Facts have been found sufficient to demonstrate that it. is the purpose of nature to reduce the force which operates upon the excitability of vegetation at that period of the twenty-four hours when, from other causes, the powers of digestion and assimilation are suspended. As far as is at present known, that power is heat, and, therefore, we must suppose that, to maintain, at night, in our hothouses a temperature at all equal to that of the day, is a practice to be condemned. Plants will, no doubt, lengthen very fast, at night, in a damp heat, but what is produced at this time, seems to be a mere extension of the tissue formed during the day, and not the addition, of any new part; the spaces between the leaves are increased, and the plant becomes what is technically and very correctly called " drawn," for, as has been justly observed, " the same quantity only of material is extended to a greater length, as in the elongation of a wire".

Some observations made in the garden of the London Horticultural Society, a few years since, place this in a striking light. Certain plants were placed for some weeks in a stove, with a high night temperature supposed to average 69°; the rates of growth, in inches, showed that they grew as fast by night as by day; but, when the same kind of plants were grown in the open air, the growth was double or treble by day what it was at night, and continued observation of many plants produced the curious result that the total growth, by night, in the open air, was 119.07, and by day, 337.16.

Thus we see that plants exposed to natural circumstances only made one inch of growth by night, while they made three by day; but that, on the contrary, under bad artificial treatment, they grew equally day and night The inevitable consequence of this inversion of natural growth, is immature or unripe wood, with imperfect, ill-constructed buds, and a feeble constitution, incapable of bearing the shock of great falls of temperature. More especially, water accumulates in the system, and is never decomposed or removed by perspiration, in the requisite degree. In short, plants growing fast by night, can neither ripen their wood nor form their inner structure well, and, therefore, they are incapable of developing their natural beauty, or of resisting those extremes of temperature which are natural to them.

That greenhouses ought not to be heated at night more than Is sufficient to exclude the frost, is certain; that, if properly prepared, plants will bear frost, is also indisputable, as, indeed, is proved by the camellias, Chinese azaleas, and other plants, which are kept in cold frames through the hardest winters, and where ' they thrive far better than in greenhouses.

With stove plants it is different; experiments are needed to determine how much cold they will bear at night There seems to be no doubt that the colder they can be safely kept, the better for their health. A celebrated gardener assures me that he keeps his stove plants, during the winter months, at no higher temperature than from 40° to 50°; it is true, his employer desires late-blooming plants, but he has the roof covered with creeping stove plants; including Cambretums, Bignonias, Passi floras, Stephanotis, Ac. When the warm days of spring return, they break with unusual vigor, enjoying, as they do, almost a natural climate; his Bignonia venusta is covered with bloom, and the Stephanotis blooms in July - the Passifloras throughout the year.

These facts are deeply interesting, and may serve for hints to those gardeners whose employers reside in the city in winter; they may have a gay house when the family returns in the spring.