Development, and Structure of the Vegetable Cell. 278 
way the true nature of the apparently simple lamelle 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 affords 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 adjoming 
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- 
theléss, 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 
