DIATOMACEAE. For the knowledge we possess of these beautiful organisms, so minute as to be undiscernible by our unaided vision, we are indebted to the assistance of the microscope. It was not till towards the close of the last century that the first known forms of this group were discovered by O. F. Muller. And so slow was the progress of discovery in this field of scientific research that in the course of half a century, when Agardh published his Systema Algarum in 1824, only 49 species included umder 8 genera had been described. Since that time, however, the microscope has been greatly improved ; and eminent naturalists in all parts of the civilized world have been induced to engage in the study of these forms. The result is that the number of known genera and species has been greatly increased; the species found in Great Britain and Ireland may be estimated at little less than 1000; and Rabenhorst, in the index to his Flora Europcea, enumerates about 4000 forms which have been discovered throughout the continent of Europe. At a time when little was understood of the structure of these organisms they were, generally known among botanists by the appellation of Bacillariaceos ; but almost all recent authors are agreed in adopting the later and more appropriate designation of Diatomaceaa.
Various opinions have been entertained as to the position to be assigned to these forms. The earlier observers referred them to the vegetable kingdom. Subsequent authors, including Ehrenberg, regarded them as animals; but in consequence of their analogy to other organisms acknowledged to be vegetable, as regards their general structure, and more especially their modes of reproduction, they are now almost universally included in the vegetable kingdom, and classified with the Monocellular Algae.
FIG. 1.Coscinodiscus FIG. 2.Surirella
perforatus. X 400. ovalis. x 400.
Some are linear, as Synedra radians (fig. 3) ; others more
FIG. 3.Synedra radians. x 200. or less crescentic, as Epithemia hyndmanii (fig. 4) ; others,
The Diatomaceae exhibit great variety in form. While some species are circular, as Coscinodiscus perforatus (fig. 1), others are of an oval outline, as Surirella ovcdis (fig. 2).
FIG. 4.Epithemia hyndmanii. x 400. again, are cuneate, as Podosphenia lynghyii (fig. few have a sigmoid outline, as Pleurosigma balticun (fig. 6) ; but the prevailing forms are naviculoid, as Navicula cuspidata (fig. 7). They vary greatly also in their modes of growth,some being free, others attached to foreign bodies by gelatinous stipes, the stipes being in some species very short, while in others they are of considerable length. In some genera the forms are simple, while in others the frustules are connected together in ribbon-like filaments, or form, as in other cases, zig-zag chains. In some genera the frustules are naked, while in many others they are inclosed in a more or less definite gelatinous investment, or frond, as this covering is usually designated. The conditions necessary to their growth are moisture and light. Wherever these circumstances coexist, diatomaceous forms will almost invariably be found. They occur mixed with other organisms on the surface of moist rocks ; in streamlets and pools, they form a brownish stratum on the surface of the mud, or cover the stems and leaves of water plants or floating twigs with a furry investment. Marine forms are usually attached to various sea-weeds, and many are found molluscs, holothurians, ascidians, and other denizens of the ocean. The fresh-water forms are specifically distinct from those incidental to salt or brackish water,fresh-water species, however, are sometimes carried some distance into the sea by the force of the current, and in tidal rivers marine forms are carried up by the force of the tide. Some notion may be formed of the extreme minuteness of these forms from the fact that one the length of which is 10°0°00ths of an inch may be considered as beyond the medium size. Some few, indeed, are much larger, but by far the greater proportion are of very much smaller dimensions.
Structure. -These minute vegetables are distinguished from kindred forms by the fact of having their soft vegeta-tive part covered by a siliceous case. This covering of silex consists of two similar valves nearly parallel to each other, each valve being furnished with a rim projecting from it at a right angle. One of these valves with its rim is slightly smaller than the other, the smaller fitting into the larger pretty much as a pill box fits into its cover. This peculiarity of structure affords ample scope for the growth of the cell-contents usually known as the endochrome. As the endochrome increases in volume the siliceous valves are pushed out, and their corresponding siliceous rims become broader.
As regards the vegetative contents of this cell, in so brief a description the following parts only need to be referred to. There is first what Pfitzer, a distinguished German writer on this subject, designates the plasm-sac, consisting of a fine colourless plasm forming a closed sac of the same shape as that of the cell. The refractive power of this plasm differing but slightly from that of water, the presence of this structure is not always obvious ; but on the application of hydrochloric acid its outline may be discerned as it slowly separates from the cell wall,at first preserv-ing the shape of the cell, but ultimately contracting into a small round mass. Within the plasm-sac is the structure which the writer just named designates the endochrome-plates. They consist of a thick substance, and are of the same colour throughout, varying from bright yellow to a dark yellowish brown. The number and position of the endochrome plates vary in the different generasome having two, others only one. Within the folds of these plates is sometimes noticeable a collection of plasm which Ehrenberg describes as resembling the embryo in an egg, and which Pfitzer calls the middle plasm-mass. Within this plasm-mass oil globules and vacuoles are diffused, and in the centre of it a small vesicle may often be observed.
Motion.One of the first phenomena which comes under the notice of the observer is the extraordinary power of motion with which the frustules are endowed. Some species move slowly backwards and forwards in pretty much the same line, but in the case of Bacillaria paradoxa the motion is very rapid, the frustules darting through the water in a zig-zag course. To account for this motion various theories have been suggested, none of which appear to be altogether satisfactory. So while the extraordinary motion of the Diatomaceae excites admiration, it must bo acknowledged that the mechanical agency which produces the motion remains unexplained.
Classification.In this group, as well as in almost all others, various systems of classification have from time to time been adopted ; but that which seems to commend itself most strongly, as well by reason of its simplicity as its facility of application, is the system which has been matured by Heiberg, the distinguished Danish writer on the subject, and which he has founded on the symmetrical or unsymmetrical form of the frustule in its several aspects. A diatomaceous frustule may be regarded on what is called the front view, in which the connecting rim or hoop is seen, or on the side view, by which the valve is presented to the eye of the observer. If the outline be symmetrical both on the transverse and longitudinal axis, in both these aspects the frustule is said to be symmetrical; but if the outline be different on one side from that of the other, or if perfect symmetry does not exist as respects the longitudinal or transverse axis, the frustule is said to be unsymmetrical on the aspect or axis in which want of symmetry is found to exist.
Reproduction.In the Diatomacese, as well as in the Desmidiere, the ordinary mode of increase is by self-division of the cell (see ALGJS, vol. i. p. 508). The cell-contents within the inclosure of the siliceous case separate into two distinct masses. As these two masses of endochrome become more and more developed, the valves of the mother cell are pushed more and more widely apart. A new siliceous valve is secreted by each of the two masses on the side opposite to the original valve. When this process has been completed the hoop of the mother frustule gives way, and two distinct frustules are formed, the siliceous valves in each of these new frustules being one of the valves of the mother cell, and a newly formed valve similar and more or less parallel to it.
During the life of the plant this process of self-division is continued with an almost incredible rapidity. On this subject the observation of the late Professor Smith is worthy of special notice:"I have been unable to ascertain the time occupied in a single act of self-division, but sup-posing it to be completed in twenty-four hours we should have, as the progeny a of single frustule, the amazing number of 1,000,000,000 in a single month, a circumstance which will in some degree explain the sudden, or at least rapid, appearance of these organisms in localities where they were a short time previously either unrecognized or sparingly diffused " (British Diatomacece, vol. i. p. 25).
Some authors of reputation have been under the impression that the Diatomaceae, like other kindred forms, are sometimes reproduced by zoospores, and some few facts from time to time have been recorded by various observers which seem to bear out this view of the case. But in this group, as well as in the Desmidiese already referred to, there obtains another mode of reproduction which is gene-rally known as conjugation. It would be unnecessary here to describe in detail the various observed modes of this process. Suffice it to say that usually two parent frustules unite, invest themselves in a gelatinous sac in which their cell contents are discharged and formed into two bodies termed sporangia, which soon are developed into two frustules La all respects resembling the parents but usually double their size. In some phases of this process the gela-tinous sac bears a considerable resemblance to that lowest form of animal life known by the name of Amceba, so much so that an inexperienced observer might suppose that the object before him was an Amceba gorged with diatomaceous frustules.
Mode of Preparation.The Diatomacere are usually gathered in small bottles, and special care should be taken to collect them as free as possible from extraneous matter. Asmall portion having been examined under the microscope, should the gathering be thought worthy of preservation, some of the material is boiled in acid for the purpose of cleaning it. The acids usually employed are hydrochloric, nitric, or sulphuric, according as circumstances require. When the operator considers that by this process all foreign matter has been eliminated, the residuum is put into a pre-cipitating jar of a conical shape, broader at the bottom than at the top, and covered to the brim with filtered or distilled water. When the diatoms have settled in the bottom of the jar, the supernatant fluid is carefully removed by a syringe or some similar instrument, so that the sediment be not disturbed. The jar is again filled with water, and the process repeated till the acid has been completely removed. It is desirable afterwards to boil the sediment for a short time with supercarbonate of soda, the alkali being removed in the same manner as the acid. A small portion may then be placed with a pipette upon a slip of glass, and, when the moisture has been thoroughly evaporated, the film that remains should be covered with dilute Canada balsam, and, a thin glass cover having been gently laid over the balsam, the preparation should be laid aside for a short time to harden, and then is ready for observation.
General Remarks.Like all other organisms, the Diatomaceae doubtless have a definite function assigned to them in the grand system of creation, but a special interest attaches to them. Allusion has been made to the fact that the soft cell of these organisms is encased in a siliceous epiderm. When the plant has fulfilled its natural course the siliceous covering sinks to the bottom of the water in which it had lived, and there forms part of the sediment. When in the process of ages, as it has often happened, the accumulated sediment has been hardened into solid rock, the siliceous exuviae of the diatoms remain unaltered, and, if the rock be disintegrated by natural or artificial means, may be removed from what has been called " their stony shroud," and subjected to examination under the microscope. The forms found may from their character help in some degree to illustrate the conditions under which the stratum of rock had been originally deposited.
Vast deposits of Diatomaceae have been discovered in
various parts of the world,some the deposit of fresh,
others of salt water. Of these deposits the most remark-
able for extent, as well as for the number and beauty of the
species contained in it, is that of Richmond, in Virginia,
one of the United States of America. It is said to extend
for many miles, and to be in some places at last 40 feet
deep The material has long been used as a polishing
powder, and recently has been largely employed in the
manufacture of the powerful explosive agent known as
dynamite. It is a remarkable fact that existing species of
Diatomaceae have been traced so far down as the lower
strata of the Tertiary formation ; and, though the genera-
tions of a diatom in the space of a few months far exceed
in number the generation of man during the period usually
assigned to the existence of the race, the fossil genera and
species are in all respects to the most minute details
identical with the numerous living representatives of their
class. (E. O'M.)