The Parents' Review

A Monthly Magazine of Home-Training and Culture

The Seasons.

Diatoms.

by Mrs. Dorman.

It may seem a trite saying that by the invention of the microscope, worlds not only unrealized but even unimagined before, have been brought within our view, and yet one cannot look into a drop of pond or bog water, teeming as it is with vegetable life in its minutest developments, without being impressed by something akin to awe, so great are the variety and beauty of the forms that are revealed by the magic glass. Through that glass we see, darkly indeed, but yet unmistakably, how the same creative energy that framed the universe, has delighted to expend itself in bringing all this varied life into being, and in tracing lines of exquisite beauty forms that are all but invisible to the unassisted eye, so that the infinitely small seems as marvelous as the infinitely great. All these finely sculptured lines are, moreover, traced upon a material so fragile as to have earned for the tiny Diatoms the name of Brittleworts, and yet so indestructible that myriads of them have been preserved in a fossil state for countless ages in the so-called Diatom earths, or Tripoli powder.

It is calculated that forty millions of these delicate forms are contained in one cubic inch of Bilin slate; and a marine deposit, forty feet in depth and of unknown extend, composed for the most part of the siliceous skeletons of indestructible Diatoms, underlies the city of Richmond, U.S.A. We may find them in a living state in any pool, or ditch, or horse-trough, no less than in the surface layers of the deep sea, and even amongst the organic dust of the snowfield. Dr. Hooker found Diatoms discolouring the Antarctic seas, and also in the lava of a volcanic mountain. There is a whitish powdery substance known in Norway and Sweden as "mountain meal," and used by the peasants to mix with flour for bread, which is composed entirely of fossil Diatoms, and great tracts both in the arctic and Antarctic regions are covered with these infinitesimal relics of a former vegetation.

What, then, are Diatoms? To what class of the vegetable kingdom do they belong? What is their nature, and what is known of their life history? How may they be recognized, and how distinguished from other microscopic plants, and especially from their kindred forms the Desmids, and why, when their short lives are ended, does not their memorial perish with them? To the first question, the answer is that Diatoms are "unicellular plants which grow both in salt and fresh water and upon moist soil." To the second question, the Diatomaceae used to be placed with the Algae, that is, when they had ceased to be considered forms of animal life, but they are now regarded as occupying an intermediate position between animals and plants, and form an isolated group called Bacillariales or rod-like plants; we may therefore expect their characteristics to be such as to warrant this exclusive treatment. First, as to the limits of these minute plants, for we have seen that our Diatom is confined within a single cell, and we find that its cell-wall, instead of consisting of purely vegetable matter, is formed into a veritable coat of mail by a silicious or flinty deposit, perfectly transparent indeed, but at the same time hard enough for innumerable dots and lines to be fretted or engraved upon its surface, and notwithstanding its fragility, indestructible alike by fire or acids. Under certain aspects of the cell wall may be said to answer to the shell of a bivalve, under others to a skeleton, and it is made up of two narrow symmetrical plates, or valves, which exactly fit into each other, the living plant being enclosed in the box or shell formed by the two halves. Whether the derivation of the name Diatom from diatomos--cut through--refers to this division of the cell-wall or to the manner of propagation by division, I am not clear. The Diatomaceae may be at once distinguished from the Desmids, to which they bear a superficial resemblance, by their colour, as only the large Diatom--Pinnularia viridis--approaches in any degree to the brilliant green of the Desmids; and they are usually of a bright golden brown, as the protoplasm and chlorophyll are stained and masked by a pigment called diatomin.

In a drop of bog water, several common species will be found, some resting, some slowly moving, for Diatoms are by no means stationary. One little voyager, shaped like a miniature boat, pushes gently along through a seemingly impenetrable mass of debris, passes it, reaches some object too difficult for him, slowly retires, returns again, again is baffled, once more returns undaunted to the charge, and, perhaps by some fortuitous change in the concourse of atoms, finds the passage he seeks for, and at last reaches a clear space in the luminous drop. Here, side by side, is a row of Fragillaria virescens or Diatoma vulgare in a ribbon-like band, and here again we find the band is broken and the tiny plants attached to each other in zigzag fashion by their corners. Now we catch sight of a detached frustule of Gimphonema capitatum and begin to look eagerly for the branching filaments from which it has become detached, or a wandering Navicula of another species than the first arrests our attention, crossing a bright space in which seem to float a dozen or more dark-green crescents of the lovely Desmid--Closterium lunula--and surely, now we have a specimen of Pinnularia viridis, the largest of British fresh water Diatoms, for it shows as a giant amongst the other species, and one can easily see the box-like form of the plant case, and the striae lying at right angles to its long axis, while near it is another very long and slender Navicula, with a clearly-defined line down the centre, but no visible striae; they are there, doubtless, but cannot be seen with a quarter-inch objective.

A long and rod-like Diatom, perhaps an individual of Synedra splendens, has a rim of finest markings along the sides, but my drawing is from an old sketch-book, and was made some years ago when I was studying Duckweeds, and before I knew anything of Diatoms, but I think I was right in naming it Synedra ulva, as that species is frequently found attached to some aquatic plant; and this is the last of my acquaintance with living Diatoms.

We will take Pinnularia viridis for further study, as its comparatively large size permits us to see clearly the markings on the frustules, and the structure is more easily made out than in the smaller Diatoms. If we get an individual under observation for more than a very short period, we shall find that it has a wonderful faculty for movement, both backward and forwards, and that it frequently changes its aspect, so as at one moment to have the appearance of a long oval, and the next of a rectangle with somewhat rounded corners. These two aspects are called respectively the valve, and the girdle sides. On the valve side, furrows are seen running towards the central line, but not quite reaching it; these furrows are considered to indicate thin places on the surface of the cell-wall; on the girdle side they reappear, but only as a narrow border to the cell. With careful focusing and management of light, the box-like form of the cell, one half of which fits into the other half, may be seen, and in dead specimens still more clearly. The contents of the cell have also a different aspect as seen from the valve, or girdle side. The sort of bridge in the girdle view, is said to be the central collection of protoplasm. We noticed that the cells of Pinnularia were mobile, but they do not swim freely, and are supposed rather to creep along the substratum by means of a delicate protoplasmic edge like a foot, which is protruded through the cleft between the valves. If a living Diatom be heated over a flame, the result is that the cell contents are destroyed, and the cell itself remains perfect, as a skeleton. In this state, the very fine markings on the cell wall are better studied than when the plant is living. The variously sculptured, dotted and ribbed surfaces of Diatoms are favourite microscopic objects on account of their beauty and delicacy.

Under a high power, the striae of the Pleurosigma resolve them-selves into hexagonal dots, and their number may be imagined from the fact that they are calculated to be 40,000 to the square inch. This is a comparatively large Diatom.

Aulacodiscus formosus is wonderful for the beauty of its skeleton, and is so minute that it only shows as a dot on the microscopic slide. The structure appears to be cellular, and there are four exquisitely-formed tubercles set like a Maltese cross. On my slide, one disc is smaller than the other, and I take it to be the second of the two halves. Diatoms multiply themselves by sub-division lengthwise. The frustules, as the shell-like cells are technically called, separate, and two plants are thus formed out of one; each half keeps one parent valve, and develops a new and smaller one on the opposite side. By this process of multiplication it is evident that these already minute organisms must get smaller by degrees, and finally reach the vanishing point! In point of fact, this condition of things does not go on indefinitely, for when a certain size is reached, a different course is followed. Either the cell contents free themselves altogether from the cell wall, enlarge, and put on new valves exactly like the parent, but much larger; or two individuals unite to form what is known as an auxopore. Sometimes the two individuals become imbedded in mucilage previous to the formation of an auxopore.

Proofread July 2011, LNL