Structure and Division of the Vegetable Cell. 213 



the latter, and that its ingrowth is probably fed by the 

 protoplasm mantling its edge. What one sees after in- 

 growth has proceeded to a considerable extent is a circular 

 shelf, as thin, or even thinner, at its point of junction with 

 the wall as throughout its free part. But this of itself is 

 not enough, for there are often figured in our botanical 

 manuals drawings of this, and allied mesocarpous algte, with 

 the protoplasm retracted, but still retained to the free edge 

 of an apparent ingrowth of the cell-wall. The reagents in 

 these cases which retract the protoplasm, such as alcohol, 

 do so comparatively gently, and as a consequence it 

 shrinks from the cell-wall generally, and masses round the 

 area of division, where it is attached to the ingrowing- 

 septum. It will thus be seen that no special strain acts 

 on the septum to rupture it. On the other hand, by such 

 reagents as chromic acid the cell contents are fixed as in 

 the living state. Preparations then, which have been thus 

 treated, on being teased and twisted about, invariably have 

 one or more cells with a forming septum and the proto- 

 plasm displaced. In such a case the latter unfailingly 

 carries the former with it. One from many such prepara- 

 tions in my possession is represented in Plate X. fig. 17. 

 Again, if an undisturbed cell which has been in chromic 

 acid have its walls subsequently expanded by endosmotic 

 action, the protoplasm and chlorophyll bands will likewise 

 swell out ; but if a forming septum be present, it— instead 

 of swelling out — detaches itself from the wall, and forms a 

 clear annular constriction in the middle of the swollen 

 cell (Plate X. fig. 16). 



Even while the protoplasm is rendered firm by the acid, 

 one would still expect, according to the generally-accepted 

 theory, that the septum would part from it rather than 

 from the cell-wall, but this is not so. In Spirogyra^ there- 

 fore, a familiar and demonstrable proof exists, of the forma- 

 tion of cellulose by protoplasm, and the fusing of this with 

 the pre-existing cellulose of the wall. 



Having now passed in review the phenomena attendant 

 on division, there are some broad questions which we might 

 with advantage linger on. That the nucleolus and — very 

 probably — the nucleolo-nucleus are invariable cell-factors 

 has already been postulated. AVhile of recent years the 



