LVI. DIATOMACÆ, Ag.
The Waters and the Ice of the South Polar Ocean were alike found to abound with microscopic vegetables belonging to this Order. Though much too small to be discernible by the naked eye, they occurred in such countless myriads, as to staiu the Berg and the Pack-ice, wherever they were washed by the swell of the sea; and when enclosed in the congealing surface of the water, they imparted to the Brash and Pancake-Ice a pale ochreous colour. In the open ocean, northward of the Frozen Zone, this Order, though no doubt almost universally present, generally eludes the search of the naturalist ; except when its species are congregated amongst that mucous scum which is sometimes seen floating on the waves, and of whose real nature we are ignorant ; or when the coloured contents of the marine animals who feed on these Algae are examined. To the south, however, of the belt of ice which encircles the globe, between the parallels of 50° and 70° S., and in the waters comprised between that belt and the highest latitude ever attained by man, this vegetation is very conspicuous, from the contrast between its colour and the white snow and ice in which it is imbedded. Insomuch, that, in the eightieth degree, all the surface-ice carried along by the currents, the sides of every berg, and the base of the great Victoria Barrier itself, within reach of the swells, were tinged brown, as if the Polar waters were charged with oxide of iron.
As the majority of these plants consist of very simple vegetable cells, enclosed in indestructible silex (as other Alym are in carbonate of lime), it is obvious that the death and decomposition of such multitudes must form sedimentary deposits, proportionate in then - extent to the length and exposure of the coast against which they are washed, in thickness to the power of such agents as the winds, currents and sea, which sweep them more energetically to certain positions, and in purity to the depth of the water and nature of the bottom. Hence we detected their remains along every ice-bound shore, in the depths of the adjacent ocean, between eighty and 400 fathoms. Off Victoria Barrier (a perpendicular wall of ice, between one and two hundred feet above the level of the sea), the bottom of the ocean was covered with a stratum of pure white or green mud, composed principally of the siliceous cells of Diatomacece. These, on being put into water, rendered it cloudy, like milk, and took many hours to subside. In the very deep water off Victoria and Graham's Land, this mud was particularly pure and fine; but towards the shallower shores, there existed a greater or less admixture of disintegrated rocks and sand ; so that the organic compounds of the bottom frequently bore but a small proportion to the inorganic.
Being indebted to the works of the illustrious Elrrenberg for all I knew of these organisms, previous to the sailing of the Antarctic Expedition, I had supposed the Diatomacece to belong to the Animal Kingdom[1]; and as they are unaccompanied in the Antarctic region by any evidence of a higher order of plants, I had always supposed vegetation to cease at a much lower latitude than these productions actually attain. The species were, however, collected on every available occasion, and transmitted, on my return to England, to Professor Ehrenberg, whose determination of the genera and species is here introduced, at the suggestion of the Rev. M. J. Berkeley and other eminent Cryptogamic botanists.
- ↑ It is well known that the true nature of the Diatomacece has been long and unsuccessfully disputed, being claimed both by botanists and zoologists. No conclusive evidence on this subject had been adduced, till, within these very few days, it was the singular good fortune of my friend, Mr. Thwaites, of Bristol, a most acute observer and profound Cryptogamist, to detect several species of Diatomacece conjugating, in a manner perfectly analogous to that pursued by the Zygneiaata: a fact which leaves no doubt of their vegetable origin in the minds of persons acquainted with his interesting observations. I am indebted to Mr. Thwaites for specimens of three British species of Eunotia, and Gomphotwma, illustrating this important discovery, and mounted in fluid, after the beautiful plan invented by that gentleman for preserving vegetable tissues moist, and always ready for the microscope in the form of slides.
