Popular Science Monthly/Volume 16/April 1880/Vegetable Phosphorescence
THERE is a curious myth which gives to Castor and Pollux, twin sons of Zeus, a divine control over the phosphorescence of air and ocean. Being present by invitation at the marriage of two youths of Amyclæ with the daughters of Leucippus, they became enamored of the brides and with the doubtful courtesy of the period attempted to carry them off. In the ensuing struggle Castor was slain; but Pollux, with a boundless and heroic love that bridged the "abyss 'twixt life and death," besought their father Zeus to restore him, proposing that to meet the ends of justice they should each live only on alternate days. This he granted, and, "kindling stars upon their brows, set them for ever with the immortals."
A religious idea was therefore involved in the superstition of the ancient mariner that the pale phosphoric light, or "St. Elmo's fire," which occasionally appears upon the masts of ships, in an electrical condition of the air, denoted the presence of these divinities as they rode upon the storm-clouds through the heavens. This indicates a recognition of the phenomena of phosphorescence in a remote antiquity; but, in advancing from the philosophy of myths and symbols to an era of empirical knowledge, there was necessarily a period of confusion, when old fancies were replaced by new facts which had not accommodated themselves to the novelty of the position. Even when, through the onward roll of centuries, the wisdom of the ancients had shrunken into grotesque fables, a long period intervened which produced no marked scientific results.
Throughout the seventeenth century inquiry was directed to the phenomena of solar light and the light of incandescent bodies; but, while phosphorescence was everywhere observed, no explanation on scientific principles was attempted, until in 1675 Nicolas Lémery, in a paper on the preparation of solar phosphorus, stated that the light produced was the result of rapid molecular motion; thus accepting the doctrine foreshadowed by Huygens and Euler, in opposition to the emission or corpuscular theory of Newton. These prophetic utterances, however, of the present undulatory theory of light were not recognized; it was reserved for later experiment to demonstrate the truth of this intuitive perception.
The "persistence of force" and the "indestructibility of matter" must first be proved, before it could be shown that the phenomena of phosphorescence are due to the same causes as the light evolved by electrical discharge, chemical combination, or mechanical movement. If, as Dr. Young has shown, light be motion or vibration of a luminiferous ether which fills space and permeates all bodies, the conditions of inexorable law are as fully met in the pale glimmer of luminous plants, the flash of the fire-fly, or the radiance of the glow-worm, as in the light evolved by the heavenly bodies.
The discovery of the luminosity of plants has been attributed to the daughter of Linnæus. In the year 1762, during the months of June and July, she observed radiations of light from a cluster of garden nasturtiums (Tropœlum majus), which occurred in the morning or evening twilight. The same effect has been observed in other flowering plants, but principally in those bearing orange-colored bloom, as the corolla of the sunflower, the French marigold, and in the yellow lily. Some species of African marigolds also have manifested this property. Professor Haggern observed the luminous radiations to be most brilliant in the marigold, next in the nasturtium, and, third, in the yellow lily. A careful microscopic examination convinced him that the light did not depend upon the presence of any animal organisms. The rapidity of the flash indicated electrical action, and upon analysis he found that the light proceeded from the petals, the anthers being at an appreciable distance. He supposed that in the process of fecundation the elasticity of the anthers scattered the pollen on the petals and produced electrical disturbance by the impingement of these different substances. In bog or marsh land plants, of the genus Pandanus, the rupture of the spathe or shield of the flowers is sometimes accompanied by a loud, cracking noise and a spark of light. In 1857 M. Fries perceived light emitted by a group of poppies (Papaver orientate) in the botanical garden at Upsal, to which he called the attention of numerous witnesses.
The phosphorescence discovered by the daughter of Linnæus bore the character of an electrical spark which shot out from the corolla and was discernible at the same hour upon warm evenings when the air was surcharged with electricity. The radiance noticed by M. Fries also indicated a periodicity of movement, occurring always when the air was electrical, between ten and eleven o'clock at night. Not only the blossoms but the leaves of many of the flowering plants have been observed to emit a phosphoric light under favorable conditions of the air—even the milky juice of several vegetables becomes luminous in the dark; this was particularly noticed by M. Martins in a species of the phosphorescent spurge (Euphorbia).
The giving out of heat in the blossoming of plants was discovered by Lamarck more than a century ago in the European arum, which in opening "grows hot as if about to burn." It was afterward observed by De Saussure, and by the later appliance of the thermo-multiplico the heat generated in any cluster of blossoms is made appreciable. The development of this force is most remarkable in tropical plants, where a large number of flowers are crowded together under a covering hood of spathe. The temperature increases periodically, growing greater in the afternoon and appearing like a "paroxysm of fever" which passes off with the decline of day—the greatest intensity always occurring during the shedding of the pollen. Any great increase of temperature is necessarily prevented by the equalizing effect of evaporation from the expanded surfaces of the leaves, and the water which pervades all the substances. It is only during a period of unusual energy, as in blossoming, that the heat becomes apparent.
The light and heat set free by combustion we recognize as merely the expression of chemical change and a giving back to original elements the forces that were stored up by vegetative activity in the coal-beds of the past or in the woody fibers of later growth. In the mysterious circle of Nature's means and mechanism, light, heat, and chemical combination are alternately cause and effect—not, it is true, in the abstract sense of cause, but, being mutually convertible or corelative, maintain necessarily reciprocal action. Now, the fact that they are thus reactive and interchangeable as modes of motion offers a very simple explanation of the giving out of light by plants at a moment when the surplus amount of these garnered forces is thrown off during the vital processes of reproduction.
That increased chemical activity exists at the period of flowering is shown by the exhalation of an unusual amount of carbonic acid, and this increased action supplies the additional heat for the elaboration of the reproductive agents, whose preparation seems to be the highest expression of energy in vegetable organization. Some doubt exists as to the proximate cause of the manifestation of light by flowerless or cryptogamic plants, in which are embraced the mosses, fungi, etc. A study of the conditions under which it is presented will, I think, enable us to refer it directly to similar chemical action.
These plants were ingeniously named by Linnæus because the concealed organs of reproduction offer great diversity in structural relations—a diversity so great that they can not even be presented under one common type; we must, therefore, look for modifications in the expression of force directed by these different forms. The root-hairs which form in the germination of one of the liverworts (Hepaticæ) have been observed to be luminous in the dim light of caverns. It gathers principally upon schists, and derives its name (Schistostega Osmimdacæ) from a miniature resemblance to the royal fern Osmimda. This plant, like the true cavern-mosses, is emerald-green, and develops into root, stem, and leaf. The dainty fern-like leaves or fronds are of the very simplest organization, and a slender, threadlike stem rises from the apex, bearing upon its summit a valvular case or capsule which contains the reproductive cells or sporules. The root-hairs which give out the light appear like the tangled meshes of a spider's web; and, as the same effect has been noticed in these tenuous structures, some naturalists have attributed the appearance to reflected light. But analogy in the condition of this and other light emitting plants leads to the conclusion that it is self-luminous.
The phosphorescence of fungi has been observed in various portions of the world. Rumphius first noticed its appearance in Amboyna in a species of corticium to which the name Telephora cærulum was given. Under this specific name are grouped many minute forms of fungi, only the mycelium of which were known to Linnæus and Agardh, the discovery of other organs forming the complete plant being of more recent date.
Among mushrooms four of the genus Agaricus are luminous, and have been examined with special reference to this effect by Delille, Fabre, and Tulasne: The red or orange-colored species (Agaricus olearius), inhabiting the adjacent soil or roots of olive-trees in Central Europe; the fire-mushroom, or Agaricus igneus, which Rumphius discovered in Araboyna; the Agaricus noctilucus, found at Manila by Gandichaud; and Agaricus Gardineri of the Brazilian provinces, growing upon the dead leaves of the Pindoba-palm. The red mushroom of the olive-trees is wonderfully beautiful. The gills curve out from the pedicle and expand under the pileus into a trumpet-or bell-like form of almost vermilion hue, which changes at night into a pale blue light, gleaming, where they are massed together, like blue bells of fire.
M. de Candolle erroneously supposed that the phosphorescence of the Agaricus of the olive occurred only at the time of its decomposition. M. Fries, with equal error, attributed the effect to the presence of a secondary parasite; Tulasne, however, denies that the seat of light is in the mold, and states that he has observed the phosphorescence of the plant itself. He agrees with Delille in regarding the appearance as limited to the period of growth, and refers to it as a "manifestation of vegetation." M. Delille supposed the radiance to be intermittent, while M. Fabre observed that exposure to the sunlight appeared to have no influence whatever upon the phenomena, and that the light was exhibited at any time under cover of darkness. Dr. Phipson, in reviewing M. Fabre, remarks that this seems, however, to indicate that the light of the sun has in reality an influence upon the emission of light during the daytime, and that the phenomenon is probably a case of phosphorescence after insolation. But as we know the Agaricus belongs to that class of colored parasites which are destitute of green foliage, and consequently of proper digestive organs of their own, and draw support from the elaborated products of the foster-plant, the phosphorescence may be accounted for as the result of chemical action under conditions where the influence of solar light is not required to produce a higher combination.
Tulasne recognized that the light was not confined exclusively to the reproductive surfaces, and proved by dissection that the whole mass offered scintillations. This is probably due to a sympathetic or highly vitalized condition of the whole plant during the process of fecundation, as is indicated by the juice of the phosphorescent euphorbia. The light, however, is usually emitted by the organs of reproduction; this is particularly the case in young fungi, while, in the older ones, after the luminosity has disappeared from the gills, the surface of the stipe becomes radiant.
The mycelium of the root-shaped fungi (rhizomorpha) penetrates through the decaying organic matter of wood and coal deposits, exhibiting a clear pale light. This has been particularly observed in the mines near Dresden, Hesse, and occasionally in England. These flowerless, gleaming plants impart a weird beauty to the caverns of granitic rock in Bohemia, illuminating them with a pale mimicry of moonlight.
The most remarkable instances of cryptogamic phosphorescence have been noted by Mr. Gardner in Brazil, by Dr. Cuthbert-Collingwood in Borneo, by Mr. Hugh Low and James Drummond in Australia, and by Mr. Worthington-Smith in the Cardiff coal-mines. Rev. J. M. Berkeley cites an instance in England, where a dazzling radiance was observed upon a spruce or larch log, which continued for several days—a byssoid mycelium, yielding an unusually pungent odor, being recognized. The common potato also, in decomposing, generates a peculiarly luminous parasite; and, at one time, an alarm of fire was sounded in the streets of Strasburg from the light produced by a decaying mass stored in a cellar. An instance has recently come to my notice, where a brilliant light was thrown off by pieces of cantaloupe, after a few hours' exposure to the air.
Wherever, then, we encounter decomposing vegetable matter, we observe some form of fungi living upon and appropriating the changed substances of a former condition to the generation of a new life. What, therefore, seems to us a loss or waste, is merely change—change of form, change of condition. The absorbing roots of these parasites grow into the tissues of the host in the most intimate manner, deriving from a disorganization of the substances the elements necessary to their own being.
Vergil describes the blighting mildew on the grain as "an unbidden crew of graceless guests that choke the fields"; and De Barry writes that there is a frequent unbidden guest in every household, who lays under contribution its stores of sweets. The mold or mildew which gathers on the surface of preserves is a plant of exquisite beauty when viewed with low magnifying power and by reflected light, for what appears to the naked eye only a soft, white, woolly crust, becomes a glittering forest of graceful stems and branches, standing like fine spun silver upon the dark background of the supporting surface. This substratum is in reality a mycelium, or system of fine interlacing, thread-like roots, which form the vegetative part of the plant, and are woven into a soft, black or brown velvety substance, through which run russet, scaly hairs. The branches rise to about the fiftieth of an inch, and bear the fruit and seed-cells. Higher microscopic power reveals the spores which are the analogues of seed, although they possess neither their vestments nor organs, but are simple membranous sacs (full of liquid), which germinate at some indefinite point of their convex surface into a new plant like that which produced them.
A comparison of the condition of flowering plants and of the mosses and fungi, during a phosphorescent display, will lead us to attribute the appearance to similar action of vital forces, if we for a moment review the processes by which these forces are accumulated. In the growth of plants heat acts as a dynamic agent, which the germ of the plant directs and uses. The first action of growth consists in the conversion of the starch of the seed into a soluble form, and in chemically combining the starch, sugar, oil, and albumen by fermentation into the protoplasmic matter which supplies the material for the tissues of the plant. The development of this protoplasm into organized tissue is due to the inherent power of the germ, and marks the second stage of progress. The only action of light is employed in producing higher chemical combination; heat acts as the constructive power.
The process of growth does not absorb all the elaborated materials provided, and an additional amount of heat or force is generated by the decomposition or a "retrograde transformation" of these compounds. A much greater proportion, however, of the organized substances is stored up within the structure of the plant as a "reservoir of reserved material," to meet the exhaustive process of flowering and the maturing of fruit and seed. That these processes are sustained at the expense of an extra amount of force, and by the decomposition of their own products, is evident from the unusual production of carbonic acid. The combination of carbon, oxygen, and hydrogen, in the building up of plants, is here reversed, and by a retrograde process heat is set free, which we have seen is necessary for the elaboration of reproductive agents.
This appears to be precisely the same action which takes place in the reproduction of fungoid plants and mosses. A corresponding condition is shown by the rapid exhalation of carbonic acid; indeed, Dr. Carpenter asserts that a decomposition of a portion of the absorbed material is the only conceivable source of the large quantity they are constantly giving out, and ascribes the very rapid growth of these plants to a "retrograde metamorphosis." The substances which enter into the new growth are already prepared by the foster-plant, and we find the parasite incapable of forming any new combination through the agency of light. This necessarily awakens a doubt of any action resulting from insolation or reflection in the cases of the liverworts or mushrooms which have been referred to.
The expression of light in flowering plants seems to be through the medium of electricity, while in cryptogams it resembles the steady glow of slow combustion. High microscopic power may, however, reveal, as in some cases of animal phosphorescence, the sparkle and flash of rapid and minute scintillations. Inquiry in this direction, I think, has never been made; nor is an analysis necessary for our present purpose. For, if we regard the light either as the result of slow combustion, or catch the vivid flash of the electric spark in this reproduction (itself the product of decay), we are met by the same inexorable permanent law, that there is not a leaf that rots by the roadside, nor a spear of pale club-moss, that is not in itself a reservoir of recreative power throwing back its faint, pulsing light or its equivalent of heat into that quickening flood which the great heart of Nature sends down through the illimitable and unknown.