Development, and Structure of the Vegetable Cell. 423 
thickened, frequently in layers (fig. 45), just as the cuticle is 
nodosely thickened sometimes in the same plant at the apices of 
the branches, and, in phanerogamous plants, frequently above 
each epidermal cell, apparently independently of the contents of 
the latter. 
An independent growth similar to this of the cuticle, which 
grows out into knots, folds, and branches, is exhibited by the 
membranes of the joint-cells of Cladophora glomerata, which 
form inwardly annular folds, sometimes presenting a great re- 
semblance to those already described as produced in consequence 
of penetration (figs. 38, 39); nevertheless they are readily dis- 
tinguished, by the disposition of their parietal prolongations, 
from the latter, which are quite different from them. 
The origin of these folds (fig. 40 0) of the integument of the 
secondary cells was observed both simultaneously with the com- 
mencing thickening of the membranes of the endogenous joint- 
cells, which come into contact in the normal septum (fig. 40 a), 
and also in those cells not undergoing the act of multiplication. 
After the thickening and the chemical transformations asso- 
ciated with it have commenced in the cell-membrane, a progres- 
Sive increase in thickness, but no elongation, is to be seen in 
these folds. Not unfrequently they encroach so far into the 
cavity of the cell as almost to reach the median line and nearly 
to divide the constricted cell-contents ; nevertheless the still 
existing connexion between the two segments of the cell thus 
drawn apart continues unchanged. 
Contemporaneously with the thickening in the membranes, 
there appears to exist in these cells, as long as this formation of 
folds is taking place, a tendency to expansion, the volume of the 
mother cell and enveloping membrane remaining the same, by 
which means the expanding daughter cell is compelled to grow 
into the cell-cavity in the form of a fold: this also occurs in the 
septa of many Sprrogyre. 
The cause of the production of these folds of endogenous cells 
appears therefore different in nature from that which separates 
the daughter cells of the root-cell from one another (fig. 32). 
The mother cell of the root-cell developes its membrane 
(cuticle) (which, as it were, takes on the function of the spon- 
gioles of vascular plants) predominantly in proportion as the 
included daughter cells expand themselves; whilst in the pro- 
duction of folds in the cells of the stem and branches the mem- 
branes of the daughter cells especially become enlarged, and are 
in this way productive of the folds, different forms of which occur 
in the other Confervacee. 
The joint-cells engaged in this process of fold-construction 
(fig. 40 6) always seem to exercise a certain degree of pressure 
