Transactions of the Horticultural Society of London, 2:130-136 (1822)
XXXIII. On the ill Effects of excessive Heat in Forcing-houses during the Night.
Thomas Andrew Knight,
Esq. F. R. S. &c. President.
Read June 17, 1814.

Few gardeners, if any, have ever believed plants to be at all endued with powers of sensation and perception similar to those of animals, or to be, in any degree, susceptible of pleasure or pain; and yet it is very questionable, whether there has ever been a single gardener, who, in the management of fruit-trees in a forcing-house, did not in some respects err, by treating his trees, as he would have done, if he had supposed them to possess such powers. Being fully sensible of the comforts of a warm bed in a cold night, and of fresh air in a hot day, the gardener generally treats his plants, as he would wish to be treated himself; and, consequently, though the aggregate temperature of his house be nearly what it ought to be, its temperature during the night, relatively to that of the day, is almost always much too high. The consequences of this excess of heat during the night are, I have reason to believe, in all cases highly injurious to the fruit-trees of temperate climates, and not at all beneficial to those of tropical climates; for the temperature of these is, in many instances, low during the night. In Jamaica, and other mountainous islands of the West Indies, the air upon the mountains becomes, soon after sun-set, chilled and condensed, and in consequence of its superior gravity, descends and displaces the warm air of the vallies; yet the sugar canes are so far from being injured by this sudden decrease of temperature, that the sugars of Jamaica take a higher price in the market, than those of the less elevated islands, of which the temperature of the day and night is subject to much less variation.

During the progress of germination, in the spring, great chemical changes take place in the component parts of the sap of trees, analogous to those which have been observed in the germination of seeds. I could not detect any vestige of saccharine matter in the Alburnum, either of the stem or roots, of the Sycamore tree in the winter; but in the spring, its sap became very sensibly sweet: and I found this sap to be much more saccharine, and of greater specific gravity, in large trees, which were prepared to nourish an abundant blossom, than in small and young trees. The sap of the same tree proved also to be subject to some variations of specific gravity, at the same period of the spring, in different years; and Duhamel has observed, that the sap of the Sugar Maple becomes first saccharine, and afterwards acquires an herbaceous taste; in the latter state, it probably is best calculated to feed the blossoms and unfolded buds.

At the period of the preceding chemical changes in the qualities and properties of the sap, previous to the growth of the leaves, that fluid is found to ascend during the warm part of the day, and to flow, in many species of trees, from any recent wound, and to fall again during the night, particularly if that be cold; and as variations of temperature are the apparent cause of these motions, it appears not improbable, that the chemical changes, which take place in it at this period, are promoted by the same agents.

Some experiments, which I have made upon germinating seeds, have perfectly satisfied me, that these afford plants of greater or less vigour in proportion as external circumstances are favourable in promoting, beneath the soil, the necessary changes in the nutritive matter they contain; and I suspect that a large portion of the blossoms of the Cherry and other fruit-trees in the forcing-house often proves abortive, because they are forced, by too high and uniform a temperature, to expand before the sap of the tree is properly prepared to nourish them.

I have, therefore, been led, during the last three years, to try the effects of keeping up a much higher temperature in the day than in the night; and as experiments of this kind cannot be made by the common gardener, who must not risk the sacrifice of his employer's crops of fruit, I trust the following account will be honoured by the approbation of the Horticultural Society, though the experiments have been chiefly confined to the Peach tree.

As early in the spring as I wished the blossoms of my Peach trees to unfold, my house was made warm during the middle of the day; but towards night it was suffered to cool, and the trees were then sprinkled, by means of a large syringe, with clear water, as nearly at the temperature at which that usually rises from the ground, as I could obtain it; and little or no artificial heat was given during the night, unless there appeared a prospect of frost. Under this mode of treatment the blossoms advanced with very great vigour, and as rapidly as I wished them, and presented, when expanded, a larger size than I had ever before seen of the same varieties: which circumstance is not unimportant, because the size of the blossom, in any given variety, regulates, to a very considerable extent, the bulk of the future fruit. As soon as the blossoms were expanded, and the pollen began to shed, water was applied in less quantity, as a light shower, sufficient to wet the pollen, without washing it off; but when the pollen was chiefly shed, I again, to promote its absorption, sprinkled the trees abundantly with water, having previously often observed, that heavy showers of rain are at this period always highly beneficial to the blossoms of the Apple trees in our orchards; and almost every blossom of my Peach trees set most perfectly. The watering was regularly continued till the fruit became very nearly ripe, the roots of the trees being, at the same time, abundantly supplied with moisture and food in the manner detailed in my last paper, in which I have stated the more than ordinary size and perfection of the fruit.

*I suspect, but I am no entomologist, that two distinct species of insect are confounded under this name, one of which forms a web, which the other does not. The latter kind often abounds in the open air, upon pear-trees, and appears to be, in the forcing-house, a much hardier insect than the other.

My house had been previously much infested with the red spider;* but not a single one now appeared, nor scarcely an Aphis; and the young wood became remarkable for the shortness of its joints, and the thickness, comparatively with the length of its shoots. A gardener, who is prejudiced in favour of old customs, will possibly imagine that he supplies the place of the cool evening dews of nature, and of the water in the preceding experiment, by sprinkling his flues with water, and filling his house abundantly with steam.

But the effect of no two operations can be more different: in the one, the plant is suddenly chilled by cold water, and subsequently kept cool by the evaporation of the water during the night: in the other, the steam is precipitated upon the leaves and branches of the trees, to which it necessarily communicates much heat. The former operation nearly resembles that of the shower-bath, sometimes used in this country, in which the patient is suddenly chilled by a heavy shower of cold water; the other resembles the hot steam bath of Russia, in which he is violently heated; and if the gardener were to try each of these processes upon himself, during a single night, I suspect he would arise in the following mornings with very different feelings, unless he were blest with much peculiar hardness of constitution. It is true, that plants do not appear to possess sensation in the ordinary sense of that term, as it is applied to animals; but nature, in forming its whole organic creation, seems to have proceeded so much by substitutions and additions, that simple sensation, in its strict and limited sense, abstracted from all powers of perception, may not improbably be as widely diffused as organization itself; and animal and vegetable life may be, in consequence, susceptible of similar injuries from similar external causes. The influence of hot and damp air upon both, is greatly more powerful than that of dry air of the same temperature. In the experiments, of which Sir CHARLES BLAGDEN has given an account in the Philosophical Transactions of 1775, he, with Sir JOSEPH BANKS and others, sustained without injury a temperature of 260 degrees in dry air; but they found damp air, at half that temperature, to be scarcely supportable: and every gardener knows, how quickly the leaves of his plants are injured by the combined action of heat and moisture.

The succulent shoots of trees, however, always appear to grow most rapidly, in a damp heat, during the night; but it is rather elongation than growth, which then takes place. The spaces between the bases of the leaves become longer, but no new organs are added; and the tree, under such circumstances, may with much more reason be said to be drawn, than to grow; for the same quantity only of material is extended to a greater length, as in the elongation of a wire.

Another ill effect of high temperature during the night is, that it exhausts the excitability of the tree much more rapidly than it promotes the growth, or accelerates the maturity of the fruit; which is in consequence ill supplied with nutriment, at the period of its ripening, when most nutriment is probably wanted. The Muscat of Alexandria, and other late grapes, are, owing to this cause, often seen to wither upon the branch in a very imperfect state of maturity; and the want of richness and flavour in other forced fruits is, I am very confident, often attributable to the same cause. There are few peach houses, or indeed forcing houses of any kind, in this country, in which the temperature does not exceed, during the night, in the months of April and May, very greatly that of the warmest valley in Jamaica in the hottest period of the year: and there are probably as few forcing houses in which the trees are not more strongly stimulated by the close and damp air of the night, than by the temperature of the dry air of the noon of the following day. The practice, which occasions this, cannot be right: it is in direct opposition to nature; and I need not point out to the intelligent members of the Horticultural Society, that the more nearly nature, in its best climates and most favourable seasons, is copied as to temperature, the more perfect will be the productions of the gardener's art.