1910-1911.] A Rare British Freshwater A Iga. 3 1 3 



with a sheath from those without that feature. When a 

 sheath is absent, a row of cells forms a trichome ; but when 

 the mucous sheath is present, the term filament is then 

 applied. This distinction, however, is not always observed by 

 writers on the subject. Oscillatoria prolifica has no sheath, 

 and the trichomes are found in bundles, floating on the 

 surface of the loch, as well as congregated round the 

 margin. 



The reproduction of Oscillatoria prolifica, like its structure, 

 is exceedingly simple. In early autumn a few cells break off 

 from the extremities of the trichomes, these being termed 

 " hormogones." The hormogones then sink to the bottom of 

 the loch, and ultimately become new plants, again rising to 

 the surface about the beginning of April. The mode of ascent 

 is rather remarkable, being facilitated by the help of so-called 

 gas-vacuoles, with which 0. prolifica is abundantly supplied. 

 The process is variously termed " the water-bloom " and " the 

 breaking of the meres." Professor West remarks that the 

 few members of the Myxophyceae or Blue-green Algse which 

 exhibit this phenomenon " are generally species which nor- 

 mally occur in the plankton of lakes and rivers " ; and he 

 adds that " the extraordinary rapidity of their increase, and 

 the consequent discoloration of the water, together with their 

 equally rapid disappearance, constitute one of the most remark- 

 able facts in the whole domain of algological inquiry." Dr 

 Greville, in his ' Scottish Cryptogamic Flora,' gives the float- 

 ing period of The Haining Loch Alga as " October to April," — 

 an evident slip for April to October. On the appearance of 

 the plant in early spring, it is of a slightly purple hue. Dr 

 Greville's illustration shows the Alga of an intense purple, 

 which one cannot help regarding as in all probability some- 

 what exaggerated.^ In the passage already quoted from Dr 



^ According to M. Gomont, the various colours of the Oscillatoriae owe their 

 origin to the protoplasm of the cells. He says : " Outre le teint vert ^rugineux 

 dont le protoplasme des Oscillariees est habituellement revetu, le brun, le violet, 

 les differentes nuances du rouge s'y observent egalement. Par la dessication 

 toutes ces couleurs peuvent se transformer en bleu ardoise." — "Monographic 

 des Oscillariees" in 'Annales des Sciences Naturelles,' Septieme Serie, Tome 

 Seizieme, p. 199. M. Gomont's monograph is a most minute and painstaking 

 study of its subject, and should not be overlooked by any worker in this group 

 of Algae. 



