FEESHWATER ALGJE, 117 



Fortunately they are widely distributed ; it is rather the exception 

 than the rule to take up a dip of water fx-om a pool or marsh in which 

 some of the brilliant crescents of Closterium or the sculx:)tiu-ed discs of 

 Micrasterias do not dehght the eye. The most distinctive featiu-e in their 

 appearance is the perfect bilateral sjonmetry of the two halves into which 

 each plant is generally divided. 



The fi-onds of Closterium are morg or less crescent-shaped, from the 

 shghtly curved form of the Tartar bow to the complete crescent foi-m of 

 the young moon ; in Cosvuiriuin, Euastrum, 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 Staurastruin and 

 XaiitJiidium they assume a triangular aspect or have their edges adorned 

 with spines or other appendages. 



A glance at the figures in Kalfs' " DesmidieaB," at Plate X. in the 

 " Micrographic Dictionary," or better still, at a few specimens in a 

 friend's microscoi^e, will give the beginner a better idea of their charac- 

 teristic 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 occm- either scattered 

 here and there among larger plants, or in thin films 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 into 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 siu-face. Many fine specimens may be obtained also by 

 squeezing out the water fi'om haudfuls of clean Sphagnum moss into a 

 shallow basin, allowing a few moments for the plants to settle to the 

 bottom, and then pouiing off the sui"plu3 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, biit some of the commoner species, as Micrasterias 

 denticulata, Fenium digitus, Closterium acerosum, C. Diance, &c., and 

 the filamentous species, Hyalotheca dissiliens, are sometimes to be found 

 in tolerable abimdance 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 

 smaU bottl3s, exposed to the hght of a north window, and theii* growth 

 can thus be watched de die in diem. 



The singular modification of cell-division by which they increase will 

 te at once understood frora the accompanjdng figures of Micrasterias 

 rotata and Cosmarium ccelatum, sketched fi-om specimens in the cabinet of 

 the writer. [Plate I., Fig. 6.] 



rxo BE COXTINUED.] 



