Desmidieae, minute algae, or protophytes, which grow in fresh water, and whose forms present singularly beautiful appearances under the microscope. For a long time claimed both as animals and plants, they seem to stand on the limits of either kingdom. The controversy as to their true place has enlisted a great number of observers, who have submitted every fact to the most rigorous examination. Ehren-berg claimed them as animalcules; Dalrymple gave extended observations upon a single genus (closterium), which appeared to him to indicate animality; and Prof. Bailey and C. Eckhard arrive at the same conclusion. The latter derives his argument for their being animals partly from their motion, partly from their organization. Ehrenberg has not only given systematic descriptions of these questionable animals or plants, but his own observations, coupled with those of his predecessors, upon the nature of these bodies, will be found copiously detailed by him. It is, however, apparent that all the facts known upon the subject are interpreted as if these creations were undoubtedly animals, while the same facts would bear a very different signification if we proceeded upon the supposition that they are plants.
Meyen contended for the vegetable character of the desmidieae, and was the first to detect starch in the cells; and the accuracy of his remarks was fully confirmed by Ralfs, Jenner, and other recent algologists. It is said that no starch is to be detected in the young cell, while upon the growth of the sporangium, or spore capsule, it appears and increases rapidly, as in the seeds of the higher plants, in which it generally abounds. Of all the circumstances which indicate the vegetable nature of the desmidieae, this is the most important, since it can be so easily submitted to experiment. In certain cavities in closterium Dalrymple noticed a peculiar motion of molecules on which he laid some stress. This motion has been termed swarming, on account of the commotion which arises within the cell; as the disturbance increases, the cell opens, when the molecules, or rather germinative cells, dart about in every direction, until at length they settle down into repose. The presence and functions of these cells in plants of entirely differing families and groups, render their occurrence in those under consideration no evidence of their being animals.
The desmidieae resist decomposition, exhale oxygen on exposure to the sun, preserve the purity of the water containing them, and when burned do not emit the peculiar odor usually so characteristic of animal combustion. Berkeley, in his "Introduction to Cryptoga-mic Botany," remarks that if in some points there be anomalies, as in closterium, their whole history is so evidently vegetable, their mode of increase, growth, etc, that if we refuse them the title of vegetables, we may as well dispute that of the whole tribe of protophytes. The fact that under the influence of light they give out oxygen, added to the other characteristics, is quite convincing. - Considering the desmidieae as vegetable productions, we find them peculiar for their beauty, variety of forms, and external markings and appendages. They are mostly of an herbaceous green color, and contain a green internal matter. The frond divides into two valves or segments, by a sort of voluntary action; a mode of growth in the bisection of cells that Meyen and others have proved to be frequent if not universal in the more simple algae. In the desmidieae the multiplication of the cells by repeated division is full of interest.
The compressed and deeply constricted cells of euastrum offer most favorable opportunities for ascertaining the manner of this division; for although the frond is really a single cell, yet this cell in all its stages appears like two, the segments being always distinct. As the connecting portion is so small, and necessarily produces the new segments, which cannot arise from a broader base than its opening, these are at first very minute, though they rapidly increase in size. The segments are separated by the elongation of the connecting tube, which is converted into two roundish hyaline lobules. These lobules increase in size, acquire color, and gradually put on the appearance of the older portions. Of course, as they increase, the original segments are pushed further asunder, and at length are disconnected, each taking with it a new segment to supply the place of that from which it has separated. All the desmidieae are gelatinous. In some the mucus is condensed into a distinct and well defined hyaline sheath or covering; in others it is more attenuated, and the fact that it forms a covering is discerned only from its preventing the contact of the colored cells.
In general, its quantity is merely sufficient to hold the fronds together in a kind of filmy cloud, which is dispersed by the slightest touch. When they are left exposed by the evaporation of the water, this mucus becomes denser, and is apparently secreted in larger quantities to protect them from the effects of drought. Their normal mode of propagation seems to be by the production of single large spores or sporangia, which derive their existence from the union of the green coloring matter (endochromes) of two contiguous plants. This process is seen in the sketch of one of the species of didymoprium, in fig. 1 of the cut. These spores are mostly globular, although they exhibit a great variety of forms with reference to their external surfaces. Sometimes they bear no resemblance to the parent plant. But once formed, they are propagated by division, in the same manner as the ordinary cells, and in the third generation acquire their regular form, which they may continue to propagate for years, without ever producing a true spore. - Very little is known respecting the uses of the desmidieae. The food of bivalve mollusks belonging to fresh waters seems to be made up of them. They are found principally where there is some admixture of peat, and in clear pools rather than in running streams.
They abound in open places, and are rarely seen in shady woods or in deep ditches. So numerous are the species and so diversified their shapes and characters, that they have been divided into distinct genera as natural series present themselves in turn. In the first of these series we discover the plant an elongated, jointed filament, which may be cylindrical, sub-cylindrical, triangular or quadrangular, plane with the margins even and smooth, or with the margins incised and sinuated. In hyalotheca we have the mucous envelope alluded to above, within which are numerous joints, which are usually broader than long; and as each has a shallow groove passing round it, it resembles a small pulley wheel. The minuteness of the plant may be estimated from the length of these joints, which vary from 1/2105 to 1/1351 of an inch. H. dissiliens (Breb.) is found in North America as well as in Europe. In desmidium the joints are bidentate at the angles; the filament is fragile and of a pale green color; the length of the joint is from 1/2000 to 1/1600 of an inch. D. Swartzii (Ag.) is common throughout the United States. In micrasterias we have a simple, lenticular frond, deeply divided into two-lobed segments, each lobe inciso-den-tate and generally radiate.
Many species of this beautiful plant are common in this country. The compressed bipartite and bivalved frond of the xanthidium is represented in the fossils by one that is globose and entire. The constriction about the middle of the frond is lost in closterium, which also differs in shape, it being crescent-like or arcuate. The species of this are common and numerous. The fronds of ankistrodesmus are aggregated into faggot-like bundles. Pediastrum tetras, occurring from Maine to Virginia, has an extremely minute frond composed of four cells, which make a star-like figure; while P. bira-diatum, found in New Jersey as well as in Germany, has many more cells, yet still arranged in a stello-radiate manner. - In collecting the desmidieae, the student must seek in proper situations the sediment observable in the form of a dirty cloud or greenish scum upon the stems and leaves of filiform aquatic plants. This is to be carefully transferred to a bottle of pure water, and thus he will secure many beautiful species for his microscope.
If the bottle be exposed to the light, the little plants will continue in good condition, and thrive for several months.
1. Didymoprium Borreri, with the cells uniting to form the green matter. 2. Micrasterias crenata. 3. Euastrium oblongum. 4. Xanthidium armatum. 5. The same with a frond acquiring a new segment by division. 6. Closterium lunula. 7. Pedastrium tetras. 8. Pedas-trium biradiatum. 9. Ankistrodesmus falcatus.