in water deprived of air, at least of oxygen, or more properly speaking, they all require oxygen to be brought in contact with their blood, which oxygen is converted into carbonic acid precisely as in the animals which breathe air. This change is most usually effected by the gills, which are in fact their lungs. In some instances, however, it appears to take place differently, as, for example, in the cobitis fossilis, in which a sort of double respiration has been observed by Erman. "In water containing air, the fish breathed as usual through its gills; but if the water was deprived of its portion of oxygen gas, the fish rose above the surface, drew air through its mouth, and swallowed it. The air penetrated the intestines, the blood-vessels of which were reddened; and when it had lost its portion of oxygen gas, the fish discharged it by the rectum." It has also been lately shown by Biot, whose experiments have been still more recently confirmed by Configliachi, an Italian professor, and by Mr. Laroche,

that the air-bladders of fishes contain oxygen gas, which is usually greater in proportion as the animal inhabits deeper waters,* a circumstance which appears to indicate their use to be somewhat analogous to that of the organs of respiration. In animals inferior to fishes, the same phenomena occur. Thus it was early observed by Ray, that insects died very soon if the holes or stig. mata through which the air enters into their bodies were stopped with oil or honey. Derham found also that wasps, bees, hornets, also snails, leeches, &c. soon died under the exhausted receiver of an air-pump; and Scheele and Bergman found that like other animals, they converted the air of the atmosphere into carbonic acid. Vauquelin, however, was the first that made accurate and satisfactory experiments with insects, in which he proved beyond a doubt the accuracy of the above conclusions. This chemist also extended his experiments to the molusca, and obtained precisely the same results; as did Spallanzani, and more recently Haussman. Lastly, Sir Humphry Davy found that even the zoophytes produced similar phenomena.§ Thus then it appears, that all animals convert the oxygen of the atmosphere into carbonic acid gas; and as the blood is the fluid which appears to be operated upon, and to produce this remarkable change in the more perfect animals, we may doubtless conclude that a similar fluid, or one that performs a similar office, is the cause of this change in the inferior animals, although we cannot discover its existence.

Secondly, We come to consider whether any other gas can be substituted for oxygen in respiration. This question was very early decided in the negative. It was also found that animals could not respire even oxygen for any length of time without dilution, and in short, that no other compound, except atmospheric air, in which the proportion of oxygen is only one-fifth of the whole bulk, is capable of supporting life. Pure oxygen and gaseous mixtures containing a larger proportion of this than atmospheric air, appear to destroy life in a short time by over excitation. On the contrary, some gases of a mild and inactive character, as hydrogen and nitrogen, when pure, or in too large proportion, destroy life by the opposite means, or suffocation; while others, as carburetted hydrogen, carbonic acid, &c. seem to prove fatal simply in virtue of their deleterious properties.

gas

Agreeably to what might be expected are the effects which these different non-respirable airs produce upon the blood out of the

* See Berzelius's View of the Progress and Present State of Animal Chemistry, p. 44. Also Annals of Philosophy, vol. v. p. 40.

See an excellent paper on the respiration of fishes, recently published by MM. Provençal and Humboldt, in Mem. d'Arcueil, ii. 259. Those gentlemen found that fishes not only convert oxygen gas into carbonic acid gas, but that a considerable proportion both of oxygen and azote is absorbed during the respira

tory process.

See Johnson's History of Animal Chemistry, vol. iii.

See Davy on Respiration, in Beddoe's Medical Contributions.