Development, and Structure of the Vegetable Cell. 419 
velopment in uninjured plants; whilst their extreme fragility 
when separated from their organic connexions renders it impos- 
sible to ascertain, with regard to the tissue-cells, whether the 
binary combination obtains with them as the external pheno- 
mena of development render probable. 
It would appear, as before observed, that the cells filled with 
colourless fluid occupy the median space of the joint-cell, and 
that, towards the outside, these pass into the series containing 
chlorophyll, probably becoming converted into them. This 
mode of development would not be in favour of the prevalence 
of the binary type, if we did not know that most of these cells 
only exercise the function of secretion-cells, and that they be- 
come overgrown (that is to say, pushed aside and dissolved) by 
one or more cells becoming developed into tissue-cells : first 
their proper membrane, then the nitrogenous vesicle enveloped 
by them, and finally the secretion-vesicles, rich in carbonaceous 
compounds, are absorbed. 
In a thick-walled form of Cladophora, I repeatedly observed 
(Vegetationsorgane der Palmen, 1847, p. 30) that, in somewhat 
diseased cells, a number of these endogenous thin-walled cells 
containing chlorophyll acquired walls as thick as those of the 
joint-cell itself; and in Cladophora glomerata small interposed 
joints are not unfrequently met with (as shown im fig. 30 a), 
which I look upon as separated secretion-cells which have ex- 
ceptionally acquired thick walls and passed into the series of 
tissue-cells, instead of having been absorbed by the vegetation- 
cells in course of normal development. 
In diseased and withering joint-cells of Confervacez we fre- 
quently meet with individual examples of these endogenous 
cells, which contain only starch or are completely empty. 
§ V. 
Mode of development of the radical extremity of the cell of Cladophora.— 
Formation of folds in the coats of the adjoming withered cells.—Joint- 
cells of the root-cell.—Independent growth of their cell-membrane.— 
Folds in the assimilating membrane of the joint-cell, and their causes. 
During the previously described downward growth of the 
joint-cell of Cladophora, not only its proper membrane, so far as 
this forms a septum, but also the immediately contiguous wall 
of the neighbouring cell, extends itself in the direction of the 
cut surface. The latter is seen thrust backward within its own 
cavity as the former is extended into this (PI. VI. figs. 31 & 33). 
Very soon, however, this backward-pushed wall of the cut cell, 
which is at first somewhat swollen and thickened, as in the un- 
injured condition, becomes lost to view at the extremity of the 
advancing growth from the neighbouring cell (figs. 35, 37, 38, 
