Few subjects connected with horticulture are of more general interest than the preservation of fruit-trees, flowers, and vegetables, from injury by exposure to extremes of temperature. Numerous have been the experiments made, various the theories propounded, and the modes advocated, to avert the evil, and frequent the success, as also the failure, of the methods adopted.

Much remains to be learned, although many facts have been of recent years accumulated in furtherance of the desideratum to be attained. And in the present imperfect state of this horticultural study, it may advance the object in view if we can succeed in inducing the practical men among the readers of the Horticulturist to give the results of their past experience, and to institute experiments with the view to elucidate the subject. In view of the latter object, we venture to offer some remarks which may by possibility be found to some extent useful by those who have not hitherto given more than a passing attention to the matter.

Plants of all descriptions have fixed constitutions, which probably cannot be changed. Each individual is capable of living within certain limits of temperature. What those limits are, whether of heat or of cold, cannot be known, a priori, but must be ascertained by experiment. Whatever those limits are, so soon as the plant is subjected to a greater degree of either extreme of temperature, vegetable life ceases. But experience has shown, and shows every day, that within those extremes, but near the limits of them, a plant sustains injury; and, although not killed, it manifests in different ways unmistakable evidence of that injury, the which, however, the judicious cultivator can by a special mode of treatment remedy and remove; while, on the other hand, unless some particular mode be adopted, the same injury would have destroyed life in the plant.

The subject, therefore, may be said to embrace two principal inquiries, viz.:

1st. What is the nature of the injury sustained by vegetation when subjected to a greater extreme either of heat or cold than is consistent with its healthy state?

2d. Is there any means by which such injury can be guarded against, or remedied, when sustained?

The first of these questions again resolves itself into the inducing cause' whether of heat or cold. As regards the nature of the injury occasioned to a plant by excess of heat, it will be found that the primary consequence is the evaporation or exhalation of the sap and aqueous portions of the plant. This, if moderate in degree, can be remedied by the removal of the plant to a moist temperature, or by the supply of water to the foliage and root. But if the excess of heat be great, (and whether the atmosphere in which it is present be dry or moist,) the consequence will be, the speedy destruction of vegetable life. The process of such destruction would appear to be, first, a collapse of the vessels and tissue of the plant, and, when the heat is sufficient, combustion of the entire fabric.

When, on the other hand, the nature of the injury received is caused by extreme cold, the effect on the plant will be very different; but here, again, such effect will vary greatly according to the species and the constitution of the plant. A slight but sudden comparative lowering of temperature to a plant that is a native of a hot tropical climate, or to many hardy plants that have been growing in a hothouse or hotbed, will frequently so affect a plant that, unless returned to a higher temperature, its living principle will become extinct Many a gardener experiences this who, without] the due precaution of "hardening off," as he terms it, turns out his young plants from his propagating frames to the open ground. In this instance the injury sustained will be evidenced by the drooping appearance of the foliage and young shoots of the plant, caused by the comparative stagnation of its secretions induced by the lowering of temperature, and the check given to the activity of the vital principle by the sudden withdrawal of greater warmth. Usually such a degree of injury admits of a remedy. But if the degree of cold be increased to the freezing point, the nature of the injury sustained by the plant is altogether of a different character.

In freezing we know that water expands, and consequently that the sap contained in the vessels of plants when frozen requires a greater volume. Within certain limits the tissue of which the vessels of plants are formed is elastic: a quality which, however, varies greatly in different species. When cold is severe enough to freeze the aqueous secretions contained in the vessels of a given plant; unless the elastic principle is sufficient to cause the requisite expansion of the several parts, rupture of the tissue must ensue; and consequently, partial or entire destruction of the organization and of the vital principle follows. But two points here require to be adverted to: the one that the degree of frost may be often sufficient only to affect the outer portion of tissue of the stem; and the other, that previous to the temperature descending so low as the freezing point the plant may be affected injuriously by a less degree of cold, not only in the mode before adverted to through a partial stagnation of the aqueous secretions, but by the contraction of those secretions from the loss of temperature; since we must bear in mind that water contracts as it descends towards the freezing points, although it expands in freezing.

The effects, therefore, produced upon plants by extremes of temperature, chiefly appear to be the loss of their secretions by evaporation and exhaustion consequent upon it, in one class of cases, and in others the suspension of the vital energies of the plant, and again, the destruction of the organization by frost. And with reference to the whole it must ever be remembered, that the degrees in which these effects are produced will be varied and modified on the one hand by the intensity of the meteorological cause, and on the other by the constitutional power of each species of plant to contend against it; which experience has shown to diner very widely.