314 A Rare British Freshwater Alga. [Sess. 



Greville, descriptive of the plant, it is further said to resemble 

 " a floating bed of opaque bituminous-like scum," evidently 

 referring to its appearance later in the season; and this 

 still fairly represents its aspect on a summer day, especi- 

 ally when the growth happens to be " prolific." There are 

 about a score of British Oscillatorias, of which 0. yrolijica 

 is amongst the smallest, if not the very smallest. What 

 it lacks, however, in size, is usually compensated for by 

 quantity.-^ 



Before quitting the description of this Oscillatoria, there is 

 one other point worthy of mention — viz., the gliding motion 

 it so beautifully exhibits, when examined fresh, under the 

 microscope. These spontaneous movements are common to 

 several of the Blue-green Algae belonging to the Oscillatoriaceae, 

 and indeed have given the designation to the genus Oscilla- 

 toria, where they are most conspicuous. The motion has been 

 described as " a slow creeping or gliding of the entire filament, 

 which at the same time rotates round its axis, and this is 

 often accompanied by a slow oscillation of the extremity of 

 the filament." The movements are said to be probably due to 

 osmotic changes. 



Now that the structure and life-history of 0. 'prolifica have 

 been thus briefly considered, it may be interesting to compare 

 its specific character as given by Dr Greville with that of its 

 latest investigator, Dr E. Lemmermann. The former says : 

 " Plant extensively diffused, forming a floating stratum of a 

 rich purple colour. Filaments extremely slender, somewhat 

 rigid, yet flexible, entirely destitute of attachment, and free 

 from any mucous layer. Annuli, from the minuteness of 

 the filament, almost inconspicuous." The following is Dr 

 Lemmermann's description : " Trichomes straight or curved, 

 not contracted at the septa, slightly narrower at the end, 

 2*2-5 ju wide,^ floating often in bundles of a purple colour, 

 violet when dry. Cells nearly square or longer than broad, 



^ The Myxophycese or Blue-green Algae in general, and species of Oscillatoria 

 in particular, are very poorly represented in Scottish and English lakes, as com- 

 pared with Irish, Swiss, and Scandinavian lake-areas. See " Phytoplankton of 

 the English Lake District," by W. and G. S. West, in * The Naturalist ' for Sept- 

 ember 1909. 



2 A micron (/x) is the thousandth part of a millimetre— the milUmetre being 

 equal (nearly) to the twenty-fifth part of an inch. 



