Development, and Structure of the Vegetable Cell. 278 



way the true nature of the apparently simple lamellae which 

 form the septa is placed beyond doubt. 



The cork-cells which are found in the much-thickened fibrous 

 cells of the bark, furnished with deep porous canals (PI. V. 

 figs. 1 & 19), exhibit with particular clearness the true impres- 

 sion of the cell-cavity which enclosed them (fig. 20). The same 

 fact is also displayed in the cork-cells of the very thickened 

 parenchyma-cells and spiral vessels (figs. 2, 3, & 4). 



During the process of development of these endogenous cork- 

 cells, the substance of the walls of the parent tissue-cells becomes 

 entirely absorbed. And by means of this process of absorption 

 the various component tissues of the stalk are eventually re- 

 placed by a completely homogeneous layer of cork, in which the 

 outline of the original histological elements cannot be recog- 

 nized, as Mohl has shown in his researches on the normal cica- 

 trization of the stalk after the fall of the leaves and that of the 

 points of its terminal shoots. 



The dissolution of the walls of the tissue-cells filled with cork- 

 cells commences with the external membrane of the primary 

 cells, and terminates with the inmost layer of the secondary cells; 

 so that the pores of the very thick porous cells of the medullary 

 sheath acquire large dimensions shortly before their final dis- 

 appearance. 



In the case of spiral vessels filled with cork-cells, the spiral 

 fibre is not unfrequently left; it then coils round the serially 

 appressed cork-cells, and may be untwisted from them (fig. 3). 

 This condition afi'ords still more certain proof of the presence of 

 these endogenous cells within spiral vessels, and also of the fact 

 of their being free isolated cells which more or less completely 

 fill the cavity. 



Sometimes, however, specimens are met with which prove that 

 the filling of a spiral vessel with cells has proceeded from the 

 neighbouring tissue-cells, the endogenous cells produced in and 

 entirely filling which have extended from them into the adjoining 

 spiral vessel (fig. 4). Hence the supposition might be entertained 

 that the incompletely developed cork-cells found in porous vessels 

 and cells (figs. 2 & 5) have likewise not freely originated in them, 

 but have grown into them from neighbouring cells. Never- 

 theless, by causing the vessels set free by boiling with nitric acid 

 and chlorate of potash to rotate on their axes, we may positively 

 ascertain that their contents are really perfectly isolated free 

 cells. 



The cases in which the intrusion of a cork-cell into a spiral 

 vessel from an adjoining tissue-cell is observed are uncommon ; 

 and such are, without doubt, due to a coalescence of the woody 

 cell with the vessel before the development of the periderm, as 



