Bacillariales or Diatoms

BACILLARIALES or DIATOMS. The diatoms are micro scopic unicellular or colonial plants belonging to the Algae, and are specially distinguished by the complex structure of their cell walls which are usually strongly impregnated with silica. The majority are exceedingly minute, and one with a length of in. (o• I 2 5mm.) is well beyond the medium size. The first forms were discovered by O. F. Muller towards the close of the 18th century. With the perfection of modern microscopes and micro scopic methods, the study of these minute plants has been prose cuted with great vigour and over 15,000 species of diatoms have been described, while about 1,200 species are known to occur in the fresh waters and on the coasts of Great Britain and Ireland.

Structure.

In spite of the immense number of diverse forms included in it, the class as a whole exhibits a remarkable uniform ity of structure. The individual cells of diatoms are called frus tules, and their wall consists of two similar halves (fig. 2), each composed of a slightly convex valve to whose incurved edges is attached at right angles a broad hoop-like connecting band. The two connecting bands together form the girdle. One half is slightly smaller than the other, the smaller fitting into the larger much as a box fits into its cover (fig. 2) and, according as one visualizes a circular, oval, or otherwise shaped box, one obtains a mental picture of the varied forms assumed by the frus tules of diatoms; the sides of the box correspond to the connect ing bands. As the protoplasmic contents increase in volume, the siliceous valves are pushed apart and the connecting bands be come broader. Diatoms are usually described from two aspects, one in which the surface of the valve is exposed, the valve-view (fig. 2) ; and one in which the girdle is exposed, the girdle-view (fig. 2). The valve-view exhibits great variety of form; the girdle-view is much more uniform and commonly rectangular. The valves may be circular (fig. I), triangular (fig. 1), or oval in outline. Some are linear, as Synedra ulna (fig. 3) ; others (Eunotia), more or less crescentic ; others again are wedge shaped (Podosphenia, Gomphonema, fig. 3) ; some few have a sigmoid outline, as Pleurosigma (fig. 3) ; but the prevailing forms are boat-shaped (naviculoid), as in the genus Navicula (fig. 2), which embraces upwards of i,000 species. A few genera have dorsiventral views (Epithemia, fig. 3) .

Many diatoms are free-floating, but some (Gomplionema) are attached to foreign bodies by simple or branched gelatinous stalks. The frustules of some are connected to form ribbon-shaped col onies (Fragilaria) or zigzag chains (fig. 3 D). In a few genera numerous individuals are enclosed in a tube-like gelatinous envelope.

The valves are thin and transparent and generally ornamented with a variety of markings. The latter often appear as fine lines Reproduction.—The ordinary mode of multiplication is by cell-division, which usually occurs at night. The protoplasm divides into two masses and the halves of the cell commence to separate; thereupon fresh valves, which are at first very delicate, are secreted on the surfaces of the new protoplasmic masses op posite to the valves of the parent. The new valves are situated within the girdle of the original frustule, but as their own con necting bands develop, those of the parent separate. Each of the new individuals thus has one valve derived from the parent, and one that is newly formed and more or less parallel to it. This process of division is repeated at frequent intervals, and it has been calculated that from a single frustule i,000,000,000 new indi viduals may arise in the course of a month.

The individual diatom, owing to the rigidity of the siliceous wall, is ordinarily incapable of any increase in length and, since (striae) on the face of the valve, and the best lenses show them in many cases to consist of a series of separate dots (punctae) which correspond to small cavities within the siliceous wall. The valves of certain marine genera exhibit a beautiful areolated structure due to the presence of larger chambers within the wall (fig. I). A good many diatoms, especially those of the navicu loid type, have thickenings at the centre and at each extremity of the valve, known as nodules (fig. 2) . These possess a com plex structure and are generally connected by a long narrow cleft, the raphe (fig. 2), which appears as a straight or slightly un dulating line in the valve-view. Some diatoms, while possessing nodules, lack a raphe, although in these cases the narrow area be tween the two rows of striae often simulates one (pseudoraphe).

The contents of diatom cells are very similar to those of other Algae. Beneath the wall there is a lining protoplasmic layer en closing the cell-sap, and connected either by two broad bands or by a number of anastomosing strands with the central proto plasm in which the nucleus is embedded. The chromatophores are either one or two of large size, or numerous small lobed discs (fig. I). The former often include a variable number of glis tening pyrenoids. The characteristic brown colour is due to dia tomin, a pigment resembling that of the brown Algae and masking the chlorophyll. The cells often contain abundant oil.

Many diatoms, but only those which possess a true raphe in their valves (Navicula, etc.), are able to move through the water, sometimes with considerable rapidity. The movements are doubt less in all cases related to the presence of the raphe, which in all probability contains streaming protoplasm connected with that in the interior of the cell by a complex system of canals within the nodules. Some believe that the movements depend upon an extrusion of mucilage from the region of the raphe. In Cocconeis, whose disc-shaped frustules are found attached in enormous num bers to Cladopliora and other freshwater plants, only the valve in contact with the substratum possesses a raphe.

the new valves are always formed within the girdle of the old ones, it would follow that one individual of every succeeding gen eration is reduced in length by the thickness of the girdle. This is not, however, strictly true, as daughter cells are sometimes formed which are larger than the parent cell, so that the reduc tion in size is not always proportional to the number of divisions.

It seems that often, when the diminution in size has reached a limit, the maximum size is regained by the formation of auxo spores, but other conditions than this no doubt also lead to auxo spore production. Auxospores in many cases arise as a result of the fusion of two individuals; prior to this conjugation, the proto plasmic contents of each may divide into two parts. The two halves of each frustule then separate, and the liberated contents fuse to form one (fig. 4 B) or two (fig. 4 A) auxospores, which are surrounded by the empty valves of the original individuals. In other cases, however, auxospores are formed merely from the contents of a single individual without any process of fusion (fig. 4 C) , and this is invariably their mode of formation in cen tric diatoms (fig. I). The auxospores usually undergo enlarge ment and then, after secreting new valves, constitute a new indi vidual.

In some marine centric diatoms the protoplasmic contents at times divide to form numerous microspores (fig. 4 D), which are liberated as naked cells, in some cases provided with cilia like the sexual cells (gametes) of Algae. It is not impossible that they represent the sexual cells of these, but the evidence is inconclusive.

Affinities and Classification.

The diatoms have been held to be allied to the brown Algae (Phaeophyceae) but, apart from the similarity in colour, there is little to support such a view. On the other hand there are some marked points of resemblance to Heterokontae (see ALGAE) which may indicate a remote rela tionship. The obvious resemblances to Conjugatae seem to be the result of a parallel development.

The system of classification of diatoms usually followed is one put' forward by Schutt (1896). He separates them into two pri mary divisions, the Centricae and the Pennatae. The former in clude the diatoms in which the valve view possesses a radial sym metry around a central point, and which are destitute of a raphe (or pseudoraphe) (fig. I) . The Pennatae comprise those in which the valve-view is boat- or needle-shaped, with the markings ar ranged on either side of a median line (raphe or pseudoraphe) (fig. 3) . The Centricae are much more abundant in the sea than in fresh waters. It is possible that these two groups are not as nearly related to one another as has hitherto been supposed. In particular the fact that the sexual reproductive process appears quite different in the two cases makes a close affinity doubtful.

Mode of Preparation.

Diatoms are usually gathered in small bottles, as free as possible from extraneous matter. The material is next boiled with acid (either hydrochloric, nitric, or sulphuric) in order to eliminate all foreign matter and dissolve away the non siliceous parts of the frustules. The residuum is treated with an excess of water, well shaken and allowed to settle, after which the supernatant liquid is carefully removed with a syringe ; it is nec essary to repeat this treatment till all traces of the acid have been removed. Subsequently the sediment may be boiled with car bonate of soda, the alkali being removed in the same way as the acid. A small portion is then placed on a glass slide and, when the moisture has fully evaporated, the remaining film is covered with dilute Canada balsam, dammar, or styrax, and a thin cover glass gently laid on top.

Occurrence and Distribution.

The conditions necessary for the growth of diatoms are moisture and light, and wherever these coexist, these forms will almost invariably be found. They occur abundantly in cultivated soils, and mixed with other forms on the surface of moist rocks ; in pools and other small pieces of water 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 seaweeds, and many are found in the stomachs of molluscs, holo thurians, ascidians, and other denizens of the ocean. Both in fresh waters and in the sea, moreover, there are myriads of free-floating diatoms which at times indeed make up the main bulk of the plank ton. Diatoms are most abundant in cold latitudes, having a gen eral preference for cold water, and exist in prodigious numbers in the Arctic and Antarctic oceans. The freshwater species are al most always distinct from those found in salt or brackish water. Large numbers of fossil diatoms are. known. Since the siliceous wall is practically unperishable, it persists after the death of the individual, so that where diatoms occur abundantly there is an unceasing rain of their minute valves on to the bottom of sea or lake as the case may be. In this way extensive deposits of dia tomaceous earth may arise, and such are not only being formed at the present time, but have been produced abundantly in the past. Even when such deposits have been hardened into solid rock, the frustules remain unaltered, and from their character it is of ten possible to arrive at conclusions as to the conditions under which the deposit was formed. These earths are generally of a white or grey colour, and mostly soft and friable. They are used as polish ing powders (Tripoli), as absorbents for nitroglycerine in the manufacture of dynamite (Kieselguhr), in the preparation of dentifrices, and in the manufacture of non-conducting and sound proof materials. Most of the fossil deposits are in Tertiary rocks, although there are records of diatoms in the Trias.

Vast deposits of diatomaceous earths have been discovered in various parts of the world, some of freshwater, others of marine origin. That at Richmond in Virginia (U.S.A.), extends for many miles and is in places at least 4of t. deep, whilst in the western states of America beds of 3oof t. thickness have been discovered. Other, though less extensive, deposits are known at Dolgelly (Wales) and at Bilin in Bohemia. It is a remarkable fact that the fossil genera and species are very similar to and in part quite identical with the living representatives of the class.

BIBLIOGRAPHY.-For

a general account of diatoms see G. S. West and Bibliography.-For a general account of diatoms see G. S. West and F. E. Fritsch, British Freshwater Algae (1927) ; or the section dealing with those forms in F. Oltmanns, Morphologie and Biologie der Algen (Jena, 1922). There are numerous systematic works for the determina tion of diatoms, e.g., W. Smith, A Synopsis of the British Diatomaceae (1853-56) ; Adolf Schmidt, Atlas der Diatomaceenkunde (Leipzig, 1874 and onwards) , a very extensive iconographical work ; H. van Heurck, Synopsis des Diatomees de Belgique (Antwerp, 1880-85) ; H. von Schonfeldt, "Bacillariales" in A. Pascher, Siisswasserflora Deutschlands, Osterreichs, and der Schweiz (Jena, 1913) and F. Meister, "Kieselal gen der Schweiz" in Kryptogamenflora der Schweiz (Part iv. 1912).

(F. E. F.)