Fig. 11.—A, Diatoma vulgare:—a, side view of frustule; b, frustule undergoing self-division.
B. Grammatophora serpentina:—a, front and side view of single frustule; b, front and end view of divided frustule; c, frustule about to undergo division; d, frustule completely divided.
The diatom or frustule is considered to present its front view when the joint or suture of the valves is turned to the eye, as in [fig. 11] B, b, whilst the side view is seen when the centre of either valve is directly beneath the eye, as in [fig. 11] A, a. When the diatoms are young the valves are in close contact, but as they increase in size by a secretion round their edges, the valves separate from one another, and the cell membrane which is left exposed is immediately consolidated by silex, and forms a kind of hoop between the valves, as in [fig. 12]. This hoop increases in breadth as the cell increases in length. When the two valves are circular discs, they are separated by a circular hoop, round the edges of which water is admitted to nourish the plant; but when the diatom has an elongated form, the water enters through depressed points in its extremities which are free from silex.
Fig. 12. Biddulphia pulchella.
Fig. 13. Pleurosigma angulatum.—A, entire frustule; B, its hexagonal areolation; C, the same more highly magnified.
Numerous as these plants are, the valves of each genus have their own peculiar ornaments, consisting of the most beautiful and symmetrical designs, which are impressed upon the young valves when they are in a plastic state. The genus Navicula and others are marked with the finest striæ, some diagonally, others transversely. Rows of round or oval spots disposed in parallel lines are peculiar to some; the valves of others are covered with hexagonal forms of the most perfect structure, as those of the Pleurosigma angulatum, [fig. 13], where A is the magnified diatom, and B and C its hexagonal areolations, seen under higher and higher microscopic powers; but the figures on the discoid genera are the most beautiful of all. There is generally a small ornamented circular space in the centre of the valves, from whence rays extend to the circumference, dividing the surface of the valves into eight, ten, or more equal parts, the alternate segments being differently and highly ornamented, as in the Actinocyclus undulatus ([fig. 14]), where A is the side view, and B is the front view. The Arachnoidiscus Ehrenbergii takes its name from the likeness of the figures on its circular valves to a spider’s web. According to the observations of Mr. Shadbolt, each valve is formed of two superposed layers; on the uppermost of these, which is a thin horny transparent substance, the spider’s web is engraven; and the undermost silicious layer, which forms the supporting frame-work, is like a circular Gothic window. The genus Triceratium, nearly allied to the preceding in general characters, though differing in having a triangular shape, has many species in a fossil state, while others are still existing in the ocean, and in tidal rivers. The Triceratium favus, one of the largest and most beautifully marked, occurs in the mud of the Thames, and that of the estuaries of other rivers on our coasts; it is also frequently found on the surface of uncleaned shells.[[32]] From the few examples given, a faint idea only can be formed of the variety and beauty of the engravings on the diatoms. It had long been doubted whether those on the valves of Coscinodiscus, Triceratium and others, were elevations or depressions, but Professor Rood of New York, United States, has proved them to be depressions by an optical arrangement which will be useful for the investigation of microscopic forms.
Fig. 14. Actinocyclus undulatus.
Diatoms increase by spontaneous bisection, by conjugation, and by the resolution of their endochrome into minute spores, called gonidia. When bisection is about to take place the cell elongates, the hoop increases in breadth, the endochrome divides into two equal parts, and the coating of the cell bends in between them, which gives the diatom the appearance of an hour-glass. At last they separate, and upon each of the new surfaces a new silicious half is formed, usually the exact counterpart of the old one, so that there are two diatoms instead of one; and the process may be continued indefinitely. In most cases, the new diatoms thus produced are free and independent. Sometimes, however, they adhere to one another by a fragment or connecting membrane, and if they happen to be slender and rectangular, and attached side by side, they form a slender filament, or if attached by alternate angles they form a zigzag chain, as in [fig. 11].