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at one time supposed to belong to this family, have been since proved not to be members of it. It has been said that hardly a specimen of fresh water can be found that does not serve as the habitation of Desmids, but such is not strictly the case, although it is true that they are very widely distributed, and one intending to study them should have no difficulty in procuring specimens for examination. In clear pools, in open exposed situations, they occur in the greatest abundance, the largest species being generally found nearest the bottom. Sometimes they are to be found adhering in large quantities to some of the submerged aquatic plants that grow in such localities, forming investing films of a bright green color, which can be removed from its support, or is best gathered along with it. At other times they rest as a thick coating upon the bottom, or float in the form of a bright green scum upon the surface; but the last mode of occurrence is by no means common, the green-colored film seen so frequent upon pools not being Desmids but members of a group into which have been placed the Protococcus, Euglenia, and the so-called "Red-snow.” Of these we may have something farther to say hereafter, as they are possessed of wondrous characteristics, and present subjects well worthy the study of any one having a microscope. The brownish scum which is so commonly seen in marshes and ponds does not consist of Desmids either, but is mostly made up of myriads of plants very nearly related to them, and familiarly known as Diatoms. These, again, are of extreme beauty, and at the present day hundreds of microscopes are turned towards them endeavoring to fathom their mysteries, and the optician's skill has been brought to bear upon the construction of lenses specially for the purpose of studying their life, history, and structure.

The Desmids, Desmidiece, or, more correctly speaking, Desmidiacea, bave had this designation applied to them from their form, that is to say, on account of their being made up of two symmetrical halves, united together by


means of a band or bridge, so to speak. They are very striking and beautiful objects when examined by means of sufficiently powerful magnifying glasses, many of them requiring for the elucidation of their structure to be amplified at least five hundred diameters, or two hundred and fifty thousand times superficially ; microscopists being in the habit of speaking of the magnification of an object in diameters, that having been found to be the most convenient method of expressing the fact, the number of times which the object is amplified superficially, being, of course, formed by squaring the diameter. But a power much less than five hundred diameters, say about two hundred and fifty, is often sufficient to exhibit the general characteristics of most of the Desmids and their allies, the other Protophyta. Thus examined they present most striking objects, and at once become favorites with the amateur microscopist on account of their very marked peculiarities, great beauty, and the variety of forms which they exhibit in outline, as well as the mathematical symmetry of their markings and appendages. The most distinctive characteristic which they at once present is the bilateral structure of their so-called fronds. In the more complex water-plants, or algæ, the term frond is used to designate the whole plant, which in that case is of some degree of complexity, but here is extremely simple, and yet the same name must be made use of, as the entire individual is enclosed in one envelope and constitutes but a single cavity. As such cavities are called cells the Desmids are hence known as unicellular plants. The individual plant among the Desmids and their near relatives, the Diatoms, is often spoken of as a frustule, as the frustule of Closterium, a frustule of Navicula, these being the distinctive names given to two groups, or genera, of Desmids and Diatoms respectively. So in the organisms under consideration, the frustule is said to be a single cell, and this is shown to be the case by the fact that when a fracture takes place of the investing membrane, at any one part, the whole contents escape therefrom. In a few instances this apparent bilateral symmetry is not so evident as in others, or even seems to be entirely absent, but on careful examination it will still be seen to be present, for the constriction in the outer coat, which is made of the substance called cellulose, may be slight or very great, cutting the individual, as it were, into two parts. External warty or spinous protuberances, or processes, are very commonly present, and then the outline of the plant is of great beauty, the green cellcontents, made up for the most part of the same material as constitutes the coloring matter of the leaves of larger plants, and there called chlorophyl, but in the Desmids known as endochrome, causing them to appear almost like brilliant gems of great purity of tint and configuration. In some cases no such external projections are present, but yet the outline of the cell is, nevertheless, extremely graceful. In the Diatoms the cell-wall is strengthened and supported by having deposited within it a mass of silicious material which then becomes marked with wonderfully fine tracings and sculpturings, but in the Desmids no such stony and indestructible substance is present, stiff

' cellulose only constituting the skeleton of the plant. Hence we do not find the remains of these organisms occurring fossilized in the older strata of the globe as is very commonly the case with the Diatoms. It is true that in some of the flints, hornstones

. and cherts, certain curious forms have been detected which have been supposed to be the remains of Desmids, but careful examination by competent authorities has tended to prove

that such is not the case, but that these are most likely only the skeletons of animals very nearly allied to, if not identical with, the sponges. The true cellulose character of the cell-wall of the Desmids is proved by the action upon it of iodine assisted by sulphuric acid, in which case it is colored blue. In all cases this tough membranous material is surrounded by a perfect and distinct, although not always readily seen, sheath of a gelatinous character, which in some cases, is very broad, but in others is extremely thin.

The outline of the Desmids, although always preserving a more or less perfect bilateral symmetry, varies very greatly. Thus in Closterium, a genus of very general distribution, and one at the same time which includes a great number of species, the general form is a round tube, more or less pointed at both ends, and with the apices both bent over in the same direction so that the individual is somewhat moonshaped, or more like two cows' horns united base to base. When Closterium is examined with care by means of a good microscope, it is found to have its bright green cell-contents arranged longitudinally in seeming uncertain bands, which coalesce more or less, and hence are not always to be distinguished. But at the ends of the frustule are to be seen apparent organs of wondrous characters, and whose office has not as yet been determined. And the extreme minuteness of the whole plant presents great difficulties to its proper study, so that it is hardly to be wondered at that the functions of its integral parts should not be thoroughly comprehended. These seeming organs are spaces or vacuoles separated from the rest of the cell-contents, and generally of a spherical form, transparent and colorless. Within them, however, are observed numerous minute granules formed of a material of different density, as is shown by their effect upon light. And these are continually, in the healthy individual, in motion, moving about with a trembling and seemingly excited action, putting one in mind of the swarming of a crowd of bees, and hence it is often spoken of as swarming. Besides this, however, there is still another kind of motion to be seen within the Closterium cell-wall, and one at the same time perhaps of greater wonder and perplexity than that already mentioned, as the mode of motion is a problem as yet unsolved. This is the circulation or rotation of much of the liquid contents of the individual Desmid; more especially that transparent and colorless portion which lies just within the membranous cell-wall and its lining tissue, called by the German naturalists the primordial utricle, and overlying the more solid green mass of endochrome and starchy matter; for it has been found that these wonderful little plants contain starchy matter very much after the manner of their gigantic fellows of the field and forest. Members of the genus Closterium have been found to afford the best subjects for witnessing this phenomenon, but the use of a good microscope, and a very careful arrangement of the focus of the lens, are always necessary to display it in a manner at all satisfactory. Some observers assert that they have observed this circulation of fluid, not only within the primordial utricle, but between it and the cellulose covering; however this must be a difficult thing to see, as these membranes are very closely united in most cases. Along the convex edges of the cell, when a magnifying power of about four hundred diameters is employed, it is not very difficult to see indications of this, what may be called "sap-motion” first spoken of, especially if the specimen under examination be one in a vigorous state of growth. Then there may be seen broad streams of fluid flowing over the whole surface of the endochrome, passing from the ends towards the centre and back again; and these streams seem to detach and carry with them, from time to time, little oval or globular bodies, which, on account of their action upon the light, doubtless resulting from their peculiar chemical composition, are readily seen, and any of them singled out and its whole course from one part of the frustule to another traced. Some observers state that these minute granules, which seem to be starchy in their composition, are thus carried on to the chambers or cavities at the end of the Closterium, and there join the bodies which are in trembling motion, as has been described; but my experience has been that such is not the case, as the number of the terminal granules does not increase, as would certainly be the case if this addition took place. On the contrary I

may be

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