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DIATOMACEAE

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Originally appearing in Volume V08, Page 171 of the 1911 Encyclopedia Britannica.
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DIATOMACEAE . For the knowledge we possess of these beautiful See also:

plants, so See also:minute as to be undiscernible by our unaided See also:vision, we are indebted to the assistance of the See also:microscope. It was not till towards the See also:close of the 18th See also:century that the first known forms of this See also:group were discovered by O. F. See also:Muller. And so slow was the See also:process of See also:discovery in this See also:field of scientific re-See also:search that in the course of See also:half a century, when Agardh published his Systema algarum in 1824, only See also:forty-nine See also:species included under eight genera had been described. Since that See also:time, however, with See also:modern microscopes and microscopic methods, eminent botanists in all parts of the civilized See also:world have studied these minute plants, with the result that the number of known genera and species has been greatly increased. Over Io,000 species of diatoms have been described, and about 1200 species and numerous varieties occur in the fresh See also:waters and on the coasts of See also:Great See also:Britain and See also:Ireland. Rabenhorst, in the See also:index to his See also:Flora Europaea algarum (1864) enumerated about 4000 forms which had up to that time been discovered throughout the See also:continent of See also:Europe. The diatoms are more commonly known among systematic botanists as the Bacillarieae, particularly on the continent of Europe, and although such an immense number of very diverse forms are included in it, the group as a whole exhibits a remark-able uniformity of structure. The Bacillarieae is one of the large See also:groups of See also:Algae, placed by some in close proximity to the A and B, Melosira arenaria. C-E, Melosira varians.

E, showing formation of auxospore. Conjugatae and by others as an See also:

order of the See also:Brown Algae (or Phaeophyceae) ; but their characters are so distinctive and their structure is so See also:uniform as to See also:warrant the separation of the diatoms as a distinct class. The See also:affinities of the group are doubtful. The diatoms exhibit great ~-' variety of See also:form. While some FIG. 2.—Synedra Ulna. species are circular and more or less disk-shaped, others are See also:oval in outline. Some are linear, as Synedra Ulna (fig. 2); others more or less cres- centic; others again are cuneate, as Podosphenia Lyngbyii (fig. 3); some few have a sigmoid outline, as Pleurosigma balticum (fig. 4) ; but the prevailing forms are naviculoid, as in the large See also:family Naviculaceae, of which the genus Navicula embraces upwards of See also:I000 species. They vary also in their modes of growth,—some being See also:free-floating, others attached to See also:foreign bodies by See also:simple or branched gelatinous stalks, which in some species are See also:short and thick, while in others they are See also:long and slender. 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, zigzag chains.

In some genera the individuals are naked, while in many others they are enclosed in a more or less definite gelatinous investment. The their growth are moisture and See also:

light. Wherever these circum- stances coexist, d i at o m a c e o u s FIG. 4.-Pleurosigma balticum. forms will almost invariably be found. They occur mixed with other organisms on the See also:surface of moist rocks; in streamlets and pools, they form a brownish stratum on the surface of the mud, or See also:cover the stems and leaves of See also:water plants or floating twigs with a furry investment. Marine forms are usually attached to various See also:sea-weeds, and many are found in the stomachs of molluscs, holothurians, ascidians and other denizens of the ocean. The fresh-water forms are specifically distinct from those incidental to See also:salt or brackish water,—fresh-water species, however, are sometimes A-C, Tetracyclus lacustris. D and E, Tabellaria fenestrata. F and G, Tabellaria flocculosa. carried some distance into the sea by the force of the current, and in tidal See also:rivers marine forms are carried up by the force of the See also:tide. Some notion may be formed of the extreme minuteness of these forms from the fact that one the length of which is - -- -th of an See also:inch may be considered as beyond the See also:medium See also:size.

Some few, indeed, are much larger, but by far the greater proportion are of very much smaller dimensions. Diatoms are unicellular plants distinguished from kindred forms by the fact of having their soft vegetative See also:

part covered by a siliceous See also:case. Each individual is known as a frustule, and the See also:cell-See also:wall consists of two similar valves nearly parallel to each other, each See also:valve being furnished with a rim (or connecting-See also:band) projecting from it at a right See also:angle. One of these valves with its rim is slightly smaller than the conditions necessary to other, the smaller fitting into the larger See also:pretty much as a pill-See also:box fits into its cover. This peculiarity of structure affords ample See also:scope for the growth of the protoplasmic cell-contents, for as the latter increase in See also:volume the siliceous valves are pushed out, and their corresponding siliceous rims become broader. The connecting-bands although closely fitting their respective valves are distinct from them, and together the two bands form the See also:girdle. An individual diatom is usually described from two aspects, one in which the surface of the valve is exposed to view—the valve view, and one in which the girdle See also:side is exposed—the girdle view. The valves are thin and transparent, See also:convex on the outside, and generally ornamented with a variety of sculptured markings. These sculptures often See also:present the aspect of striae across the See also:face of the valve, and the best lenses have shown them to consist of a See also:series of small cavities within the siliceous wall of the cell. The valves of some of the marine genera exhibit a beautiful areolated structure due to the presence of larger See also:chambers within the siliceous cell-wall. Many diatoms possess thickenings of the cell-wall, visible in the valve view, in the centre of the valve and at each extremity. These thickenings are known as the nodules, and they are generally connected by a long median See also:line, the raphe, which is a cleft in the siliceous valve, extending at least some part of its length.

The protoplasmic contents of this siliceous box-like unicell are very similar to the contents of many other algal cells. There is a living protoplasmic layer or primordial utricle, connected either by two broad bands or by a number of anastomosing threads with a central See also:

mass of See also:protoplasm in which the See also:nucleus is embedded. The greater part of the cavity of the cell is occupied by one or several fluid vacuoles. The characteristic brown See also:colour of diatoms is due to the presence of chromatophores embedded in the lining layer of protoplasm. In number and form these chromatophores are variable. They contain See also:chlorophyll, but the See also:green colour is masked by the presence of diatomin, a brown pigment which resembles that which occurs in the Brown Algae or Phaeophyceae. The chromatophores contain a variable number of pyrenoids, colourless proteid bodies of a crystalloidal See also:character. One of the first phenomena which comes under the See also:notice of the observer is the extraordinary See also:power of See also: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 zigzag course. To See also:account for this motion various theories have been suggested, none of which appear to be altogether satisfactory. There is little doubt that the movements are connected with the raphe, and in some diatoms there is much See also:evidence to prove that they are due to an exudation of See also:mucilage. See also:Classification.—The most natural See also:system of classification of the Bacillarieae is the one put forward by Schutt (1896), and since generally followed by systematists.

He separates them into two See also:

primary divisions, the ` Centricae' and the `Pennatae.' The former includes all those diatoms which in the valve view possess a radial symmetry around a central point, and which are destitute of a raphe (or a pseudoraphe). The latter includes those which are zygomorphic or otherwise irregular, and in which the valve view is generally See also:boat-shaped or See also:needle-shaped, with the markings arranged in a sagittal manner on each side of a raphe or pseudoraphe. See also:Reproduction.—In the Diatomaceae, as well as in the Desmidieae, the See also:ordinary mode of increase is by simple cell-See also:division. The cell-contents within the enclosure of the siliceous case See also:separate into two distinct masses. As these two daughter-masses become more and more See also:developed, the valves of the See also: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 See also:original valve, the new valves being situated within the girdle of the original frustule. When this process has been completed the girdle 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 See also:life of the plant this process of self-division is continued with an almost incredible rapidity. On this subject the observation of See also:Professor See also:William See also:Smith, See also:writing in 1853, is worthy of See also:special notice:—" I have been unable to ascertain the time occupied in a single See also:act of self-division, but supposing it to be completed in twenty-four See also:hours we should have, as the progeny of a single frustule, the amazing number of 1,000,000,000 in a single See also:month, a circumstance which will in some degree explain the sudden, or at least rapid, See also:appearance of these organisms in localities where they were a short time previously either unrecognized or sparingly diffused " (See also:British Diatomaceae, vol. i. p. 25). Individual diatoms when once produced by cell-division are incapable of any increase in size owing to the rigidity of their siliceous cell-walls, and since the new valves are always formed within the girdle of the old ones, it would follow that every succeeding See also:generation is reduced in size by the thickness of the girdle. In some diatoms, however, this is not strictly true as daughter-cells are some-times produced of larger size than the See also:parent-cells.

Thus, the reduction in size of the individuals is not always proportionate to the number of cell-divisions. On the diminution in size having reached a limit in any species, the maximum size is regained by the formation of an auxospore. There are five known methods of reproduction by auxospores, but it is unnecessary here to enter into details of these methods. Suffice it to say that a normal auxospore is produced by the conjugation of two parent-cells, its distinguishing feature being a rejuvenescence accompanied by a marked increase in size. These auxospores formed without conjugation are parthenogenetic. Mode of Preparation.—The Diatomaceae are usually gathered in small bottles, and special care should be taken to collect them as free as possible from extraneous See also:

matter. A small portion having been examined under the microscope, should the gathering be thought worthy of preservation, some of the material is boiled in See also: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 precipitating See also:jar of a conical shape, broader at the bottom than at the See also: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 See also:syringe or some similar See also: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 See also:boil the sediment for a short time with supercarbonate of soda, the See also:alkali being removed in the same manner as the acid.

A small portion may then be placed with a pipette upon a slip of See also:

glass, and, when the moisture has been thoroughly evaporated, the film that remains should be covered with dilute See also:Canada See also: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. See also:General Remarks.—Diatoms are most abundant in See also:cold latitudes, having a general preference for cold water. In the pelagic waters of lakes and of the oceans they are often very abundant, and in the cold waters of the See also:Arctic and See also:Antarctic C Auxospores. A. Navicula limosa. B. Achnanthes flexella C. Navicula See also:Amphisbaena. D. Navicula viridis. Oceans they exist in prodigious:See also:numbers. They thus form a large proportion of both the marine and the fresh-water See also:plankton.

Large numbers of fossil diatoms are known. Not only are these minute plants assisting at the present time in the See also:

accumulation of oceanic and See also:lake deposits, but in former ages they have been sufficiently active to give rise to considerable deposits of diatomaceous earths. 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 See also:rock, the siliceous frustules of the diatoms remain unaltered, and, if the rock be disintegrated by natural or artificial means, may be removed from the enveloping See also:matrix 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. These earths are generally of a See also:white or See also:grey colour. Some of them are hard, but most are soft and friable. Many of them are of economic importance, being used as polishing powders (" See also:Tripoli "), as absorbents for See also:nitroglycerin in the manufacture of See also:dynamite (" Kieselguhr "), as a dentifrice, and more recently they have been used to a large extent in the manufacture of non-conducting and See also:sound-See also:proof materials. Most of these diatomaceous earths are associated with rocks of See also:Tertiary formations, although it is generally regarded that the earliest appearance of diatoms is in the Upper Cretaceous (See also:chalk). Vast deposits of Diatomaceous earths have been discovered in various parts of the world,--some the See also:deposit of fresh, others of salt water. Of these deposits the most remarkable for extent, as well as for the number and beauty of the species contained in it, is that of See also:Richmond, in See also:Virginia, one of the See also:United States of See also:America. It extends for many See also:miles, and is in some places at least 40 ft. deep.

It is a remarkable fact that though the generations of a diatom in the space of a few months far exceed in number the generation of See also:

man during the See also:period usually assigned to the existence of the See also:race, the fossil genera and species are in most respects to the most minute details identical. with the numerous living representatives of their class. (E. O'M.; G. S.

End of Article: DIATOMACEAE

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