FROM the letters which we have received asking for advice as to lower night temperatures in forcing-houses since J. S. directed attention to the subject in our December issue, we are led to believe that there is at present a very considerable amount of interest on the part of the owners of hothouses in this important matter. Apart altogether from the soundness of the practice viewed from a purely horticultural stand-point, no doubt the present exceptionally high price of coal, with but' slight hopes of a fall to any considerable extent, is, from its effects on the pockets of many, prompting to inquiry as to the soundness of the practice of keeping up the night temperatures which have been generally aimed at in our plant and fruit houses. Within certain limits, there can scarcely be a question as to the wisdom of standing by the theory of low night temperatures so ably advocated by some of the physiologists of the last generation, and notably the late Dr Lindley, whose mantle seems to be lost to the horticultural press.

"We have only to go to the school of nature to learn most unmistakably, from the nocturnal fall of temperature, how erroneous is the practice of hard firing to maintain high night temperature, even in the case of those plants which in their native habitats are subject to the most intense sun and the highest degree of tropical heat, and where the variation between the sweltering heat of day and the chilliness of night is most forcibly experienced. Some may perhaps be inclined to tell us that nature is not now marching according to primeval law and order in this as well as in other respects. This, however, would be "drawing the subject a little too fine" in reference to the case in point. True, the most successful horticultural practice does not invariably homologate the teachings of nature, but it does corroborate what we are taught in these nocturnal variations; and surely it is not necessary, at this era of horticulture, to point out how erroneous is the artificial application in excess of the stimulating power of heat throughout the long hours of darkness of a British winter night.

The experienced cultivator, at any rate, knows well that such a combination of circumstances is productive only of debility, and the utter want of that stamina in plants which is only attainable under a corresponding amount of light and sunshine, with which we are never favoured in this country during our season of early forcing.

If plants are kept continuously at a high pitch, of excitability by the stimulating agency of heat, irrespective of the variations of day and night - of light and darkness - their whole system becomes impaired; and nature has provided against such a result, not only by the less sudden variation of summer and winter, rainy .seasons and dry ones, when a long season of activity is followed by a long repose, but by the more sudden variation from a high temperature by day with light, to comparative coolness by night with darkness. Were it possible to reverse this order of things for a single month, when plants are in full tide of growth - could we have light and a low temperature, darkness and excessive heat - we should learn a lesson from the appearance of the vegetable world that would impress us with the beneficence and wisdom of Nature's order of things, and would teach us a great and lasting lesson in early forcing if in nought else.

It is no part of our present intention to enter into the nature and results of the distinct functional operations of plant-life by day and night. Our object, and all that is possible for us, is, to throw out a few hints which we hope may stimulate our young and inexperienced readers to study vegetable physiology - the structure and functions of plants; and we are not aware that we can direct them to a better authority than Dr Lindley, in his 'Theory of Horticulture.' Suffice it here to say, that in the absence of sunshine at night, there is a cessation in plants of that evaporating and decomposing process by which plant food is perfected and rendered fit for augmenting in a proper manner the growth of plants and trees; and that all excess of heat at night, in the absence of these processes, which are dependent on light, only tends to gorge the system with an overdose of crude sap, producing a mere attenuation of imperfect and unfruitful growth, which by day does not bear the strain of sunshine in a manner so as to result in the production of wood and foliage, flowers and fruits, of which plants are capable when subject to that nocturnal repose which is as necessary to plants as it is to animal life.

Hence all experienced forcers of early flowers and fruits avoid high night temperatures when the days are short and dull, and endeavour, on the contrary, to do the - what may be termed - hard forcing by day with light. Experience has taught that the growth that is squeezed out in mid-winter with a high temperature is soft and flabby to a degree that will not bear with impunity that sunshine which is absolutely necessary to restore it to a proper state of tissue.

The too common practice of fixing rigidly any given temperature in hothouses, irrespective of the state of the external atmosphere, we regard as bad practice, and, so far as we are concerned ourselves, we invariably fix the range of temperature over at least 5° or 7°, according to the coldness or mildness of the weather. This not only saves fuel, but it is better for the plants than highly-heated surfaces. Moreover, we have cause to regard the fluctuation of the thermometer, even in steady weather, with much more complacency than we did at one time. And we are at a loss to know from whence such rigid lessons as to heat have been learned. We have several correspondents in the tropics who have remarked to us that if cultivators of tropical Orchids at home saw how amazingly they luxuriate with the night temperature frequently below 40°, they would not be so careful about high night temperatures. There is, however, another side to this question. We know that the Peach sometimes gets killed with a British frost, while it stands that of the United States with impunity, owing to the more thorough maturity that the wood attains under an American sun.

The same may no doubt be applicable to even many Orchids. This, however, teaches us that it is not from wide differences of temperatures in the twenty-four hours that plants suffer, but that it is much more from unnatural growth in the absence of light.

There can be no doubt that this is a question well worthy of discussion while we are face to face with the price of coal nearly tripled within the last two years, and that the hours of darkness are those in which most money can be saved or wasted in connection with our practice in maintaining night temperatures. It is therefore from this, as well as from other points of view, that we would invite further discussion on the subject. The tendency of the present generation of gardeners has been to recede from the night temperatures advocated by those who have gone before them; and our conviction is, that there are yet some steps, not only within the limits of safety, but to be attended with improved culture in many things. The subject has many sides in practice, and a change in this calls for change more or less in other conditions as well. Our space forbids reference to these at present, but we may recur to the subject ere long.