Fresh-Water Algce of Canada. 457 



invariably attracts the contents of the smaller : sometimes this will 

 happen in the one and sometimes in the other filament (fig. 6, b.)„ 

 The same thing is observable in the conjugation of adjacent cells in 

 the same filament; the cell wall of one is everted into the other 

 and a channel of communication is thus formed, through which the 

 smaller mass of endochrome passes into the larger. The united 

 contents of two cells form a large circular or oval cist, (fig. 6, a.) most 

 frequently of a deep green colur, but occasionally becoming reddish 

 brown. The endochrome soon becomes a mass of granules ; and 

 by and by the cist, or sporangium as it is called, breaks away from 

 the filament and floats free in the water. The granules (zoospores) 

 finally burst the cells walls, and, after moving about freely in the 

 water, germinate into filaments. 



A curious organ has been observed in this family of plants by 

 Hassall, J. Agardh, J. S. Bowerbank, Kutzing and others, 

 which they call a cytoblast. " It is solitary, and usually occupies 

 a central situation in each cell of the genus Zygnema. It consists 

 generally of two membranes, but sometimes there are three ; the 

 innermost of these being either circular or elliptical, and present- 

 ing a nucleated appearance, (as may be seen in Fig. 5, p. 458) The 

 surface of the enclosed membrane is smooth, while that of the exter- 

 nal is rendered irregular by the giving off of numerous tubular 

 prolongations or radii which terminate in the spiral threads 

 formed by mucous, and containing endochrome and large bright 

 granules, which I regard as unfertilized spores." 



" The structure of this curious organ explains with apparent 

 satisfaction one of the offices which it is destined to discharge, 

 viz., that of a laboratory or stomach, in which the materials neces- 

 sary for the growth and vitality of the cell and its contents are 

 received and digested, and from which they are conveyed by 

 means of the tubular radii to those organs by which the materials 

 are to be assimilated." 



" The cytoblast, therefore, is at first fixed in the centre of the cell 

 by the prolongations which proceed from it (see Fig. 5) ; but it 

 happens that at a certain epoch these radii disappear, and then 

 the cytoblast floats freelv within the cavity of the cell ; the dis- 

 appearance of the rays, the cessation of the growth of the cells, 

 and the assumption of the characters of reproduction, being almost 

 contemporaneous, the two latter being readily accounted for by 

 the disappearance of the radii." 



" In addition to the organs above described two others have 



