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 Palraen, 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 in 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 Confervacese 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 adjoining 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, 



