FRESH-WATER ALG^. 93 



to be found in the widely-distributed Protococcus phtvialis^ which 

 may be found in ahuost any house-gutter. An aUied species, 

 Protococcus nivalis^ the so-called red snow, was long supposed to 

 be confined to the arctic regions and to snow mountains, but it 

 has been found in this country on the borders of lakes, especially 

 affecting calcareous rocks. 



Glmocapsa shows a little advance in organisation, the thallus 

 being gelatinous and enclosing cells and families irregularly dis- 

 posed, and presenting fine example of cell division. The colour 

 of the cell contents is sometimes greenish yellow, but more 

 commonly red ; G. sa7igut?iea, an example of the latter, is a 

 favourite slide with microscopists. 



There are numerous other simple genera included under this 

 class, but it is probable that many of them are not independent 

 species, but only stages in the development of some higher form, 

 and that in course of time such families as the PahtiellacecE^ 

 ProfococcacecB, and Chroococcacea. will disappear altogether from the 

 catalogue. 



As we go up in the scale we find the derivative cells, instead of 

 separating and carrying on an independent life, remaining united 

 and forming slender rows of cells, or thin lamellae. 



Thus in the Nostocs we find a more or less firm jelly, in which 

 chains of small rounded cells are imbedded, with here and there a 

 large cell, termed a heterocyst, which has to do with the propaga- 

 tion of the species. Some of the Nostocs are supposed to be 

 parasitic (especially Nostoc lichenoides)^ and are described as such 

 in the leaves of Hepaticae and Mosses, by various French and 

 German observers. 



The Oscillarice consist of rigid cylindrical filaments of varying 

 thickness, divided into disc-like cells by delicate transverse septa. 

 The filaments are straight or a little curved, rarely spirally convo- 

 lute, and mostly brightly coloured in various shades of green and 

 blue ; they revolve on their axis, and on this account the filaments 

 often get matted together in large masses. 



O. cenigesce7is is mentioned by Dr. Drummond as existing in 

 such quantity in Glasslough lake, Ireland, as to impart a decided 

 green tinge to the water. The plant seemed diffused all through 

 the water of the lake, but in a ditch extending from the lake he 



