Development, and Structure of the Vegetable Cell, 425 



velopment, and left only their final and most highly developed 

 products, in the shape of small vesicles freely moving within the 

 juices of the cell, and destined for the nutrition of new joint- 

 cells. 



If such joint-cells, furnished with annular folds (fig. 40 b), 

 be examined for some time, progressive, though very gradual, 

 changes are observed to take place in the position of the secre- 

 tion-vesicles. Here and there in the vicinity of the annular fold 

 the sharp outline of a cell, apparently filled with colourless fluid, 

 will make its appearance among the mass of secretion-material 

 which occupies the joint-cell. This enlarges, and progressively 

 advances towards the middle line of the fold. A similar process 

 may be frequently witnessed going on simultaneously on the 

 other side of the fold. 



Lastly, the entire and somewhat rounded end of the cell may 

 be observed free from chlorophyll close to the fold ; the mem- 

 brane, as compai-ed with that of other young cells, is thick, but 

 of a gelatinous aspect, whilst the contiguous thick membrane of 

 the fold resembles that of woody tissue. 



The two endogenous cells at last approach so closely together 

 in the middle line of the fold, that, by their mutual pressure, 

 they form a septum which, to those who have not followed its 

 development, would appear as if it had been forcibly separated 

 at its periphery by the annular fold. 



In other cases, a cell arises on one side only of the fold, and 

 continues to elongate itself beyond this, until it reaches the next 

 septum. 



The membrane of the fold appears, during the subsequent 

 existence of the endogenous cells, to become absorbed ; for we 

 find that where these latter have thick walls, the former is thinner 

 than when those cells were newly formed. 



This mode of formation of septa by tolerably thick-walled 

 endogenous cells, readily observable during their slow growth, 

 is, however, of comparatively rare occurrence in Cladophora 

 glomerata. Usually, in this plant, the commencement of thick- 

 ening first calls attention to a pre-existent and fully developed 

 although very delicate septum, this having been previously con- 

 cealed from observation by the great quantity of secretion- 

 materials with which it is covered. 



In my account of the structure of Vaucheria, I showed that 

 this so-called unicellular plant consisted of a composite system 

 of endogenous cells, the innermost of which develope themselves 

 and enclose organized secretory materials simultaneously with 

 the absorption of the mother cells with their contents, but that 

 the membranes of all these joint-cells do not become thickened, 

 and therefore may easily be overlooked. 



Ann. ^ Mag. N, Hist. Ser. 3. Vol. xiii. 28 



