it is found to be in proportion to the temperature and brightness of that day. Should it be cold and gloomy the injury is less; but when warm and bright, the leaves of the vegetable become black and never revive. Had the effect been produced by the expansion of the water or juices of the vegetable by frost, it would have been uniform and not affected by the succeeding day; but here is an effect corresponding to the degree of the returning stimulus. It will be recollected that I stated the stimulus of heat to be a leading cause, thereby intending not to exclude the other stimuli of light and moisture. The great and sudden evaporation of moisture, independent of its generating cold, I am persuaded, is alone sufficient to bring on vegetable death."
Upon this it may be observed, if the disease arises merely from the too sudden return of the abstracted stimuli of warmth and light, and moisture, that when these are slowly and gradually restored; when the two or three days which succeed these biting frosts are "cold and gloomy," the plants ought not, according to Mr. Egremont, to receive any injury at all. But it seems, from the very case itself, that they do receive some injury; they only receive less than when the following days are warm and bright. The case, therefore, does not prove that no injurious effect was produced by "an expansion of the water or juices of the vegetable by frost," or by the sudden abstraction of stimuli; it rather proves that the sudden return of abstracted stimuli increases the injury which the plants had already received. Restore to a frozen limb as slowly as you please the abstracted stimulus of warmth, you will not restore it to soundness. The injury has been already sustained.
But it is not worth while to contend further about the influence of the abstraction of stimuli which, in his zeal to illustrate that of the too sudden restoration of them, Mr. Egremont seems, indeed, rather to have overlooked than to deny. The fact is, that plants, like animals, are morbidly affected by any violent transition either from heat to cold, or from cold to heat; by the too sudden abstraction or the too sudden accumulation of stimuli. Laurels and other shrubs, which have been carefully planted on the south side of a house for the sake of protection against the cold, are very likely to suffer in the winter for want of protection against the sun, while others which are exposed to the uniform severity of a northern aspect pass uninjured through the rigors of the season.[1] The former experience more rapid changes of temperature than the latter are exposed to. Sometimes in winter the sun breaks out very
- ↑ See White's Natural History of Selborne.
