ZOOLOGICAL AND BOTANICAL ASSOCIATION. 99 



complete, when ultimate somewhat sudden separation is effected by a 

 from-side-to-side sawing motion of the segments, which is highly cu- 

 rious to observe ; the blunt convex new end of each now separated 

 individual cell afterwards grows out, till the symmetrical more or less 

 attenuated arcuate form of the frond is restored. Although I have 

 not witnessed it in Peninm digitus, I am greatly inclined to think 

 that the mode described in Closterium holds good in this species. In 

 Penium BHbissonii, although it doubtless follows the same rule as the 

 other Desmids, the mode of increase can hardly be distinguished, and 

 Borne botanists have included that species amongst the Palmellaceous 

 Alga? ; but its central pale space and elongate form easily distinguish 

 it In such bipartite forms as Micrasterias, Euastrum, Cosmarium, Stau- 

 rastrum, &c, to produce two exactly similar fronds from one, it is 

 obvious that two new segments must be formed ; but in these the growth 

 of the new segments takes place, always to some, and often to a consi- 

 derable, extent before their separation. The constricted portion of the 

 old frond expands by the formation of a connecting tube, " of which the 

 interposed new coat is the direct continuation of that which lines the 

 internal surface of the cracked halves of the old shell" (Hofmeister),* 

 and which is soon dilated into two globular or roundish enlargements. 

 These are the rudiments of the two new segments, which soon increase 

 in size, and in doing so push asunder the two older segments, acquire 

 colour, and ultimately assume a like appearance, with all the characte- 

 ristic lobes, sinuosities, or processes of the species, similar to those pos- 

 sessed by the older segments. Plate XI., Pig. 12, happens to afford an 

 example of this. I have seen this process of division in specimens of 

 Micrasterias rotata (obtained with the division just commenced), ac- 

 complished in between twenty-four and thirty-six hours, at the end of 

 which time separation had taken place, the new segments being of full 

 form and dimensions. In the free genera they are at length disconnected, 

 each new segment bearing with it the old one, thus producing two dis- 

 tinct fronds, and when this transverse division is incomplete a filament 

 is formed. This mode of division docs not appear to differ essentially 

 from that which holds good in other Alga;, and seems to be only a ne- 

 cessary modification resulting from the exigencies produced by the con- 



• Translated in " Annals of Nat. Hist," third seriea, vol.i., No. 1, January, 1-858. 



