132 Prof. H. Karsten on the Vegetable Cell. 
The other extreme of this fold-formation is represented in 
fig. 82, from a plant which had lain for some time in water con- 
taming carbonic acid; hence the thickening of the primary 
membrane of its joint-cells. The endogenous cells were here 
in complete apposition, forming a perfect septum. Squeezed in 
between them 1s a fold of the wall of the mother cell, which in 
this case did not grow regularly from the whole periphery into 
the cavity of the cell between the daughter cells, but only pro- 
jected far into it in a part of its extent, whilst another portion 
of the circumference remained unaltered. 
At this part, which was not affected by the folding, the 
chlorophyll-saes, which were elongated simultaneously with the 
formation of the fold, are seen bent and crooked, as if they had 
been acted upon by a mechanical constriction. Others are 
separated into fragments, as in the normal formation of a 
septum. <A joint-cell of this kind, seen from the side on which 
the fold is perfect, may readily be regarded as completely 
divided ; and this illusion may be incre eased by the position of 
the new cell-nuclei, when, as in the case figured, they are large 
and filled with oranular mucilaginous matter, situated in each of 
the new cells, not in the middle, as in the examples described at 
p- 125, but close to the new septum, and when seen in a parti- 
cular direction appear like a cell-nucleus cut through by the 
fold. 
It is rarely that, as in Cladophora, these folds appear to pro- 
ject freely to a greater or less distance into the cell-cavity ; 
nevertheless T have repeatedly observed this on apparently per- 
fectly healthy plants, especially of S. orthospira. 
In diseased and dying plants, the joint-cells of which are 
often disproportionately ‘short, the folds of the membrane are 
usually more developed, so that it would almost appear that 
the development of the two parts stands im a certain mutual 
relation. 
These folds may be most readily seen when Spirogyre are 
allowed to lie for a long time in water containing carbonie acid 
until all the endogenous cells of the joint-cells are destroyed. 
By the action of dilute endosmotic solutions, the membrane of 
the secondary cell then readily retracts itself, together with its 
still adherent chlorophyll-sacs, from the folded primary cell- 
membrane, producing appearances which would certainly appear 
well fitted to confirm the constriction-theory, if we were not 
undeceived by developmental history and analogy. 
These folds, which occur in all degrees of breadth and differ- 
ence of form, are, however, not destined to effect a multiplica- 
tion of the joint- “cells by the growing together of their central 
margin, any more than those of Cladophora, many of which 
