424 Pi'of. 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 Desmidiese, 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 b) 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 infolded 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 has been withdrawn), that its contents on either side of the fold 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. 



This phenomenon shows that no actual, complete septum, 

 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- 



