430 Prof. H. Karsten on the Formation, 



this stage, no chlorophyll-vesicles are to be seen outside the 

 endogenous cell undergoing enlargement ; but almost as often 

 secretion-vesicles are distinctly visible in abundance externally to 

 the central cell which has been arrested in its development, and 

 not uncommonly so dispersed and irregularly arranged and 

 mingled among the colourless and distended vesicles, that the 

 impression, at first sight, is that they are not enclosed within a 

 special membrane. 



It would appear that the secretory material of the chlorophyll- 

 layer in process of absorption serves, on the one hand, for the 

 thickening, and on the other for the enlargement, of the neigh- 

 bouring cell-walls (p. 427). 



The nature of the cell which probably exists in the interior, 

 and becomes enlarged simultaneously with the thickening of the 

 septum, has not been ascertained. 



Conditions like that shown in fig. 46 q render it probable that 

 the next younger pair of joint-cells is already developed and 

 undergoes division in the cavity of the joint-cell that is just 

 making its appearance, from the walls of which they are sepa- 

 rated only by a small quantity of chlorophyll and starch-vesicles, 

 which are subsequently dissolved gradually. 



In the unthickened septum itself, even when it is visible in 

 this state, as in the example represented in fig. 46 q, it is very 

 seldom that, with the reagents hitherto employed, the two cell- 

 membranes of which it was composed (fig. 46 x) can be demon- 

 strated. Still less can it be decided by observation whether the 

 membrane of these cells in this stage of development belongs to 

 two cells nested one within the other. 



Those septa, on the other hand, in which thickening has 

 commenced are capable, in proportion to the extent to which 

 this process has advanced, of being resolved into the diflferent 

 cell-membranes of which they are composed by the action both 

 of endosmotic fluids and of solvents of the recently thickened 

 membrane. 



Transitory endosmotic phenomena are produced in many Con- 

 ferva-cells even by the action of difi'erent kinds of water met with 

 naturally, such as spring-, river-, or rain-water, and indeed some- 

 times by pure distilled water in the case of plants previously 

 grown in other water. Water containing carbonic acid, and 

 other mineral waters, are more energetic. 



The action of concentrated carbonic-acid water upon many 

 Confervaceae (for instance, on Spirogyra) is of a complex nature ; 

 for, besides diosmosis, a change in the constitution of the cell- 

 membrane takes place : in this plant it causes a sweUing up of 

 the young, and an increase of the ligneous condition of the older, 

 cell-membranes. 



