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 #) 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 different 
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 different 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 
Confervaceze (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 swelling up of — 
the young, and an increase of the ligneous condition of the — 
cell-membranes. 
