30 THE ELEMENTARY STRUCTURE OF PLANTS. 



whatever manner, has the power of propagating itself by division 

 into parts, each of which forms a new cell. The modes by which 

 cells are thus multiplied, diverse as they appear to be in the vari- 

 ous processes of vegetable growth, are evidently reducible to two ; 

 and even these, if they are now rightly understood, are only two 

 modifications of one and the same process of division, or meris- 

 matic multiplication. Taking the most distinct cases for examples, 

 we may say that, in the first mode, 



31. The cell is propagated by the division of its living contents 

 into two, four, or sometimes a greater number of free new cells ; 

 the wall of the original cell perishing or losing its vitality in the 

 process. This can occur only in cells whose walls have not been 

 thickened by internal deposition (39), and while yet lined with the 

 vitally active layer of protoplasm * (26, 27). This mucilaginous 

 lining becomes constricted or infolded around the middle, and the 

 fold extends inward until it is divided, with the whole contents, into 

 two parts (Fig. 64) ; at the same time, or immediately following 

 the division, a wall of cellulose is deposited around each portion. 

 The two new cells thus produced may at once divide again in the 

 same way, giving rise to four cells in a parent cell (as in Fig. 65) ; 

 or the division may be again and again repeated. The delicate 

 wall of the parent cell is either absorbed or obliterated as the 

 new ones it incloses enlarge, or it remains, for a while at least, 

 although no longer in a living state. By this method the cells 

 of pollen formed in the anther of all Flowering plants (110), and 



* This layer, according to Mohl, is a delicate and soft membrane of proto- 

 plasm (called by him the primordial utricle), formed earlier than the cellu- 

 lose cell-wall which is soon deposited around it. Schleiden has not been able 

 to satisfy himself that this matter is organized into a membrane, or that it pre- 

 cedes the proper wall of cellulose. By terming it, without reference to these 

 points, the mucilaginous lining, or vitally active layer of protoplasm, inter- 

 posed between the proper wall of the cell and its contents (nucleus, gelatinous 

 mass, endochrome, or whatever they may be called), their views are brought 

 into agreement with each other. Those of Mr. Thwaites do not essentially 

 differ, except in his pushing too far, as I should suppose, the inference, " that 

 cell-membrane is quite a subordinate part of living structure ; that its func- 

 tions are of a purely physical character; that its principal office is to protect, 

 locate, or isolate the matter it contains, and that any vitality, it possesses is 

 derived from the presence within it of its endochrome." Ann. fy Mag. Nat. 

 Hist., Vol. 16. The movement of the cilia on the surface of the cell-wall, 

 seen in so many spores, surely shows that this possesses for a time a vitality 

 of its own. 



