110 DUBLIN [JNIVEBSITY 



segments. In the bipartite forms (such as Micrasterias, Euastrum, Stau- 

 rastrum, &c.), the mode of increase is necessary in order to restore the 

 symmetry of the dividing frond ; but the form in question being desti- 

 tute of lobes, inflations, or processes, and straight, the normal symmetry 

 of the cylindrical dividing joint is not disturbed. Nevertheless (while 

 it seems to me that the characters before detailed are abundantly suffi- 

 cient, in fact do prove, that this organism is a true Desmid, for it will 

 be noticed that the mode of cell-increase referred to does not form part 

 of Ralfs' diagnosis of the Desmidian group), though it cannot be de- 

 duced, from any alteration in the outward form of the joints, that the 

 new growth takes place in the manner described, yet I think the fol- 

 lowing circumstance indicates sufficiently definitely that this is really 

 the mode which holds good, and to my mind it leaves no doubt. 



On looking over a mass of the filaments upon a slide, in by far the 

 greater number of the joints it may be seen that the pale interruption 

 of the endochrome occupies the centre of the joint, and is distant from 

 each extremity precisely, or very nearly precisely, the same interval. 

 But in a few cases, it may sometimes be noticed, that in two neighbouring 

 joints of a filament the pale space is not central, but in both is very con- 

 siderably nearer the adjacent extremities of the two contiguous joints. 

 It will also be remarked that the two joints displaying this peculiarity 

 are also shorter than the remaining joints of the same filament : Fig. 3, 

 representing a few joints of a filament (magnified 200 diameters), shows 

 at its upper part the state alluded to. Now I believe that this occurs 

 in the following way. A septum is formed — as in the other Desmidians 

 under the isthmus at the constriction — here, at the pale space in the 

 centre of the cylindrical joint, which represents the isthmus in the bi- 

 partite forms. Close to this new septum, at each side, there now 

 exists an interruption of the cell-contents forming the pale space, 

 which, by the growth (perhaps pretty rapidly) of the interval between 

 it and the lately formed septum, and by a fresh accession of endochrome, 

 soon, in place of being, as hitherto, eccentric, becomes removed to its 

 normal or regular central position ; and this taking place in each joint, 

 the equilibrium of both is restored, because, moreover, the segments 

 remote from the new septum, in both cases, seem to remain un- 

 altered. This is as precisely similar to the mode of increase which is 

 usually seen, here and there, in a few of the joints in the filaments of 

 Spharozosma vertebratum, as is compatible with the great difference of 



