292 THK PHENOMENOK OF 



striate and truncated ends,* seem to exhibit a similar 

 behaviour. 



/3. The halves of the dehiscent cell only separate a 

 little on the side turned towards the other individual, at 

 which the internal membrane emerges, while they remain 

 connected on the outer side; the internal membrane 

 remains as an internal investment in the outer; the 

 spore mostly fills the uniting canal so completely, and is 

 in such close contact with its walls, that the latter is 

 scarcely distinguishable. This is the condition in Penium\ 

 (with the exception of the species mentioned previously), 

 and the unstriped ClosteriaX (with the exception of the 

 above-mentioned CI. Lunula). 



Closterium lineatum^ also belongs here, but with the 

 peculiarity, known only in it, that two spores are formed 

 between two individuals. As I have had an opportunity 

 of observing the process of conjugation minutely in this 

 very species, || I am able to explain the mode of origin of 

 the really double spore (not two-lobed, as Ralfs terms it). 

 The very much elongated, only slightly cm-ved specimens 

 of Clost. lineatum, lie close together in pairs before 

 conjugation, sometimes with both the convex sides in con- 

 tact, sometimes with the convex side of one touching the 

 concave side of the other. The cell-membrane dehisces 

 transversely, either simultaneously in both, or in one 

 earlier than the other, in the middle of the cell, previously 

 marked outside by an annular line, and inside by an 

 interruption of the green contents,^ but this takes place 

 in such a manner that the two halves only separate a 

 little on the side turned towards the other individual, 

 remaining connected on the opposite side, so that the 



* Ralfs, 1. c, t. 29, f. 2, {Closterium striolatnm) ; fig. 6, 7, {Closteriam 



juncidum.) 



f Ibid., t. 25, f. ] and 6, Penium margaritacmim and (runcatum.) 

 % Ibid., t. 27, f. 2, {Clost. acensum;) t. 28, f. 4, {Closl. Leibleinii.) 

 § Ehrenberg, ' Infusionsth./ t. 6, f. viii, 4.) RaKs, 1. c, p. 174, t. 30, f. 1. 

 li In the middle of May, 1848, in specimens from the peat-holes of the 



Mooswald, near Freiburg, especially rich in Desmidiaceae. 

 ^1 The two pieces are mostly of unequal length. 



