PART II. ANATOMY AND HISTOLOGY. 



25 



arrangement. The following are the principal modes of cell-forma- 

 tion : 



I. Cell-division. The protoplasm of the mother-cell divides 

 into two or more parts, each of which constitutes a new cell. The 

 division of the cytoplasm is usually preceded by that of the 

 nucleus. 



In the simplest case of cell-division the nucleus divides into two, 

 the cytoplasm does the same, and a cellulose-wall or septum is 

 formed in the plane of division. In other cases the secondary 

 nuclei and their investing cytoplasm may again divide before any 

 cell-wall is formed. Finally, the formation of a cell-wall may be 

 postponed until the division of the 

 nuclei and of the cytoplasm has 

 been repeated an indefinite number 

 of times. The varieties of cell- 

 division which thus arise may be 

 arranged as follows : 



1. In growing vegetative organs, 

 a division of the cell takes place, 

 such that the whole of its proto- 

 plasm, without any rounding-off 

 or contraction, is divided into two 

 parts : the new wall is formed be- 

 tween the two masses of proto- 

 plasm only along the plane of 

 division (Fig. 60). The wall is 

 sometimes formed simultaneously 

 at all points of the plane of divi- 

 sion, as in the development of 

 stomata, and sometimes, as in cer- 

 tain Algae, e.g. Spirogyra, it grows 

 as a ring from without inwards. 



2. The formation of the cells 

 which subserve reproduction (see 15) is always accompanied by 

 a rounding-off of the protoplasm. These cells are generally set 

 free, and may or may not have a wall when set free : the wall, 

 when present, is always formed over the whole surface of the 

 young cell. 



a. The whole cytoplasm of the mother-cell may become aggre- 

 gated around four newly-formed nuclei ; this process occurs 

 principally in the formation of the pollen of phanerogamous 



PIG. 00. Cell-division in the cortex of 

 the growing stem of Vicia Faba ( x 300). 

 At a the division hag just taken place, 

 the nucleus still adheres to the new 

 wall ; at b it has retreated to the older 

 wall : fc the nucleus. 



