126 Prof. H. Karsten on the Formation, 
habitat when these have been long upon the slide, and brought 
into contact with different kinds of water. 
A less degree of this depression appears, however, to occur even 
in plants growing in a natural state; and this is of interest here, 
because it induces the fold-formation which was formerly re- 
garded as the cause of cell-multiplication, when the depression 
occurs at the precise period at which the two more or less sphe- 
rical daughter-cells, touching the large secretion-cells with their 
peripheral surface, and hampered by these in their rapid growth, 
bring the previously free parts of their central surface into 
complete contact, and thus enclose this impressed membrane 
between them. The depth to which the folds of the secondary 
cell is enclosed in the septum in course of formation depends 
upon the greater or less extent of contact of the central surfaces 
of the daughter cells at the time of this process. 
By acurvature or depression of one or the other of the chloro- 
phyll-sacs, the side of the joint-cell is already perceptible, on 
which the liquefaction of the small secretion-cells situated about 
the nucleus takes place more rapidly than the enlargement of 
the neighbouring young joint-cells, which usually occurs simul- 
taneously with it. 
Newly formed septa not unfrequently occur, which on one 
side do not enclose the smallest trace of a fold of the mother cell 
between them, but show the well-preserved chlorophyll-saes 
distinctly at their circumference (as represented in fig. 58 a, 
in S. nitida), whilst on the other side of the periphery of the 
mother cell a fold of this kind is engaged, more or less deeply, 
between the two plates of the septum. 
These enclosed folds of the membrane of the secondary cell, 
which are no doubt subsequently absorbed, are at first thickened, 
reminding us of the folds of Cladophora, described at pp. 420 
and 425 (vol. xii.), as well as its peripheral portion, whilst the 
chlorophyli-saes appressed to them are immediately absorbed. 
The thickening of the membranes of the daughter cells, which 
takes place immediately, and their amalgamation with those of 
the mother cell commence in the portions forming the septum 
even before the completion of the absorption of the chlorophyll- 
sacs which surround them. 
When the absorption of these secretion-materials is much de- 
layed, the new, half-thickened septum may be seen, in certain 
positions, already united on each side to the membrane of the 
mother cell, after the joint-cell has been treated with endosmotic 
fluids, whilst it is still free beneath the chlorophyll-saes. Figs. 
74 and 75 show this in one sac. 
But phenomena do occur which seem to show that in the 
Spirogyre the development into new joint-cells does not always 
