ZOOLOGICAL AND BOTANICAL ASSOCIATION. 109 



"molecular" movement displayed by the pressed-out granules is only 

 just a continuation of the same movement to be seen within the joint, 

 and that the additional granules moving about when forcibly expelled 

 would have also moved in the same manner inside, could they have been 

 disassociated from the mass of endochrome in situ. "When by violence 

 one of these joints is broken, the separation takes place by a suture at 

 the centre over the pale space, and by a smooth line of division. 



Of such joints as I have thus endeavoured to communicate an idea 

 are the filaments composed of which the plant is constituted, and which 

 for some time, attached to aquatic plants, maintain their connexion as a 

 filament (Fig. 3). There does not appear to be any dilated or scutate base 

 by which the first joint is attached to the foreign objects, but on which 

 the filaments seem to stand directly, and with which the truncate apex of 

 the first joint appears merely to be in apposition. The joints frequently 

 separate, however, and can be met with in the water singly. Indeed, 

 my first acquaintance with this organism was made with a single detached 

 cell. They increase in length, too, sometimes after separation. 



The division of the joints into two segments by a suture, although 

 there is no transverse stria or other perceptible indication in the unbroken 

 cell- wall of its existence, coupled with the interruption of the endo- 

 chrome into two distinct portions, as well as with the active granules, 

 seem at once to decide that this organism belongs to the Desmidiaceae. 



The particular mode of increase by cell-division which prevails 

 amongst the other Desmids (by the formation of a septum and by the 

 interposition of new growth between the old, unaltered segments 

 pushing them asunder, and afterwards becoming divided at the middle, 

 each half of the newly grown portion ultimately attaining the size and 

 form of the old segments, and usually becoming cut off, and separation, 

 taking place), I need hardly repeat, is abundantly evident and unmis- 

 takeable amongst the more elaborately formed bipartite genera; whilst 

 even in many of those of more simple, even cylindrical outline, there is 

 often a difference of colour in the cell- wall marking the newly grown 

 portion. For example, Penium cylindrus, in which the cell- wall of the 

 older segments being reddish, the newly grown portion is well marked 

 by its absence of colour. So also in Closterium and others. But in the 

 form under consideration, the sides being parallel and straight, and the 

 cell- wall destitute of colour, I do not see any external means of proving 

 that the new growth of each joint takes place only between the older 



