586 



PHYSIOLOGY. 



are most satisfactorily studied. Cell- division can only take place in a 

 cell which retains its protoplasm in an active state, as in the cells 

 constituting the meristem (p. 514). When the parent cell is about 

 to produce two (or four, rarely more) new cells, the protoplasm 

 separates from the cell-wall at the line bounding the plane of divi- 

 sion, and advances inwards in the form of a narrow fold, until the 

 portions of the fold coming from the different sides of the cell 

 coalesce, so that the protoplasm is resolved into two (or more) closed 

 utricles, together completely occupying the place of the original 

 utricle. "While the protoplasm is folding inward, it forms thicken- 



Fig. 593. 



Cell-division in Cladophora glomerata. A. Part of a filament in a natural condition : a, cell- 

 membrane ; c, primordial utricle or protoplasm; x, situation where division is about to 

 take place. B & C. Stages of the formation of a septum at x, the filament having been 

 treated with alcohol: a, wall of the parent cell; b, walls of the new cells; c, protoplasm. 

 D. Septum of old filament treated with dilute sulphuric acid, to swell up and. separate 

 the laminae of the cell-wall and contract the protoplasm : a, wall of parent cell ; b, 

 wall of daughter cells; c, protoplasm. Magn. 200 diam. 



ing cellulose laminae on the wall of the parent cell and on either side 

 of the septum separating the cavities of the two new cells, formed 

 by the infolded portions of the protoplasm (fig. 593). This takes 

 place not only in the vegetative cells but also in the pollen. At 

 other times the new dividing cell-wall is formed gradually, after 

 the sudden division of the protoplasm into two or more portions, 



