A Selection from the Physiological and Horticultural Papers,
published in the Transactions of the Royal and Horticultural Societies by the late Thomas Andrew Knight, Esq. 1841

XXXIX. UPON THE EFFECTS OF VERY HIGH TEMPERATURE ON SOME SPECIES OF PLANTS.
[Read before the HORTICULTURAL SOCIETY, December 7th, 1819.]

HAVING constructed a forcing-house for the purpose of attempting the culture of the mango, and a few other species of tropical fruits, I have endeavoured to ascertain, with accuracy, the advantages and disadvantages, of employing very high temperature during the day in bright weather, and of comparatively low temperature during the night, and in cloudy weather; and I communicate the following account of my experiments, considering the results to have been generally very favourable, and where unsuccessful, not wholly uninteresting.

A fire of sufficient power, only, to preserve in the house a temperature of about 70, during summer, was employed, but no air was ever given, nor its escape facilitated, till the thermometer, perfectly shaded, indicated a temperature of 95; and then only two of the upper lights, one at each end, were let down about four inches. The heat of the house was consequently sometimes raised to 110, during the middle of warm and bright days, and it generally varied, in such days, from 90 to 105, declining during the evening to about 80, and to 70 in the night.

Late in the evening of every bright and hot day, the plants were copiously sprinkled with water, nearly of the temperature of the external air; and the following were the effects produced upon the different species.

The Melon. Plants of this species were trained upon a trellis near the glass, which was of the best quality, and these exhibited a greater degree of health and luxuriance, than I had ever before seen; but not a single flower ever unfolded; a great profusion of minute blossoms, nevertheless, appeared in succession at the points of the shoots, all of which perished abortively. I was much disappointed at the result of this experiment; from which I confidently expected to obtain fruit of the greatest excellence.

The Water Melon. A plant of this species, treated in the same manner as the melon plants above mentioned, grew with equal health and luxuriance, and afforded a most abundant blossom; but all its flowers were male. This result did not, in any degree, surprise me; for I had many years previously succeeded, by long continued very low temperature, in making cucumber plants produce female flowers only; and I entertain but little doubt, that the same fruit-stalks might be made, in this and the preceding species, to support either male or female flowers, in obedience to external causes.

The Guernsey Lily. I transferred plants of this species, from the open air to the hot-house, in the summer, with the hope of obtaining seeds, in which I was wholly disappointed. The flowers expanded very beautifully; but their pollen never shedded. The plants have, nevertheless, subsequently grown with more than ordinary vigour; and I entertain scarcely any doubt that the same roots which afforded flowers in the present season, will blossom strongly in the next. It appears therefore from this, and the two preceding experiments, that the same degree of temperature, which may promote the growth, and exuberant health of the plant, may, at the same time, render it wholly unproductive of fruit or offspring.

The Fig Tree. Several varieties of this species were subjected to experiment; but the trees, although planted in pots, grew with so much luxuriance, and afforded me so little prospect of fruit, that I removed all except those of the large white variety, from the house. The white fig tree succeeded perfectly, first ripening its spring-figs, (those which usually ripen in the open air in this country,) and afterwards its summer figs. The trees then produced new leaves and branches: and the fruit, which would have appeared in the next spring, ripened in high perfection in September. Subsequently also a few of those, which, in the ordinary course of the growth of the tree, would have appeared as the summer crop of next year, have ripened, and these, though far inferior to those of the preceding crops, have not been without merit.

The Nectarine. A seed of this species of fruit was planted in a hotbed, in January last, and it vegetated in the succeeding month. It was subsequently removed to the hot-house, in which it continued to grow through the summer, without being in the smallest degree drawn by the high temperature in which it was placed: its wood, on the contrary, is remarkably short-jointed, and is covered with blossom-buds; from which I think it will be practicable to obtain ripe fruit, within sixteen months of the period, at which the plant first sprang from the ground.

The Orange and Lemon. A very high temperature appeared peculiarly favourable to plants of these species, or, I believe, more properly of this species; for I consider both, with the citron and shaddock, to be varieties only of the lime. A plant which sprang from seed in March, had, in the end of August, attained the height of more than four feet, with proportionate strength; when wanting the place it occupied for another purpose, it was removed from the house. I obtained in April a plant of the China orange, with one very small fruit upon it, which has ripened in much apparent perfection, and the tree exhibits every appearance of the most exuberant health.

The Mango. (Mangifera Indica.) This species of fruit-tree appears to possess great peculiarity of constitution; for, although a native of a very hot and bright climate, and capable of bearing, with apparent benefit, the hot drying winds of Bengal, it vegetates freely, and retains its health in comparatively low temperature, and under a cloudy atmosphere. The plants I possess sprang from seeds in October 1818; and the leaves acquired, during winter, their proper dark colour, and remained in perfect health till spring; although, not possessing at that period, a hot-house, I was very ill prepared to preserve them. In March they began to shoot a second time, without having been, I believe, at any period subjected to a higher temperature than 60, and some of them are now shooting strongly; although the temperature of my house during the last five weeks, except once or twice in very bright days, has rarely been so high as 60. The mode of growth of this plant appears also to be very singular; it extends a few inches, and then closes its terminal buds, as if its growth for the season were ended. One of my plants has done so nine times within the last thirteen months, without having acquired a greater height than two feet seven inches. I am much inclined to believe that the mango might be raised in great abundance and considerable perfection in the stove in this country, for it is a fruit which acquires maturity within a short period. It blossoms, in Bengal, in January, and ripens in the end of May; and Mr. Turner, in his journey to Thibet, states that he found the mango growing in latitude 27 50' in Boutan, in the same orchard with the apple-tree; the apples ripening in July, and the mangoes in September. And another Eastern traveller of credit (I think it is Mr. Barrow), mentions an instance in which a frost, sufficiently severe to have injured the crops of barley, had proved fatal to the blossoms (only) of the mango-trees.

The Alligator, or Avocado pear. (Laurus Persea.) The plants of this species have grown with rather troublesome luxuriance in my house, though they have been generally confined to small pots; one plant to which a larger pot was given is more than six feet high, with branches extending five feet wide; and a stem, the growth of a single year, exceeding, at its base, an inch in diameter. To obtain fruit of this species within the narrow limits of a forcing-house, it would be necessary to propagate from buds or grafts taken from the extreme branches of trees of considerable age.

The Mammee-tree. (Mammea Americana.) Very contrary to my expectations, this plant, a native of Jamaica, proved extremely impatient of heat and light, and its young leaves always required to be shaded when the temperature of the house exceeded 90. But with proper attention to screen the leaves from the mid-day sun, till they acquired maturity, the young trees of this species have succeeded as well as those of any of the preceding species.

Several other plants, part of them natives of temperate climates, grew in my house through the whole summer, without any one of them being drawn, or any way injured, by the very high temperature to which they were occasionally subjected; and from these, and other facts, which have come within my observation, I think myself justified in inferring, that, in almost all oases in which the object of the cultivator is to promote the rapid and vigorous growth of his plants, a very high temperature, provided it be accompanied by bright sunshine, may be employed with great advantage; but it is necessary that the glass of his house should be of good quality, and that his plants be placed near it, and be abundantly supplied with food and water. In the preceding experiments, water was made the vehicle of food to the roots of the plants, in the manner I have described in a former communication*, and with similar good effects.

My house contains a few pine-apple plants, in the treatment of which I have deviated somewhat widely from the common practice; and, I think, with the best effects; for their growth has been exceedingly rapid, and a great many gardeners, who have come to see them, have unanimously pronounced them more perfect than any which they had previously seen. But many of the gardeners think that my mode of management will not succeed in winter, and that my plants will become unhealthy, if they do not perish, in that season; and as some of them have had much experience, and. I very little, I wish at present to decline saying more relative to the culture of that plant.

*See above, page 211. (Recommending liquid manure for potted plants)