94 FRESH-WATER ALG.E. 



found large masses of it, several inches thick and above a foot and 

 a half in length. 



O. thermalis has been described by Hassall as having been 

 found in warm water, and O. tejiuissima as peculiar to the Bath 

 Mineral Water, but neither species has been identified by Cooke. 



In Rivularia the frond has a tendency to a hemispherical or 

 bladdery form, the filaments of rounded cells are agglutinated by 

 mucilage and radiate from the base of the frond ; at the free end 

 the filament runs out into a long hyaline hair, while at the central 

 end is a large heterocyst, which gives the whole filament the form 

 of a riding whip. 



This is one of the Algge associated with the phenomenon called 



" Breaking of the Meres," which is thus described by Professor 



Dickie in his " Botanists' Guide : " — " For some years excursions 



were made with students of my botanical class to a loch on the 



estate of Parkhill, about four miles north-west from Aberdeen. 



The sheet of water in question is about a quarter of a mile in its 



greatest length ; on almost all sides it is surrounded by extensive 



deposits of peat, with the soluble matter of which a great proportion 



of the water passing into the loch is impregnated. The locality 



was generally visited in the beginning of July. Nothing particular 



had ever been observed till the summer of 1846, when my attention 



was arrested by a peculiar appearance of the water, especially 



near the edge, but extending also some distance into the loch. 



Numerous minute bodies, with a spherical outline, and varying in 



size from 1-2 4th to i-i2th inch in diameter, were seen floating at 



different depths, and giving the water a peculiar appearance. In 



some places they were very densely congregated, especially in 



small creeks at the edge of the loch. A quantity was collected by 



filtration through a piece of cloth, and on examination by the 



microscope there could be no doubt that the production was of a 



vegetable nature and a species of Rivularia^ associated however 



with another genus, Trichor7mcs^ in small quantity. In the first 



week of July, 1847, the same species were observed similarly 



associated, but the Trichornms was now more plentiful. In July, 



1848, it was observed that the Rivularia was as rare as the 



Trichornms had been in 1846 ; to the latter consequently the water 



of the loch now owed its colour, which was a very dull green. 



