Development, and Structure of the Vegetable Cell. 429 



lamina of deposit (thickening layer), advances into the cavity of 

 the cell and unites at its centre. 



Between the secondary cell and the chlorophyll-layer a colour- 

 less mucous fluid collects during this process. This fluid is held 

 by Dippel (Vegetabilische Zellenbildung, 1858, p. 32) to be 

 plasma, which originates in this situation on account of the 

 active generation of tissue going on in it, and which forces the 

 green contents towards the interior as a consequence of its ac- 

 cumulation. Not unfrequently, when this plasma is first seen, 

 there is no perceptible trace of the septum, which subsequently 

 makes its appearance, commencing from the periphery in such a 

 manner that, as it has been described, it seems to be a product 

 of crystallization. 13ut that the delicate walls of the daughter 

 cells may be here actually present, though hidden in the mucoid 

 plasma, the previously recorded action of solution of tannin upon 

 the mucous contents of the cells of Mougeotia afford sufficient 

 evidence. 



Concurrently with the first indications of the presence of a 

 septum (at times, indeed, previously), the mass of chlorophyll re- 

 cedes towards the middle line of the cell : it seems as if the green 

 contents were separated all round by the colourless plasma and 

 on each side by the narrow lamina from the neighbouring walls 

 of the joint-cell, and pressed towards the middle line of the 

 latter, — not divided, but for the present constricted (fig. 40). 



In this case, however, the constriction or compression of the 

 chlorophyll-mass is only apparent ; for a closer examination of 

 this condition shows that the vesicles which form the chlorophyll- 

 layer do not actually withdraw, but have become colourless in 

 the spot occupied by them, whilst the adjoining septum makes 

 its appearance more distinctly. Sometimes a green vesicle re- 

 mains somewhat longer in the colourless mass, and may be 

 distinctly seen to grow progressively more and more colourless. 



The mode of deposition of the elongated starch- and chloro- 

 phyll-vesicles in the so-called constriction (fig. 40 a) often gives 

 rise to an appearance as if these corpuscles were deposited on 

 the wall of a tube, continually constricted at that part, and 

 particularly so in the last stages of thickening of the septum, 

 where they stand at right angles to it ; nevertheless it may be 

 observed, especially in wide cells, that here also at their centre 

 there exists a lighter cavity elongated in the direction of the 

 septum which is undergoing alteration, — a cylindrical body, a 

 cell which exerts its influence upon the deposition of the chloro- 

 phyll during absorption. 



However, this arrangement of the secretion-vesicles is liable 

 to very great variation : the condition represented in fig. 40 cr, 

 and previously described, is very often met with ; frequently, in 



