PROTOCOCCUS PLUVIALIS. 523 



by Flotow, the author differs from him, at the com- 

 mencement, in one important particular. Flotow looked 

 upon Profococcus pluvialis as a multicellular plant, the 

 individual cells of which are held together by a common 

 parent vesicle. Although he does not express himself 

 particularly with respect to the kind and mode of this 

 connexion, it may be taken that he regarded it some- 

 thing like that which obtains in Nostoc, or rather, per- 

 haps, like the structure ofPofycoccus punctiformis,Kntz., 

 in which numerous cells are said to be surrounded by a 

 thin, common cell -membrane. (Kiitz., 'Phycol. Germ./ 

 p. 148.) 



This view, however, is undoubtedly erroneous. The 

 P. pluvial is is in all its phases a unicellular plant. 



The idea of a unicellular plant, as first propounded by 

 Nageli, as a systematic principle (' Gattung. einzelliger. 

 Algen.' Zurich, 1849,) does not apply throughout to the 

 extent in which he employs it. He certainly includes 

 under the term a number of genera, having a certain degree 

 of internal relationship, as indicated by the fact that pre- 

 vious phycologists had united them in large natural 

 orders, (CtamapAyct<s,Kg.; UZvace<e,}:larvey;PaImelle<2, 

 Decaisne, Endlicher, &c.) On the other hand, the 

 definition of a unicellular Alga, "that in it the indi- 

 vidual is a single cell," is too wide or too narrow, (1. c., 

 p. 1.) Too wide, because this definition may be applied 

 with equal right to other Alga3, which must be reckoned 

 among the multicellular, such as CEdogonium, which is 

 a Conferva, or Prasiola, one of the Ulvacea?, as Desmi- 

 dium and Gallionella, or as Tetraspora, all of which are 

 declared to be unicellular Algae. Too narrow, because it 

 is only by straining the definition that such things as 

 Pediastrum, Sphterastrum, which form definitely bounded 

 bodies, and many Palmellece, can be included under it. 

 Just as little do the characters assigned to a unicellular 

 plant bear a comparative scrutiny. The unicellular Alga3 

 are said 1 . to present merely a reproductive, and, nor- 

 mally, only a double kind of cell-formation ; this is, how- 



