BRITISH DESMIDIE^. 161 



junction of the two segments, and then the newly-formed portion 

 continues to enlarge until the original segments are separated by a 

 cell of an irregular four-sided figure. The contents of the fronds, 

 being collected in this cell, become a dense seed-like mass, which is 

 sometimes globular resembling the sporangium in Mougeotia, and 

 sometimes square like that in Staurospermum. The newly- formed cell 

 is thinner and generally paler than the segments of the frond ; in some 

 species it looks like a prolongation of the segments, and in others these 

 are so loosely attached that their connection is scarcely perceptible. 



Two species having cruciform or quadrate sporangia permanently 

 attached to the empty segments of the conjugated fronds are separated 

 by Kiitzing to form a new genus, Stauroceras. To these others have 

 been added by Brebisson. Whilst fully admitting the soundness of 

 the principle on which this separation has been made, I regi'et that I 

 am unable to adopt it here, not only because I am uncertain to which 

 genus some of the species should be referred, but because I have 

 decided to employ in this monograph only such generic characters as 

 can be determined in the ordinary state of the plant, and will not 

 oblige the observer to depend on conditions which he may not be able 

 to discover. 



According to Meneghini the coupling of the fronds takes place 

 from the convex margin. This is generally, but not invariably, the 

 case, for I have observed that they are connected sometimes on the 

 convex and sometimes on the concave margin ; in some instances, in- 

 deed, I have seen the convex margin of one frond connected with the 

 concave margin of another. Mr. Jenner has noticed the same diversity, 

 but it appears to him to characterize different species. 



As there is no constriction in Closterium, although its frond divides 

 like that of other Desmidiefe, the process of division is less evident. 

 It is best seen in the striated species, in which the central suture is 

 most distinct. The transverse line becomes double, and by the in- 

 termediate growth the frond at length consists of three portions. As 

 the newly-formed central one continues to elongate, another transverse 

 line becomes visible at its middle, where a complete separation at 

 length takes place. At first however the new segment is often merely 

 a rounded protuberance, and the frond is consequently unequal ; and 

 when it is perfected, if the covering is a coloured one, the newer seg- 

 ment can still be distinguished by its greater paleness. 



