Page:Midland naturalist (IA midlandnaturalis01lond).pdf/139
Fortunately they are widely distributed; it is rather the exception than the rule to take up a dip of water from a pool or marsh in which some of the brilliant crescents of Closterium or the sculptured discs of Micrasterias do not delight the eye. The most distinctive feature in their appearance is the perfect bilateral symmetry of the two halves into which each plant is generally divided.
The fronds of Closterium are more or less crescent-shaped, from the slightly curved form of the Tartar bow to the complete crescent form of the young moon; in Cosmarium, Enastrum, and Micrasterias they consist of thin discs of a more or less oval or oblong shape, deeply constricted in the middle, and with their edges cut, crenated, or sinuated into forms of exquisite beauty and endless variety; while in Staurastrum and Nanthidium they assume a triangular aspect or have their edges adorned With spines or other appendages.
A glance at the figures in Ralfs' "Desmidieæ," at Plate X. in the "Micrographic Dictionary," or better still, at a few specimens in a friend's microscope, will give the beginner a better idea of their characteristic appearance than any description, and enable him at all times to recognise them among his own gatherings.
These plants rejoice in peaty bogs, where they occur either scattered here and there among larger plants, or in thin film encrusting their submerged stems; or floating in delicate clouds in the recesses of shallow pools, where the eye only detects them when it has become accustomed to the dim light by steadily gazing info the water for some minutes.
From such positions Desmids are best removed by carefully passing a watch glass under them, and raising the contents with slow and steady motion to the surface. Many fine specimens may be obtained also by squeezing out the water from handfuls of clean Sphagnum moss into a shallow basin, allowing a few moments for the plants to settle to the bottom, and then pouring off the surplus water, and transferring the greenish residue to a tube.
Some of the best habitats in Sutton Park have been destroyed by the railway and by drainage, but some of the commoner species, as Micrasterias denticulata, Penium digitus, Closterium acerosum, C. Diane, &c., and the filamentous species, Hyalotheca dissiliens, are sometimes to be found in tolerable abundance in boggy ground by the side of the streams. One morning's search in any bog on a Welsh moorland will, however, yield more and rarer species than any amount of hunting in this district.
It is worth adding that Desmids flourish for years in cultivation in small bottles, exposed to the light of a north window, and their growth can thus be watched de die in diem.
The singular modification of cell-division by which they increase will be at once understood from the accompanying figures of Microsterias rotata and Cosmarium colatum, sketched from specimens in the cabinet of the writer. [Plate I., Fig. 6.]