230 DESCRIPTION OF [Pof^fjctsfrica. 



and who would work at them independently, should obtain Mr. 

 Ealfs' book, in which cvcrj^ British species is described and figured, 

 its special localities named, and many further details given, such as 

 can be admitted only in a Monagraj^h. 



Before entering upon the Demnidiacea, the reader M-ill do well to 

 examine the different drawings, illustrative of them given in Plate 1 . 

 f. 63 to 67; P. 2. f. 99 to 123; P. 13. f. 1 to 44; P. 18. f. 10 

 to 19. 



" The Desmidiem are of an herbaceous green coloui" ; a few only of 

 the Closteria have the rigid integument coloui-ed, but in all, the 

 internal matter is green. They inliabit fresh water. 



Their most distinctive character is that " Each cell or joint, con- 

 sists of two sjTumetrical valves or segments, and the sutm^c or line 

 of junction is in general well marked ; in a few instances, however, 

 as in Scenedesmus, it is determined principally by analogy. In 

 Pediastrum, its situation is shewm by a more or less evident notch 

 on the outer side, but no separation has been noticed. In the other 

 genera the suture eventually opens and allows the escape of the con- 

 tents, and it is indicated by either a ti'ansverse line or a pale band, 

 and usually also by a constriction." An uninterrupted gradation 

 may be traced from species in which these characters are incon- 

 spicuous to those in which they are fully developed : — from Clos- 

 terium, in which there is no constriction to Bpltarozomia, Micras- 

 terias, and other genera, where the constriction is so deep that the 

 segments appear like distinct cells, connected by a mere cord. By 

 Ehrenberg, indeed, such forms were regarded as binate cells, but 

 Ml'. Ralfs for several reasons, concludes that each bipartite frond is a 

 distinct, simple cell, merely constiicted. 



The Besmideoi multiplying by repeated tranverse division ; the 

 nature of which is well seen in the compressed and deeply constricted 

 cells of Eumtrum. Here, as the connecting portion is so small, and 

 necessarily produces the new segments, which cannot arise from a 

 broader base than its opening, these are at first very minute. 

 " The segments are separated by the elongation of the connecting 

 tube, which is converted into two roundish hyahne lobules. These 

 lobides increase in size, acquu'e colour, and gradually put on the 

 appearance of the old portions. Of course, as they increase, the 



