INTRODUCTION. 3 



vestigators, and handed down to posterity. Sucli expecta- 

 tions, however, it is impossible to realise; and I agree with 

 Vaucher in thinking, that the wisest course to adopt would be 

 (except in some few cases, where the productions can with cer- 

 tainty be determined by otlier characters,) to notice only those 

 species whose reproduction has been satisfactorily made out. 



In the present Avork, the necessity for which is in a 

 measure indicated by the preceding remarks, the characters 

 developed in the state of reproduction are relied upon, in the 

 framing not merely of the families and genera, but also in the 

 definition of species, for which they are even more valuable. 



In this Introduction it is not intended that a, full descrip- 

 tion should be given of the different modes of reproduction 

 and of the structure of the freshwater Al(j(S, the details of 

 these coming under consideration with more propriety when 

 the divisions into families, genera, and si)ecies are treated of. 

 The general particulars of each will, liowever, be now noticed. 



LinuKus supposed that all vegetable productions owed 

 their perpetuity to sexes : he did not, however, assign in his 

 system any fructification to the Conferva;. Had Linna3us, 

 nevertheless, been aware of the highly curious and interesting 

 facts which more recent investigation has made knoAvn, viz., 

 of the phenomenon of the union of two cells, either in dif- 

 ferent or in the same filaments, which so frequently occurs 

 amongst the Conferva (^ee Plates 30 — 50. and 33.), he would 

 doubtless have regarded this commingling as not merely 

 strengthening, but proving the correctness of his views of the 

 sexual character of all plants. But it is to be questioned 

 how far the fact just alluded to would bear any such inter- 

 pretation, its tendency in support of the opinions of the illus- 

 trious Swede being completely neutralised by our acquaintance 

 with otlier facts, and chiefly with this, viz., that in a consi- 

 derable proportion of freshwater species, and probably in the 

 entire of the marine Conferva, no such conjunction of filaments 

 or commingling of the contents of tAvo cells occurs, all the re- 

 quisites for the continuance of these being indisputably con- 

 tained within each cell, no exterior organs of reproduction ever 

 having been discovered in the vast majority of these. Tlio 

 u 2 



