60 



BOTANY 



CELL-FORMATION 



New cells may arise by division, or by the union of two (occasion- 

 ally several) into a single cell. 



Fission. The commonest form of cell-multiplication is the divi- 

 sion of the cell into two, usually equal, parts. This mode of division, 

 or Fission (Fig. 42), is the only method by which new cells are formed 

 in the lowest organisms, such as Bacteria. In the Bacteria, where a 

 distinct nucleus cannot be certainly demonstrated, the cell-division 

 consists merely in the constriction of the protoplast, and its division 

 without the complicated changes in the nucleus which characterize 

 cell-division in the higher plants. Sometimes there is no evident con- 

 striction of the protoplast, but a division-wall cuts the cell into two 

 parts, which may remain connected, and by repeated divisions give rise 

 to a cell-row. In these lowest forms, all the cells are alike, and there 



FIG. 42. A, cell of a Bacterium, Chromatium Weissii, in process of division 

 (X 1000). B, a living cell of Cladophora glomerata, in process of division; the 

 division-wall is not complete. C, the same cell an hour later (X 200). D, cells of 

 Yeast, Saccharomyces cerevisise, multiplying by budding (X 700). 



is no distinction between vegetative and reproductive cells. In some- 

 what more specialized forms, certain cells may be somewhat changed, 

 and become modified into thick-walled resting spores, which are, 

 however, derived from ordinary vegetative cells. 



Where a definite nucleus is present in the cell, as occurs always in 

 the cells of the typical plants, the division of the protoplast is pre- 

 ceded by a division of the nucleus. The only exceptions to this are 

 multinucleate cells, or Coenocytes, in which nuclear division and cell- 

 division are quite independent. The formation of the division-wall 

 may begin as an equatorial ring of cellulose, which grows centripetally, 

 until it cuts the protoplast in two ; or there may be formed simulta- 

 neously in the protoplast an equatorial cell-plate, which extends com- 

 pletely across the cell. 



Karyokinesis 



The division of the protoplast is preceded by extensive changes in 

 the nucleus, which finally become divided into two daughter-nuclei. 

 These changes are known as Mitosis, or Karyokinesis. 



