6 W. Hofmeister on the Propagation 



sion of the cell by a septum passing across it, whose composition 

 of two layers could not be directly detected, was always followed 

 by a separation of the daughter-cells, proceeding from without 

 inwards. The annular grooves formed in this way became 

 deeper and deeper, until at length the two halves appeared com- 

 pletely detached (figs. 13, 14). The formation of a cross-septum 

 and the mode of division of the daughter-cells occur therefore in 

 the same way as appears general in the Desmidiese*. 



When the cells are about to conjugate, they protrude from 

 one of the lateral surfaces, mostly not accurately in the middle, 

 a process of not inconsiderable length, often far exceeding the 

 transverse diameter of the cell (fig. 18), differing in some degree, 

 by gTcater slenderness and sharper apiculation, from the short, 

 blunt process of P. macrococca-\. When two such processes 

 meet, the firm cell-membrane disappears at the point of contact, 

 and the pellicular coats of the contents of the two cells unite. 

 The long canal connecting the two individuals (fig. 19) some- 

 times inci'eases in thickness, at the same time becoming con- 

 siderably shorter, and thus approximating the conjugated cells 

 (fig. 20). The granular layers of the two cells only now make 

 their way into the connecting canal, at first still evidently sepa- 

 rate at their point of contact (fig. 19). This boundary dis- 

 appears; the connecting piece of the cells continues to grow 

 broader and simultaneously shorter; a continually greater por- 

 tion of their adjacent side-walls enters into the composition of 

 the canal, until at length the two individuals appear blended 

 into a single body, at first of irregular quadrangular shape (fig. 

 21), just as in P. macrococcaX. Subsequently the spore thus 

 formed acquires a regular quadrate form ; its contents gradually 

 lose their green colour, and tui-n to a red-brown. Of solution 

 of the walls of the conjugated cells, or formation of new mem- 

 brane around the conjugation-cell, nothnig whatever can be seen 

 here, as may be the case also in P. macrococca. The conjugation 

 of the Palmogloeese, as I have already advanced elsewhere §, un- 

 doubtedly afi"ords an example, in the course of development, of 

 cellulose membranes, capable of very considerable contraction, 

 diminishing their superficial dimensions. This diminution of 

 the surface to |rds of the original extent is accompanied by 

 great increase of thickness of the membrane. This increase of 

 thickness of the coat depends, if not exclusively, yet certainly in 

 part, upon its contraction. 



* Clearly recognized iu Euastrum, Cosmarium, Staurastrum, Docidium. 

 t Al. Braim, I.e. (Translation, p. 328). 



+ Al. Braun, I. c. pi. 1. figs. 22-26 (Translation, pi. 1. fig. 22; pi. 2. 

 figs. 1-4). 



§ Flora, 1855, p. 534. 



