Heat is the prime agent employed by the Almighty Creator to call vegetable life into existence, to develop vegetable form, to effect all vegetable changes, and to ripen all vegetable produce. All these effects are performed most efficiently, in the case of every plant, at some different temperature or degree of heat; and he who ascertains most correctly those heats, has taken a gigantic step towards excellence as a gardener. An uncongenial heat is as pernicious to vegetables as to animals. Every plant has a particular temperature without which its functions cease; but the majority of them luxuriate most in a climate of which the extreme temperature does not much exceed 32° and 90°. No seed will vegetate - no sap will circulate - at a temperature at or below the freezing point of water. No cultivation will renderplants, natives of the torrid zone, capable of bearing the rigours of our winters, although their offspring, raised from seed, may be rendered much more hardy than their parents. Others are capable of resisting the greatest known cold to which they can be exposed; yet all have degrees of temperature most congenial to them, and if subjected to lower temperatures, are less or more injured proportionately to the intensity of that reduction.

If the reduction of temperature be only slightly below that which is congenial, it only causes the growth of the plant to diminish and its colour to become more pale; this effect being now produced by the plant's torpidity, or want of excitement to perform the requisite elaboration of the sap, as it is by over-excitement when made to vegetate in a temperature which is too elevated.

If blossoms are produced at all, they are unfertile, and the entire aspect o the plant betrays that its secretions are not healthy and its functions are deadened. Mr. Knight says, "that melon and cucumber plants, if grown in a temperature too low, produce an excess of female blossoms; but if the temperature be too high, blossoms of the opposite sex are by far too profuse."The drier the air the greater is the amount of moisture transpired; and this becomes so excessive, if it be also promoted by a high temperature, that plants in hot-houses, where it has occurred often, dry up as if burned. The justly lamented Mr. Daniell has well illustrated this by showing, that if the temperature of a hot-house be raised only five degrees, viz. from 75° to 80°, whilst the air within it retains the same degree of moisture, a plant that in the lower temperature exhaled fifty-seven grains of moisture, would in the higher temperature, exhale one hundred and twenty grains in the same space of time.

Plants, however, like animals, can bear a higher temperature in dry air than they can in air charged with vapour. Animals are scalded in the latter if the temperature is very elevated, and plants die, under similar circumstances, as if boiled. MM. Edwards and Colin found kidney-beans sustained no injury, when the air was dry, at a temperature of 167°; but they died in a few minutes if the air was moist. Other plants under similar circumstances, would perish probably at a much lower temperature; and the fact affords a warning to the gardener to have the atmosphere in his stoves very dry whenever he wishes to elevate their temperature for the destruction of insects or other purposes.

Some plants, like some animals, are able to endure a very high degree of tem-perature. Sir Joseph Banks and others have breathed for many minutes in an atmosphere hot enough to cook eggs; and I have myself travelled in Bengal breathing air, without inconvenience, which rendered the silver-mountings of my green spectacles too hot to be borne without their occasional removal.

So do certain plants flourish in hot-water springs of which the temperature varies between the scalding heats of from 150c to 1S0° of Fahrenheit's thermometer; and others have been found growing freely on the edges of volcanoes, in an atmosphere heated above the boiling point of water. Indeed, it is quite certain that most plants will better bear, for a short time, an elevated temperature which, if long continued, would destroy them, than they can a low temperature. Thus a temperature much above the freezing point of water, to orchidaceous and other tropical plants, is generally fatal if endured by them for only a few minutes; whereas a considerable elevation above a salutary temperature is rarely injurious to plants. But this is not universally the case; for the elegant Primula marginata is so impatient of heat that, although just about to bloom, it never opens a bud, if brought into a room in which there is a fire.

The temperature should always be regulated, in our hot-houses, with a due regard to the light. At night it should be so low as to put the circulation of the sap into a comparative state of rest; and in dull days the temperature should be full HP lower than in those of bright sunshine.