98 DUBLIN UNIVERSITY 



usual pale band at the centre of the endochrome) ; in Hyalotheca, va- 

 rious Cosmaria, Tetmemorus, &c., the constriction is but slight ; in Des- 

 midura and Didymoprium it is indicated by only a notch at each angle ; 

 while in various other genera the constriction becomes deeper and more 

 obvious, until at last in Sphaerozosma, Euastrum, Micrasterias, &c, the 

 constricted portion becomes like a mere isthmus between the segments, 

 giving them the appearance of distinct cells, and as such they were for- 

 merly considered. Each frond or individual, however, is always a single 

 cell, as is often evidenced by the whole contents escaping through a 

 single accidental rupture. The cells frequently possess warty or spi- 

 nous processes, and the cellulose coat often presents minute markings 

 or pun eta, caused by the presence of little elevations. The cells are 

 usually more or less surrounded by a gelatinous sheath, — in Hyalotheca, 

 Didymoprium, and many others, this is well defined, but in some species 

 it is so attenuated as to have its existence made known only by the 

 gelatinous investment preventing the contact of the fronds. The con- 

 tents of the cells of the Desmidiaceae appear to be similar to the green 

 confervoids generally, that is, protoplasm coloured green by chlorophyll, 

 and entirely enclosed in a "primordial utricle," which organ appears 

 more evident as a real and distinct utricle than in any other vegetable 

 cells I have seen. The contents of the cells often contain starch gra- 

 nules. A circulation of their fluid contents has been noticed in 

 various species ; in Closterium and Penium, &c, it is of a very re- 

 markable character. In Cosmarium Ralfsii (Br 6b.), after the contents 

 had lost their characteristic somewhat radiate appearance, and had be- 

 come broken-up, I have seen a regular current rotating somewhat rapidly 

 round-and-round the internal margin of each segment, and carrying the 

 chlorophyll granules with it, very strikingly like that in Anacharis, &c. 

 The ordinary mode of multiplication of the individual cells is by re- 

 peated transverse division, which is effected by the interposition of new 

 growth between the original segments, the older segments remaining 

 unaltered, except (when they remain for some time attached) by being 

 pushed asunder by the enlarging young segments. The exact manner 

 in which this takes place differs slightly in detail in the different ge- 

 nera. In Closterium, which in the various species is more or less of an 

 arcuate or lunate form, the original cell acquires a constricted appear- 

 ance at the middle ; a separation of the endochrome having taken place, 

 the new constriction gradually becomes deeper, until at length it is 



