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 difi'erent from them. 



The origin of these folds (fig. 40 h) 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 Spirogyra. 



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 Confervacese. 



The joint-cells engaged in this process of fold-construction 

 (fig. 40 b) always seem to exercise a certain degree of pressure 



