468 Mr. J. Ralfs on the British Desmidieae. 



Pediastrum, Meyen (Micrasterias, Ehr.). 



Fronds plane, circular, composed of several cells, which form by 

 their union a flattened star, and are generally arranged either 

 in a single circle or in two or more concentric ones ; marginal 

 cells bipartite. 



Fronds minute, composed of four or more cells united together 

 into the figure of a flattened star ; when only four in number they 

 are not arranged in a circle, and the star is somewhat angular ; 

 in most species however they form either a single circle or two 

 or more concentric ones, and one or two of them usually occupy 

 the centre. The cells are thus combined by a mucous matrix 

 which is generally colourless, and constitutes hyaline interstices. 

 Occasionally some are ruptured, but their endochrome alone 

 escapes, and the others are not affected. In all the species the 

 outer cells have the free margin bipartite, a character which I 

 consider important, and in fact a modification of the form ob- 

 served in Cosmarium and other genera with constricted fronds. 

 The cell in Pediastrum is different, since its division into two seg- 

 ments exists only on one side, and in the inner cells is nothing 

 more than a slight concavity of the external margin. The oc- 

 currence of this division on one side only of the outer cells, 

 whilst in the inner ones it becomes obsolete, is an additional 

 reason for considering the frond in the preceding genera to be 

 really a single cell, which constricted in the middle forms two 

 segments, and not the result of the union of two cells. 



The flat, starlike fronds of Pediastrum are so characteristic, 

 that there is no risk of mistaking it for any other genus, Gruci- 

 genia perhaps excepted ; that however differs by having entire 

 quadrangular cells. 



It is far more difficult to distinguish its species ; Ehrenberg 

 relies chiefly on the number of the circles, but this character, as 

 Meneghini and Professor Bailey have observed, cannot always be 

 depended on ; the latter says, " There appears to me to be much 

 confusion in the specific characters, arising from the circumstance, 

 that the number of corpuscles in the different rows has been made 

 a character of specific importance. From what I have seen of the 

 species, I am satisfied that the number of corpuscles in a star is 

 liable to great variation in the same species." 



I have myself noticed that, in one species (P. Boryanum) at 

 least, the number of circles varies from one to three, and the same 

 observation has been made by Mr. Jenner. He has also remarked 

 that the number of cells in the inner circle, on which I was in- 

 clined to place greater reliance, is subject to variation. Some 

 species have one cell in the centre, and others two ; these I have 

 found constant, but they are said to be sometimes deficient. 



