Page:The American Cyclopædia (1879) Volume VI.djvu/51

 DESMIDIE^E DES MOINES 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 seg- ment to supply the place of that from which it has separated. All the desmidiese are gelat- inous. In some the mucus is condensed into a distinct and well defined hyaline sheath or covering ; in others it is more attenuated, and t-he 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 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. of filmy cloud, which is dispersed by the slight- est 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 ex- hibit 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 desmidieaD. The food of bivalve mollusks be- longing 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-cylin- drical, 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 -^fa to T ^Vr f an inch. E. dissiliens (Breb.) is found in North America as well as in Europe. In desmidium the joints are bidentate at the angles ; the fila- ment is fragile and of a pale green color; the length of the joint is from ^^ to ^Vrr of an inch. D. Swartzii (Ag.) is common through- out 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 coun- try. 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 spe- cies of this are common and numerous. The fronds of anMstrodesmus are aggregated into faggot-like bundles. Pediastrum tetras, oc- curring from Maine to Virginia, has an ex- tremely minute frond composed of four cells, which make a star-like figure ; while P. 'bira- diatum, found in New Jersey as well as in Ger- many, has many more cells, yet still arranged in a stello-radiate manner. In collecting the desmidieee, 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. DES MOOES, a S. E. county of Iowa, bor- dering on Illinois, washed by the Mississippi on the E., bounded S. by Skunk river, and