of the Freshwater Alga. 27 



when a cell lias discharged its contents its cavity is empty, and 

 no resistance can be offered by it to tlie protrusion of the inverted 

 portion of the adjacent cell or cells, replete as it or they are with 

 fluid and endochronie. This explanation applies likewise to the 

 fact, that when a number of cells have either emptied themselves 

 of their contents or have been the recipients of those of other 

 cells at the same time no eversion takes place, for in this case 

 there is no inequality of internal pressm-e. 



But while a correct exjiosition may be given of the cause of this 

 protrusion and intromission, it is not so easy to offer a satisfactoiy 

 explanation of the purpose to be attained. The eversion doubt- 

 less assists in effecting the dislocation of the cells, and thus, repro- 

 duction being perfected, hastening the destruction and dispersion 

 of the species ; processes, which, from the greater length of the 

 cells and consequent continuity of the enveloping sheath, would 

 possibly occupy, were it not for some special provision of the na- 

 tm-e indicated, a much longer time than in the short-celled spe- 

 cies. A subordinate and not unimportant use of this provision 

 is, the assistance which it affords in the determination of allied 

 species. 



It is remarkable that no similar conformation presents itself to 

 our notice in the genera Tyndaridea and Mougeotia, so closely 

 allied to Zygnema, for in these the cells terminate by plane sur- 

 faces, which however may be either everted or inverted to a slight 

 extent. 



. This peculiar formation of the cells of some Zygnemata was first 

 noticed by me in the spring of 1842, but its true nature only 

 became apparent to me in the early portion of the present year. 

 WTien viewed through a low power of the microscope, and in a 

 Zygnema whose filaments are as yet separate, it exhibits the ap- 

 pearance of two cmTcd knife-blades shghtly approximating to 

 each other at their apices, near to which usually lies the divided 

 spiral thread, and strongly impressing the superficial observer, 

 from the position and aspect of these blades, with the idea that 

 they are the instruments which effect its separation, and remind- 

 ing him of the beautiful provision whereby the section of pollen 

 granules is brought about. 



On transmitting a short time since a specimen of the Zygnema 

 now figm-ed (quadratiim) , in a state of reproduction, to the Rev. M. 

 J. Berkeley and jMr. Ralfs, but miaccompanied by any remarks in 

 reference to the structure of the cells, both these gentlemen no- 

 ticed their peculiar conformation, and from the former I received 

 correct sketches of their appearances. 



The structm-e of the joints in Zygnema was long since noticed 

 in one species of the genus by Mohl, who thus describes it in his 

 paper upon the midtiplication of cells by division inserted in the 



