DUBLIN UNIVERSITY ZOOLOGICAL AND BOTANICAL ASSOCIATION. 239 
sessed by tbe older segments. Plate XXI., Fig. 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 does not appear to differ essentially 
from that which holds good in other Algae, and seems to be only a ne¬ 
cessary modification resulting from the exigencies produced by the con¬ 
stricted form of the cells; while it does not appear improbable that other 
Algae not Desmidian may have a like mode of cell-increase. 
The mode of true reproduction in the Desmidiaceae is by conjugation, 
very like the same process in the Zygnemaceous group of Confer voids. 
This process consists essentially in the pouring out and amalgamating 
of the contents of two distinct cells, resulting in a definitely formed 
spore or sporangium; and in the different genera of Desmidiaceae pre¬ 
sents some slight modifications. In the filamentous forms, such as Hy~ 
alotheca, Didymoprium, &c., conjugation does not take place till the 
constituent joints of the filament become disunited, apparently any two 
of which meeting in the water, couple and blend together their contents, 
the old cell-wall of each pair of conjugated joints remaining, as it were, 
fused together, and surrounding the sporangium. In Closterium, Penium, 
Cosmarium, Staurastrum, &e., the free cells conjugate in pairs. In them 
the process differs somewhat from that which takes place in the Zygne- 
maceae. In that group, the cells of two filaments lying side-by-side, 
bud-out, as it were, pouch-like protrusions towards each other, which, 
meeting half-way, become inosculated, forming ladder-like transverse 
tubes between the connected filaments; the contents of each conjugated 
cell of one side either pass over by the transverse tube into the cavity 
of the opposite cell, or the contents of each meet half-way in the centre 
of the connecting tube, and in either case become ultimately massed to¬ 
gether into a rounded compact spore. In the most of the Desmidiaceae 
there is no transverse tube formed; but the fronds about to conjugate 
approach and dehisce by the transverse suture, and each pours out its 
contents in one mass into the water, which, gradually becoming conso¬ 
lidated with the contents of the other conj ligating individual, assumes a 
membrane, and becomes a perfect sporangium, surrounded by gelatine, 
and destined for reproduction. The sporangium, which is usually sphe¬ 
rical (sometimes quadrate) either remains smooth or ultimately by de¬ 
grees produces, in every direction from its surface, variously formed 
spines or forked processes, and bears, in this state, no resemblance to 
the parent species. Plate XXI., Fig. 13, is an example. Any observations 
that have been made in regard to the subsequent development of the 
sporangium go to prove that its contents ultimately give birth, by binary 
segmentation into a definite number of portions, to forms which at first 
are very minute, but when set free by the solution of the wall of the spo- 
