268 NOSTOCHINKdE. 



true Nostoc, whether young or old, present one uniform dia- 

 meter ; there are no gradations of size. It cannot therefore be 

 supposed that the threads are increased in number by the 

 effusion of the minute contents of the cells. 



The multiplication of the threads in a frond having been 

 as it seems to me satisfactorily accounted for, the manner of 

 the formation of new fronds remains to be determined. When 

 a Nostoc has arrived at the full and last period of its develope- 

 ment, the pellicle formed by the inspissation of the mucous ma- 

 trix bursts : the mucous contents and the filaments are eifused : 

 these last become disarticulated, so as to form short fragments, 

 each of which retains about it a portion of mucus, so that in 

 this state it corresponds with a Nostoc in the first period of its 

 developement. In this mode of multiplication, remarkable as 

 it is, there is nothing generically peculiar. A Conferva multi- 

 plies itself occasionally by the disarticulation of the filaments. 

 The only difference between the case of the Nostoc and the 

 Conferva is, that the process in the first is natural, and in the 

 second artificial. While, however, the separation of the pri- 

 mary filaments accounts amply and satisfactorily for the mul- 

 tiplication, not only of the threads of a frond, but also for 

 the increase of a number of the fronds themselves, it falls 

 short of explaining the first developement of the first thread. 

 The species of the genus Nostoc, like the freshwater Algce in 

 general, are short-lived : in the course of a few months they 

 pass through the stages of their developement ; they die, dis- 

 appear ; the filaments themselves are destroyed ; and then are 

 seen no more until the advent of another season. It is plain, 

 therefore, that the true mode of reproduction of the species of 

 this family is something more than a mere separation of the 

 filaments into fragments. It is now, that in my view we 

 learn the real nature of the enlarged vesicles : these are the 

 true reproductive bodies, at the season not proper for their 

 developement, lying concealed in the earth, awaiting their 

 appointed time to start into growth, activity, and life 

 all else of the plant, the mucus and the filaments, utterly 

 perishing. 



In the " Annales des Sciences Naturelles," third series, 



