Note 75 — IDEAS FOR A PHYSIOGNOMY OF PLANTS.

p. 212—“Like luminous stars.”

The phosphorescence of the ocean is one of those splendid phenomena of nature which excite our admiration, even when we behold its recurrence every night for months together. The ocean is phosphorescent in all zones of the earth, but he who has not witnessed the phenomenon in the tropics, and 246especially in the Pacific, can form but a very imperfect idea of the majesty of this brilliant spectacle. The traveller on board a man-of-war, when ploughing the foaming waves before a fresh breeze, feels that he can scarcely satisfy himself with gazing on the spectacle presented by the circling waves. Wherever the ship’s side rises above the waves, bluish or reddish flames seem to flash lightning-like upwards from the keel. The appearance presented in the tropical seas on a dark night is indescribably glorious, when shoals of dolphins are seen sporting around, and cutting the foaming waves in long and circling lines, gleaming with bright and sparkling light. In the Gulf of Cariaco, between Cumana and the Peninsula of Maniquarez, I have spent hours in enjoying this spectacle.

Le Gentil and the elder Forster ascribed these flames to the electrical friction of the water on the vessel as it glides forward—an explanation that must, in the present condition of our physical knowledge, be regarded as untenable.^[KD]

There are probably few subjects of natural investigation which have excited so many and such long-continued contentions as the phosphorescence of sea-water. All that is known with certainty regarding this much disputed question may be reduced to the following simple facts. There are many luminous mollusca which possess the property when alive of emitting at will a faint phosphoric light; which is of a bluish tinge in Nereis noctiluca, Medusa pelagica var. β,^[KE] and in the pipe-like Monophora noctiluca, discovered in Baudin’s expedition.^[KF] The luminosity of sea-water is in part owing to living light-bearing animals, and in part to the organic fibres and membranes of the same, when in a state of decomposition. The first-named of these causes of the phosphorescence of the ocean is undoubtedly the most common and the most widely diffused. The more actively and the more efficiently that travellers engaged in the study 247of nature have learnt to employ powerful microscopes, the more our zoological systems have been enriched by new groups of mollusca and infusoria, whose property of emitting light either at will or from external stimulus has been recognised.

The luminosity of the sea, as far as it depends on living organisms, is principally owing, among zoophytes, to the Acalephæ (the families of Medusæ and Cyaneæ), to some Mollusca, and to an innumerable host of Infusoria. Among the small Acalephæ (Sea-nettles), the Mammaria scintillans presents us, as it were, with the glorious image of the starry firmament reflected in the surface of the sea. When full-grown this little creature scarcely equals in size the head of a pin. The existence of siliceous-shelled luminous infusoria was first shown by Michaelis at Kiel. He observed the coruscation of the Peridinium. (a ciliated animalcule,) of the Cuirass-monad (Prorocentrum micans), and of a rotifer, which he named Synchata baltica,^[KG] the same that Focke subsequently found in the lagoons of Venice. My distinguished friend and fellow traveller in Siberia, Ehrenberg, succeeded in keeping two luminous Infusoria of the Baltic alive for nearly two months at Berlin. I examined them with him in 1832; and saw them coruscate in a drop of sea-water on the darkened field of the microscope. When these luminous Infusoria (the largest of which was only ⅛ and the smallest from ¹⁄₄₈ to ¹⁄₉₆ of a of a Parisian line in length) were exhausted, and ceased to emit sparks, they would renew their flashing on being stimulated by the addition of acids or by the application of a little alcohol to the sea-water.

By repeatedly filtering fresh sea-water, Ehrenberg succeeded in procuring a fluid in which a large number of these light-emitting animalcules were accumulated.^[KH] This acute observer has found in the organs of the Photocharis which give off flashes of light (either voluntarily or when stimulated), a cellular structure of a gelatinous character in the interior, and which manifests some similarity with the electric organ of the Gymnotus and the Torpedo. “When the Photocharis is irritated, in each cirrus a kindling and a gleaming of separate sparks may be observed, which gradually increase and at length illuminate the 248whole cirrus; until the living flame runs also over the back of this nereid-like animalcule, making it appear under the microscope like a burning thread of sulphur with a greenish-yellow light. In the Oceania (Thaumanthias) hemisphærica, the number and position of the sparks correspond accurately, at the thickened base, with the larger cirri or organs which alternate with them, a circumstance that merits special attention. The manifestation of this wreath of fire is an act of vitality, and the whole development of light an organic vital process, which exhibits itself in Infusorial animals as a momentary spark of light, and is repeated after short intervals of rest.”^[KI]

The luminous animals of the ocean appear, from these conjectures, to prove the existence of a magneto-electric light-generating vital process in other classes of animals besides fishes, insects, mollusca, and acalephæ. Is the secretion of the luminous fluid which is effused in some animalcules, and which continues to shine for a long period without further influence of the living organism (as, for instance, in Lampyrides and Elaterides, in the German and Italian glow-worms, and in the South American Cucuyo of the sugar-cane), merely the consequence of the first electric discharge, or is it simply dependent on chemical composition? The luminosity of insects surrounded by air assuredly depends on physiological causes different from those which give rise to a luminous condition in aquatic animals, fishes, Medusæ, and Infusoria. The small Infusoria of the ocean, being surrounded by strata of salt-water which constitutes a powerful conducting medium, must be capable of an enormous electric tension of their flashing organs to enable them to shine so vividly in the water. They strike like the Torpedo, the Gymnotus, and the Electric Silurus of the Nile, through the stratum of water: whilst electric fishes which, in connection with the galvanic circuit, are capable of decomposing water, and of imparting magnetic power to steel needles. (as I showed more than half a century ago,^[KJ] and as John Davy has more recently confirmed,^[KK]) yield 249no indications of electricity through the smallest intervening stratum of flame.

The considerations which we have here developed render it probable that one and the same process operates, alike in the smallest living organisms invisible to the naked eye, in the contests of the serpent-like Gymnoti, in the flashing luminous Infusoria which impart such glorious brilliancy to the phosphorescence of the sea, in the thunder-cloud and in the terrestrial or polar light (the silent magnetic flashes), which, caused by an increased tension of the interior of the earth, are announced, for some hours previously, by the sudden variations of the magnetic needle.^[KL]

Sometimes one cannot, even with high magnifying powers, discover any animalcules in the luminous water; and yet, wherever a wave breaks in foam against a hard body, and, indeed, wherever water is violently agitated, flashes of light become visible. The cause of this phenomenon depends probably on the decomposing fibres of dead Mollusca, which are diffused in the greatest abundance throughout the water. If this luminous water be filtered through finely woven cloths, the fibres and membranes appear like separate luminous points. When we bathed at Cumana, in the gulf of Cariaco, and walked naked on the solitary beach in the beautiful evening air, parts of our bodies remained luminous from the bright fibres and organic membranes which adhered to the skin, nor did they lose this light for some minutes. If we consider the enormous quantity of Mollusca which animate all tropical seas, we can hardly wonder that sea-water should be luminous, even where no fibres can be visibly separated from it. From the endless subdivision of the masses of dead Dagysæ and Medusæ the whole ocean may, in fact, be regarded as a fluid containing gelatine, and, as such, luminous and of a nauseous taste; unfit for the use of man, but capable of affording nourishment to many species of fish. On rubbing a board with a portion of the Medusa hysocella, the surface thus rubbed recovers its phosphorescence when friction is applied by means of the dry finger. During my voyage to South America I occasionally placed a Medusa on a tin plate, and I then observed that if I struck the plate with another metallic 250substance the slightest vibrations of the tin were sufficient to cause the animal to emit light. How do the blow and the vibrations here act? Is the temperature momentarily augmented, or are new surfaces presented? or, again, does some gaseous matter such as phosphuretted hydrogen, exude in consequence of this impulse, and burn when it comes in contact with the oxygen of the atmosphere, or with that dissolved in the sea-water, and by which the respiration of the Mollusca is maintained? This light-exciting effect of the blow is most remarkable in a cross or sugar-loaf sea, (mer clapoteuse,) where the waves, clashing from opposite directions, rise in a conical form.

I have seen the ocean, in the tropics, luminous in the most opposite kinds of weather, but most strongly so before a storm, or in a sultry and hazy atmosphere with thick clouds. Heat and cold appear to exercise but little influence on this phenomenon, for, on the Bank of Newfoundland, the phosphorescence is frequently very brilliant in the severest winter. Occasionally, too, the sea will be highly luminous one night, and not at all so on the following, notwithstanding an apparent identity of external conditions. Does the atmosphere favour this development of light? or do all the differences observed during this phenomenon depend on the accidental circumstance of the sea being more or less impregnated, in some parts, with the gelatinous portions of mollusca? Perhaps these phosphorescent social animalcules only rise to the surface under certain conditions of the atmosphere. It has been asked, why our fresh-water swamps which are filled with polyps are not phosphorescent. It would appear that, both in animals and plants, a peculiar mixture of organic particles favours this development of light; thus, for instance, the wood of the willow is more frequently found to be luminous than that of the oak. In England, salt-water has been rendered luminous by mixing herring-brine with it; indeed, it will be easy for any one to convince himself by galvanic experiments, that the luminosity of living animals depends on nervous irritation. I have observed strong phosphorescence emitted from a dying Elater noctilucus, on touching the ganglion of its fore leg with zinc and silver. Medusæ also occasionally emit a stronger light at the moment the galvanic circuit is completed.^[KM]