The two groups of Desmidiaceæ and Diatomaceæ differ so little in their general characters that they may be spoken of as members or representative families of microscopic and unicellular algæ alike in their remarkable beauty and bilateral symmetry, and of such peculiar interest as to call for special notice. Desmids differ from diatoms chiefly in colour, in lacking a non-silicious skeleton, and in their generative process, which for the most part consists in the conjugation of two similar cells. Diatoms, on the other hand, have dense silicious skeletons and a general absence of green colouring matter. Ralfs, in his systematic monograph, enumerates twenty genera of desmids. The limiting membrane is alike firm and flexible, since it exhibits some elasticity and resistance to pressure, and is not readily decomposable. Traces of silica are found in only a few of the desmids, while the frustule of the diatom is chiefly composed of this substance; both have an external membranous covering, so transparent and homogeneous in structure as to be in danger of being entirely overlooked, unless some staining material is used, together with a high-power objective possessing considerable penetration. In some species, however, the mucous covering is more clearly defined, as in Staurastrum and Didymoprium Grevelli. Openings occur in the outer membrane of other species, as the Closterium.
PLATE X.
DESMIDIACEÆ.
Many species of desmids have a power of motion, the cause of which must be due either to cilia or a flagellate organ. This, however, is denied by some observers, who regard their movements as due to an exudation of the mucilaginous contents of the cell, to exosmose, or diffusion, neither of which hypotheses will at all help us to understand the gliding movements of the Oscillariæ or the sharp jerky movement of the Schizonema. The movements of desmids are especially exerted when in the act of dividing, and by sunlight, towards which they are always observed to move. The force with which some diatoms move about is very great, and this can only be satisfactorily explained by admitting a specialised organ.
The appearance of the Desmidiaceæ ([Plate X].) is much modified by their eminences, depressions, and processes, as well as that of the surface, the margin of the fronds, and the depth and width of the central constriction. The surfaces may be dotted over irregularly, the dots themselves being elevated or depressed points in their structural character. The margins of some have a dentate appearance, as in Cosmarium. In the elongated forms, such as Penium, the puncta are disposed in lines parallel to the length. In several these lines are either elevations or furrows, it is not always easy to say which; they are peculiar, however, to the elongated forms of Closterium. When the lines are fine they produce a striation of the surface, but in order to discover this the fronds should be viewed when empty and by a fairly good power. The modification of surface in several genera seems to be due, not to mere simple appendages, but to expansion of the limiting membrance into thickened processes, and which may terminate in spines, as in Xanthidium and Staurastrum ([Plate X]., Nos. 8-19 and 22). A general distribution over the surface is characteristic of the former, but in Euastrum the surfaces are very irregular, and therefore described as “swellings or inflations.” Micrasterias has its margin deeply incised into lobes, which in some have a radiating arrangement; when the lobes on the margin are small they constitute crenations or dentations. The fronds of Euastrum binatum are bicrenate on the sides, as are those of Desmidium and Hyalotheca and other species. Another variety of margin exists, known as undulating or wavy, while the general concavity or convexity of the margins furnish other specific characteristics.
Pediastreæ ([Plate X]., Nos. 24-29).—The members of this family formerly included the Micrasterias and Arthrodesmius of Ehrenberg. From their arrangement of cells in determinate numbers and definite forms, it has been thought by some observers that they should be removed from the desmids to a special or sub-family. The points of difference consist in the firmness of the outer covering, in the frequent interruptions on the margin of the cells, and in the protrusion of “horns,” or rather a notch more or less deep. It is true that the cells are not made up of two symmetrical halves, and that they are in aggregation, which is not (except in the Scenedesmus, a genus that distinctly connects this group with desmids) in linear series, but in the form of discoidal fronds. They, however, divide into 8, 16, or 32 gonidia, and these move about for some time before the formation of a new frond. It was Nägeli who first instituted a sub-genus of Pediastrum, under the designation of Anomopedium, the chief characteristic of which is the absence of bilobed peripheral cells. In Cœlastrum the cells are hexangular, the central ones very regularly so; in Sorastrum they are wedge-shaped, or triangular, with rounded-off angles. Viewed laterally the cells appear oblong. The cells of Pediastrum are considerably compressed, so that when aggregated they form a flattened tubular structure; in figure they are polygonal, frequently hexagonal, a shape owing, in all probability, to mutual lateral pressure during growth. There is a pervading uniformity in the contents of the cells of the different genera, which consist of protoplasmic endochrome. At first the colour is pale green, but it becomes deeper with full maturity, while the decaying cells are seen to change to a deep reddish-yellow or brown. The protoplasm is also clear and homogeneous, but in time granules appear, enlarge, and multiply in number; moreover, each cell presents a single bright green vesicle, around which are collected clear circular spaces or globules, recalling those of Closterium, and varying in number from two to six or more, their position not being regulated by the partition wall as in Palmellæ, but by the centre of the entire frond. Oil globules are also contained in the cells; their presence is indicated by the addition of a drop of tincture of iodine. On one occasion Nägeli saw in Pediastrum boryanum the endochrome disposed in a radiating manner, an arrangement which often obtains in algals and in other vegetable cells with a central nucleus. The cells of Pediastreæ are always united together in compound fronds, as represented in [Plate X]., Nos. 24 and 29.[56]
The differences pointed out in no way constitute a claim to remove Pediastreæ from among Desmidiaceæ, certainly not to rank as a distinct species.
Reproduction of Desmidiaceæ.—A true reproductive act is presented by the conjugation or coupling of two fronds, and by the resulting development of a sporangium and subsequent interchange of the contents of the two cells. At another time self-division is frequently seen to take place in all respects as in the cells of other algæ. The proceeding is varied in some essential particulars by the form of the fronds and by other circumstances; as in fission of Euastrum, for instance (seen in [Plate X]., Nos. 1, 2, and 12), when the narrow connecting bands between the two segments of the fronds are rapidly pushed aside by growth and finally divide. Two modes of conjugation of fronds are represented in [Plate X]., Nos. 25 and 33, in Closterium and Penium. The act of conjugation admits of variations in character, as shown in Staurastrum and Microsterias; the contents of both fronds are discharged into a delicate intermediate sac; this gradually thickens and produces spines ([Plate X]., Nos. 8 and 19). In Didymoprium the separate joints unite by a narrow process pushed out from each other, often of considerable length, through which the endochrome of one cell is transferred to the other, and thus a sporangium is produced within one of two cells, just as in the conjugatæ (No. 5). In Penium Jennereri the conjugation takes a varied form; the fronds do not open and gape at the suture, but couple by small but distinct cylindrical tubes (No. 27).
Among those enumerated, the compressed and deeply constricted cells of Euastrum offer the more favourable opportunities for studying the manner of their division; for although the frond is really a single cell, in all its stages it appears like two, the segments being always distinct, from the earliest stage. The segments, however, are separated by a connecting link, which is subsequently converted into two somewhat round hyaline bodies. These bodies gradually increase and acquire colour, and as they grow the original segments are further divided, and at length become disconnected, each taking a new segment to supply the place of that from which it is separated. It is curious to trace the progressive development of the newer portions, which at first are devoid of all colour; but as they become larger a faint green tint is observed, which gradually darkens, and then assumes a granular appearance. Soon the new segments attain their normal size, while the covering in some species shows the presence of puncta. In Xanthidium, [Plate X]., Nos. 9, 10, and Staurastrum, Nos. 15-18, the spines and processes make their appearance last, beginning as mere tubercles, and then lengthening until they attain their perfect form and size, armed with setæ; but complete separation frequently occurs before growth is fully completed. This singular process is repeated again and again, so that the older segments are united successively, as it were, with many generations. When the cells approach maturity, molecular movements may be at times noticed in their contents, precisely similar to what Agardh and others aptly term “swarming.” Meyen describes this granular matter as starch.[57] Closterium, early in the spring, when freshly secured and exposed to light, presents a wonderful appearance, these bodies being kept continually in motion at both ends of the frustule by the ciliary action within the cell, and the whole frond is seen brilliantly glittering with active cilia. When a gleam of stronger light is allowed for a moment to fall on the frond, the rapid undulations of the cilia produce a series of most delicate prismatic Newton’s rings. The action and distribution of the cilia, together with the cyclosis of the granular bodies in the frond, are better seen by the aid of Wenham’s parabola or a good condenser with a central stop. One of the wide angular objectives shows the circulation around the marginal portions of the whole frond. The stream is seen to be running up the more external portion, internal to which is another stream following a contrary direction; this action, confined to the space between the mass of endochrome and the outer portion of the cell-wall, is seen to pass above or around the space in which cyclosis of the spores is taking place.