204 



M. C. COOKE ON SOME DESMIDS 



rious mode of growth, the cells being united end to end in a con- 

 tinuous chain. These threads are at first attached to some object 

 after the manner of Confervee. We may, for convenience, call these 

 the Jixed Desmids. 



Others, and by far tlie greater number, are unattached, free, float- 

 ing cells, and these we may term free Desmids. 



We have nothing further to do with the Jixed Desmids on this 

 occasion. 



The free Desmids may also be subdivided into two artificial sub- 

 groups, in one of which the cell is elongated so as to be almost 

 cylindrical, or fusiform, and in the other oval, or with a length but 

 little exceeding the breadth. To the former belong as types Clos- 

 terium, Docidium, and their allies ; to the latter, Euastrum, Cosma- 

 rium, and Staurastnim, with some others. 



We may banish from our minds the cylindrical free Desmids, and 

 concentrate our attention upon the sub-group, which includes Stau- 

 rastrum, for that is the genus to which I am purposing to allude 

 more in detail. 



Before doing this you will permit me to refresh your memories 

 with the distinctive features which characterize the different genera 

 in this group. They are only six, or at any rate six will serve our 

 purpose ; and these are Micrasterias, Euastruni, Cosmariwn, Xan- 

 thidium, Arthrodesmus, and Staurastrum. The two genera Scene- 

 desmus and Pediastrum, which are included in Ralfs's book, are not 

 Desmidice at all. 



In Micrasterias the cell is flattened, and almost lens-shaped, 

 nearly circular, with deep sinuses, which are directed towards the 

 centre, and a deep central notch on each side, nearly dividing the 

 cell into two semicircular equal halves ; the side view is consequently 

 narrow and but slightly constricted at the centre. There are no 

 inflated prominences on the face, and consequently no projections in 

 the side view. 



In Euastrum the cells are usually longer in proportion to their 

 breadth, with a deep notch in the centre on each side dividing the 

 cell into two equal sinuated or lobed halves, generally with a notch 

 at each end, and wart-like inflations on the surface, which show as 

 projections in the side-view. Each segment is usually narrowed 

 from the base upwards. 



In Micrasterias and Euastrum therefore the frond or cell has a 

 more or less lobcd outline. 



