424, Prof, H. Karsten on the Formation, 
upon the neighbouring cells, which involves some impediment 
to their extension. 
In the normal production of septa (fig. 40 a) no such tension 
exists in the subdividing cell. 
Mohl, indeed, observed these folds, which he represented as 
originating in a disturbance of the process of constriction of the 
primordial utricle during continuous formation of membrane. 
Mohl saw these structures in specimens of Cladophora glomerata 
that had been kept for some time in fresh water. 
I likewise found these folds frequently in cultivated specimens 
of this plant, but have no explanation to offer respecting the 
special cause of their production. In a specimen of C. glomerata 
which has been growing for a year in a very small vessel filled 
with distilled water, both these folds and septa of the normal 
form occur; and indeed the latter are the more frequent. In 
various other specimens, however, preserved also for a long time, 
though not so long as the former, in river-water, in plants co- 
vered with Desmidiez, the normal septal development rarely 
occurs; still the multiplication of joint-cells has proceeded in 
these plants, though their differences in length are very marked. 
The production of folds has here (fig. 40) occurred very — 
largely, and in all forms ; so that the idea suggested is that these — 
inwardly growing folds are the commencement of septa. Still 
this is not the fact; for, as before stated, the folds, when once 
formed, undergo no ulterior change. 
§ VI. 
Formation of septa in the joint-cells of Cladophora by means of endogenous 
cells. The cell-membrane is rendered thicker in the vicinity of the an- 
nular folds by superposition.—Thickening of the membrane, in the ab- — 
sence of annular folds, by the superposition of extremely thin-walled 
cells.—Means of separating the superimposed cells of the thickened 
cell-wall by diosmotic fluids.—Action of these reagents upon new joint- — 
cells. 
In many stages of development of these mfolded Cladophora- ; 
cells, we may convince ourselves, by slight pressure exercised — 
upon a joint-cell engaged in the act of folding (as, for instance, © 
during the gradual drying-up of the surrounding water, or — 
during the sudden accession of fresh water in place of that which — 
has been withdrawn), that its contents on either side of the fold 
are in direct communication, and traverse freely from one part — 
to the other, which is not the case when a true septum is pro- — 
duced. Q 
This phenomenon shows that no actual, complete septum, — 
oe eS eS ee ee 
however delicate, is ever established in connexion with this fold; _ 
and it also indicates that the secretion-cells, which usually occupy 
the joint-cells, had here probably terminated their cycle of de- — 
