26,] 



CHAPTER 



-THE CELL. 



123 



but here the cells are at first free zoogonidia without any cell- wall, which 

 subsequently aggregate together so as to form the body of the plant, and 

 secrete cell-walls (see pp. 15, 92). 



A case which closely connects the foregoing with the typical 

 form of cell-division is that offered by the development of the 

 pollen-grains of Dicotyledons and Conifers, and of the spores of 

 Pteridophyta and Bryophyta in general. Here (Fig. 78) the nuclear 

 divisions are limited to the first generation; the nucleus of the 

 mother-cell divides into two, and each of those again into two, 

 so that four nuclei are produced. The protoplasm around these 

 is marked out into corresponding areas by lines, along which 

 cell-walls are formed, which meet in the middle, and abut extern- 

 ally on the wall of the mother- cell ; in this way four special 

 mother-cells are formed, in each of which a spore is developed 

 (see p. 125). 



The marking-out of the ~ . 



protoplasm into units in 

 the process of free cell- 

 formation is effected by the 

 kinoplasm. As the nuclei 

 lie free in the 'protoplasm, 

 connecting threads of kino- 

 plasm are formed between 



them or rather between FIG. 81. Helleborusfcetidus (after Strasburger : 



the Centrospheres adjacent X * >; Qaadripartitionof mother-cell of pollen; 



J in E the connecting threads and cell-plates are 



to the nuclei each of shown; in A the walls have been formed. Only 



Which becomes somewhat three are visible, the fourth not being in focus; 



. . the mode of development is tetrahedral. 



thickened in the middle 



line, the thickenings coming into lateral contact thus forming a 

 cell-plate (Fig. 81 B), which marks the limits of the future cells. 

 When free cells are to be produced, the cell-plates mark the 

 planes of separation ; when a tissue is to be produced, the cell- 

 plates both mark the position in which, and supply the material 

 out of which, the septa are formed. 



(6) Cell-division may give rise to free cells, though more com- 

 monly it gives rise to a tissue ; in either case, it consists essenti- 

 ally in the bipartition of the cytoplasm, once or repeatedly ; septa 

 mayor may not be formed, and their formation follows on the 

 division of the cytoplasm. 



The development of free primordial cells by cell-division is 

 confined to the reproductive cells of certain of the lower plants; 



