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Fig. 83.—Wake-robin (Arum maculatum).
In 1839 Vrolik and Vriese made numerous observations, and found that the maximum of several hundreds of experiments was from 48° to 57° Fahr. These bring us to the memoir of Dutrochet, in which, after recounting the labours of his predecessors, he narrates his own experiences up to 1840, chiefly on the common wild Arum, in which he found an elevation of temperature of from 25° to 27° Fahr. From all these experiments, made by different individuals and in diverse ways, we ascertain that there is a great elevation of temperature in the plants of the Arum family at the period of flowering, but the precise amount varies with the observers, the highest being from 50° to 68° Fahr. for the larger species, and proportionately less for the smaller ones. The greatest heat was obtained at or shortly after the opening of the spathe, or the climax of flowering. Temperatures ascertained at different periods of the day would necessarily be influenced more by the condition of the flowers than by the precise hour. This will account for the maximum being fixed at different hours by different observers. Subsequent experiments conducted by M. Garreau at Lille demonstrated the great consumption of oxygen which accompanied the elevation of heat and its proportionate increase. When the mean heat was seven degrees, sixteen volumes of oxygen were consumed per hour; when the mean heat rose to twelve, the consumption of oxygen increased to twenty-one volumes; and when the mean heat had attained seventeen and a half, the volumes of oxygen consumed exceeded twenty-seven. The quantity of carbonic acid evolved is in direct proportion to the oxygen absorbed, and the degree of chemical action which takes place determines the amount of heat.
We shall be prepared to concede that, after all, it is not so remarkable that here and there we meet with records of an elevation of temperature at the time of flowering, in plants where the natural conditions are favourable, as that these records are not more numerous and explicit. Mr. N. E. Brown states that on one occasion the living spadix of Philodendron williamsii, which had flowered at Kew, was brought to him in a condition in which it was uncomfortably hot to the hand, but he had no ready means of ascertaining the precise temperature. Mr. Nicholson has observed Philodendron sagittaefolium with the anthers nearly ready to dehisce, and which exhibited a rise of temperature from 69°, that of the stove, to 81° Fahr. Also of Philodendron eximium, when the house was at 82°, showed an elevation of 92°. Seeing that the latter are aroids, in which numerous flowers are associated, the rise of temperature was comparatively small.
Somewhat more striking results were shown some years ago by Mr. W. H. Tillet, of Norwich, on an aroid growing in his conservatory, in which he observed a manifest increase of temperature, and found, by testing with a thermometer, that the elevation exceeded those above alluded to. It may be accepted as a general result of numerous experiments, that, in the large aroids, an increase of temperature of fully 30° Fahr. may be anticipated, which, under exceptionally favourable circumstances, may reach as much as 50°.
In palms and their allies the flowers are produced in dense masses, and these are often wholly or partially surrounded by an envelope, so that the physical conditions are very similar to those of the aroids; yet opportunities do not often arise for determining the heat evolved during the flowering. Mr. Nicholson has recorded that on one occasion he tested the temperature in a cluster of flowers of one of the palms (Carludovica), and found an elevation of 10° above that of the surrounding air. In this case the cluster of flowers was smaller than in many other palms, and there was no envelope to retain the heat.
That heat is evolved during the flowering of the royal water-lily (Victoria regia) was determined by several observers at the time when this plant was first flowered in this country. Caspary observed an elevation of 13° Fahr. and other observers have confirmed the fact, with slight variations in the degree. This phenomenon is by no means confined to aroids and palms, but has been determined also in many other plants, although to a less extent. Dutrochet has recorded his experiences with several common plants, such as the rose, the vine, and the euphorbia, in all of which some heat was evolved, and he was of opinion that it reached its maximum at or near mid-day.
Evolution of heat, therefore, is common to all plants during the period of flowering, but it is only in those where a large number of blossoms are collected together, and where the heat is prevented from being rapidly dissipated by some kind of envelope, that its effects are so manifest as to have become an object of popular interest. The chemical processes of oxidation and decomposition which accompany the maturation of the floral organs are the physical causes of the phenomenon, and these processes are common to all plant life.
determined recently the elevation of temperature in the ivory-palm (Phytelephas macrocarpa) at Kew. "On April 20th, at one p.m., the temperature of the house was 68° Fahr., the bulb of a thermometer, which had been suspended for some time near the plant in question, was placed in the centre of the cream-coloured inflorescence, and the mercury almost instantly rose to 92°, showing an increase in temperature of 24°. The following day, at the same hour, the thermometer registered 72° in the house, and, when placed in the same position in the centre of the inflorescence, only rose to the same height as that reached the preceding day, viz. 92°. As the drawn-out end of the bulb prevented it from actually touching the convex ovaries, a small incision was made in one of these, and the thermometer then rose to 92°."
The same observer also tested another allied plant (Carludovica plumieri) and found the thermometer rise from 73° to 90°, but the plant was not in good condition, for "the long barren stamens had already changed from creamy white to cinnamon colour, and the spathe had commenced to decompose, although not three hours had elapsed since the flowers had opened."
"Development of Heat in Phytelephas," Journ. Bot., x. (1881), p. 154.
Dr. De Vriese has also referred to a high temperature obtained at Buitenzorg, in the male cones of Cycas circinalis, but does not state the precise amount; he says that the elevation always took place between six and ten p.m., and was accompanied by a strong smell.
The evolution of heat in other plants, where the flowers are produced singly, and not enclosed in a spathe or envelope, is not only less, but more difficult of determination than in agglomerated flowers. Wherever a number of flowers approximate, as in composite plants, greater heat has been detected. Amongst the species which have been tested may be mentioned the flowers of a Cactus, in which three degrees were registered above the surrounding air. In geranium as much as six degrees are said to have been determined. Saussure found by a thermometer—scarcely a satisfactory medium—that the tuberose rose half a degree, the flowers of a gourd from 1° to 3° Fahr. and a Bignonia only 1°. The flowers of Victoria regia were tested at Hamburg when the temperature of the house was 70° 7' Fahr., and the flowers found to be 80° 3'. On another occasion, when the air was 72° 5', the flower had risen to 105° 1', or the rather extraordinary increase of about 33°.
- Hooker’s “Kew Gardens Miscellany,” iii., p. 186.
- Balfour’s “Class Book of Botany,” p. 519.
If this determination is an accurate one it becomes almost inexplicable, and should at least receive some corroboration, especially when compared with the results of an examination of the flowers of Nymphaea stellata, another water-lily, in which the maximum elevation was little over 1° Fahr. Of the flowering heads of composite plants we have accounts of but two, the capitulum of the cotton-thistle (Onopordum acanthium), in which about 1° 5' Fahr. is recorded, and in a number of flower-buds of Anthemis chrysoleuca, the temperature rose to 2° 4' Fahr. One result of the great stimulus which electrical science has recently received, it may be hoped, will be an extensive series of observations, with delicate appliances, to determine the variations of temperature at different periods in a large number of plants.
Chemical change takes place so rapidly in the fleshy fungi that we should have been quite prepared to find that under certain conditions an appreciable elevation of temperature has been ascertained. It seems to us surprising rather that so small a rise in temperature has been observed, than that such changes have been recorded. The larger species of Lycoperdon, when quite mature, will become sensibly warmer to the hand when they exhibit signs of decomposition. The finger thrust into a decaying cluster of Agaricus melleus will obtain decided evidence of increase of temperature. In these cases it will be the necessary accompaniment of decomposition.
Dutrochet examined growing fungi of five species, and found in all a slight elevation of temperature, but in none so much as one degree, and in some not one fifth of a degree. Probably the most favourable period was not selected, at least the subject requires further investigation. Dr. McNab has also recorded his observations on Lycoperdon giganteum, but in this instance the rise was not so much as would have been expected, although in excess of the amount determined by Dutrochet. It can hardly be supposed that so large a mass, undergoing rapid chemical change, does not exceed about one degree per cent. in rise of temperature.
CHAPTER XVIII.
LUMINOSITY.
THE phenomena of "luminosity" in plants are evidently variable in their causes, as predicated by the variability of the results. We have brought together examples of these manifestations from flowering and cryptogamic plants, associated somewhat in accordance with their apparent relationship, but without any effort at explanation. There is strong presumption that some of the supposed cases, of flashes of light from bright coloured flowers, may be explained optically. Others can be accounted for by no such hypothesis. The different facts seem to group themselves thus:—Flowers exhibiting electrical flashes of light on sultry evenings; plants becoming surrounded by the vapour of essential oil, which readily takes fire; roots, or rhizomes, which, under certain conditions are luminous; and fungi which are either luminous in their imperfect, or "mycelium" condition, or when fully matured.
The luminosity of flowers, under certain conditions, has many times been affirmed, by different and independent observers, and yet still remains the subject of some doubt and uncertainty. The earliest instance is that of the daughter of Linnæus, who observed a "lightning-like phosphorescence" in the flowers of the nasturtium during a sultry tempestuous night. Another instance was recorded in 1843, when Mr. Dowden mentioned a luminous appearance in the double variety of the common marigold. This circumstance was noticed on the 4th August, 1842, at 8 o'clock p.m., after a week of very dry weather. Four persons observed the phenomenon. By shading off the declining day-light, a gold-coloured lambent light appeared to play from petal to petal of the flowers, so as to make a more or less interrupted corona around the disc. It seemed as if this emanation grew less vivid as the light declined; it was not examined in darkness. Dr. Edwin Lankester was strongly in favour of the verity of such exhibitions. Another contributor says, "I have observed it frequently, and have looked for it on each succeeding summer on the double marigold, and more especially the hairy red poppy (Papaver pilosum), in my garden at Mosely, in Worcestershire." Many years after, and another instance was recorded: "We witnessed (June 10, 1858) this evening, a little before 9 o'clock, a very curious phenomenon. There are three scarlet verbenas, each about nine inches high, and about a foot apart, planted in front of the greenhouse. As I was standing a few yards from them and looking at them, my attention was arrested by faint flashes of light passing backwards and forwards from one plant to the other. I immediately called the gardener and several members of my family, who all witnessed the extraordinary sight, which lasted for about a quarter of an hour, gradually becoming fainter, till at last it ceased altogether. There was a smoky appearance after each flash, which we all particularly remarked. The ground under the plants was very dry, the air was sultry and seemed charged with electricity. The flashes had the exact appearance of summer lightning in miniature. This was the first time I had seen anything of the kind, and having never heard of such appearances, I could hardly believe my eyes. Afterwards, however, when the day had been hot and the ground was dry, the same phenomenon was constantly observed at about sunset, and equally on the scarlet geraniums and verbenas. In 1859 it was again seen. On Sunday evening, July 10th of that year, my children came running in to say that the 'lightning' was again playing on the flowers. We all saw it, and again, on July 11th, I thought that the flashes of light were brighter than I had ever seen them before. The weather was very sultry."