INTRODUCTION. 3 



which appear like distinct cells, and are so considered by 

 Ehrenberg and others. 



When the papers on the Desmidieae were publishing in the 

 ' Annals of Natural History,' I stood alone in regarding the 

 frond as a single cell, differing on this point not only from 

 Ehrenberg, but from every author whose works I had seen. 

 Professor Kutzing, however, in his ' Phycologia Germanica,' 

 has by independent observations arrived at the same con- 

 clusion ; an important corroboration of the correctness of the 

 opinion I then advanced. That the frond in Euastrum and 

 allied genera is really a single constricted cell, and not a bi- 

 nate one, will, I am persuaded, be apparent to any one who 

 traces the gradations mentioned above ; but as the opinions 

 of such distinguished naturalists as Ehrenberg and Mene- 

 ghini are deservedly of much influence, and the subject is 

 so important, — since upon the view which we take of it de- 

 pends the explanation of the division of the frond presently 

 to be described, — I shall proceed to notice some facts which 

 seem to me quite decisive. 



In Navicula and other genera of Diatomacese the frustules 

 are often truly binate, and, as each frustule is complete in 

 itself, though they be separated from each other their respec- 

 tive contents will still be protected on all sides ; and even 

 should one be broken the contents of the other will be un- 

 disturbed. In the Desmidiese, on the other hand, as there is 

 no septum between the segments, if these separate, or an 

 opening be made in one, the contents of both will escape ; 

 and I have more than once observed the minute granules 

 passing from one segment through the connecting tube into 

 the other. The conjugation of the fronds and formation of 

 sporangia I believe to be altogether irreconcilable with the 

 supposition of binate cells. For in the simple Algse are many 

 examples where the contents of two cells meet and form a 

 compact seed-like mass ; but I know of no instance in which 

 the contents of more than two cells are thus united : nor does 

 it appear probable that the process of reproduction in either 

 an animal or a vegetable should require, or indeed permit, 



B 2 



