"I am inclined to refer to this cause the well-known fact, of which many cases occurred this winter, that the sudden exposure of frozen plants to warmth will kill them; though they may not suffer if warmed gradually. In such cases, it may be supposed that the air, forced into parts not intended to contain it, is expanded violently,and thus increases the disturbance already produced its by expulsion from the proper air cavities; while,on the other hand, when the thaw is gradual, the air may retreat by degrees from its new situation without producing additional derangement of the tissue. It is also possible that leaves,from which their natural air has been expelled by the act of freezing, may, from that circumstance, have their tissue too little protected from the evaporating force of the solar rays, which we know produce a specific stimulus of a powerful kind upon those organs.

"These circumstances are, in themselves alone, sufficient to account for death being produced in plants by frost; and it is chiefly to such as these, that Professor Morren has directed his attention. It however appears to me that there are some other points of im portance to which observers have not applied themselves.

"The green coloring matter of leaves, or chlorophyll, is certainly affected by so little as only two or three degrees of frost. In Stapelia, when thawed, it is found collected into clusters, and apparently half dissolved. In Euphorbia Tirucalli, when the plant is alive, it is extremely abundant, and consists of distinct spheroidal transparent particles, but, after a slight freezing, a considerable part of it disappears, and the remainder loses its transparency .becomes fusiform,is sometimes surrounded by coagulated gelatinous color-? less matter, and many of the particles appear as if burst. In the green subcutaneous parenchyma of the leaf of Hibiscus Rosa Sinensis, the vesiclesforming the sides of the air chambers are filled with distinct, angular, deep green particles, which, after freezing, become amorphous, and seem as if partially dissolved. It is possibly to the decomposition, of which these appearances are the incipient signs, that the extremely offensive odor of some frost-bitten plants, especially the Laurustinus, when thawed, is to be ascribed.

" The amylaceous matter, which is so abundant in many plants, also undergoes altera tion. This has been remarked by Professor Morren, who found that when potatoes are frozen, a part of their starch disappears, leaving the deformed integuments behind it, and he suspected that the starch thus lost had furnished the sugar formed in the process of freezing this tuber. I believe it will be found a general fact, that starch is materially altered by frost, for I have always found that the amylaceous particles seem less abundant in a plant after freezing than before, and of those which remain, a part is generally becoming amorphous, clustered together, and certainly diminished in size. This is particularly striking in Hibiscus militaris. In that plant the cells of the pith abound in amylaceous granules, and are often quite filled with them; and they also occur abundantly inside the cells of the bark, of the medullary rays, and even of the tubes of the wood, and, in short, everywhere except inside the woody tubes of the liber; so that a thin slice of the stem of this plant, treated with iodine, forms a most beautiful microscopical object. But after being frozen, a great part of the starch disappears, and the particles which remain are not more than a half or a quarter of their former size.

I have not, however, remarked among them any appearance of dissolving; neither have I been able to observe any change in the curious double-headed bodies, in form resembling dumb-bells, found in the vessels of Euphorbias, and supposed to be a state of amylaceous matter, because iodine colors them violet; they appeared to me to be in precisely the same state before and after the plant was frozen to death. M. Pay en, however, denies that any starch whatever is lost in frozen potatoes ( Comptes rendus, vi. 345;) but as only a small part of his important treatise on amylaceous matter has reached this country, I am unable to state in what way he explains the action of cold upon this substance.

"Finally, it appears that frost exercises a specific action upon the latex, destroying its power of motion. If, as Prof. Schultz supposes, this is the vital fluid of plants, such a fact would alone account for the fatal effects of low temperature. In all the cases I have observed frost coagulates this fluid, collecting it into amorphous masses. In Stnpelia, where the laticiferous vessels are easily found, the latex itself is so transparent, that it is difficult to perceive it in a living state, even with the best glasses; but after freezing it is distinctly visible, resembling half coagulated water. In the Hibiscus above mentioned, the stem is covered with long, rigid, simple hairs, filled with a plexus of capillary laticiferous vessels of extreme tenuity, but in which the motion of the latex may be seen beautifully with the one-eighth of an inch object glass of an achromatic microscope. Upon being thawed, after freezing, all this apparatus is found reduced to some misshapen separate sacs of fine grumous matter, in which no motion can be detected.

That these vessels lose their vitality after freezing, may indeed be seen withont the aid of a microscope; for if a stem of a Ficus elastics, or a Euphorbia, or any such plant, which discharges an abundance of milk when wounded, be first frozen, and then thawed, no milk will follow the incision.

"From these facts, I think we must draw the conclusion, that the fatal effect of frost upon plants is a more complicated action than has been supposed; of which the following are the more important phenomena:

"1. A distention of the cellular succulent parts, often attended by laceration, and always by a destruction of their irritability.

"2. An expulsion of air from the eriferous passages and cells.

"3. An introduction of air, either expelled from the air passages, or disengaged by the decomposition of water, into parts intended exclusively to contain fluid.

"4. A chemical decomposition of the tissue and its contents, especially of the chlorophyll.

"5. A destruction of the vitality of the latex, and a stoppage of the action of its vessels.

"6. An obstruction of the interior of the tubes of pleurenchyma, by the distension of their sides.

"These phenomena may be considered in part mechanical, in part chemical, and in part vital. The two latter are beyond our control, and probably depend, in part, upon the quality of fluid and organic matter, which may resist the action of the cold in different degrees, according to their various modifications; and, in part, upon specific vitality. Salt and water freeze at various temperatures, according to the density of the mixture, from 4° to 27°; oil of turpentine at 14°; oil of bergamot at 23°; vinegar at 28°; milk at 30° water at 32°; olive oil at 36°; oil of anise at 50°; and it is not to be doubted, that in like manner, the fluid contents of plants, which we know are indefinitely modified, will resist the action of cold in very different degrees.

"The mechanical action of frost may, however, undoubtedly be guarded against to a great extent. It is well known, that the same plant growing in a dry climate, or in a dry soil, or in a situation thoroughly drained from water during winter, will resist much more cold, than if cultivated in a damp climate, or in wet soil, or in a place affected by water in winter. Whatever tends to render tissue moist, will increase its power of conducting heat, and consequently augment the susceptibility of plants to the influence of frost; and whatever tends to diminish their humidity, will also diminish their conducting power, and with it their susceptibility; this is an invariable law, and must consequently be regarded as a fundamental principle in horticulture, upon attention to which all success in the adaptation of plants to a climate less warm than their own, will essentially depend. The destructive effects of frost upon the succulent parts of plants, or upon their tissue, when in a succulent condition, may be thus accounted for, independently of the mechanical expansion of their parts; indeed, it is chiefly to that circumstance, that Dr. Neuffer ascribes the evil influence of cold in the spring; for he found that at Tubingen, nearly all the trees contain eight per cent more of aqueous parts in March, than at the end of January: and the experience of the past winter shows, that the cultivation of plants in situations too much sheltered, where they are liable to be stimulated into growth, and consequently to be filled with fluid, by the warmth and brightness of a mild protracted autumn, exposes them to the same bad consequences as growing them in damp places, or where their wood is not ripened, that is to say, exhausted of superfluous moisture, and strengthened by the deposition of solid matter, resulting from such exhaustion".