18 PLANT LIFE. 



as fission. The material of the nucleus passes through a 

 complex series of changes and separates into two parts. In a 

 plane between these daughter-nuclei particles are deposited 

 to form a cell- wall (A, fig. 22A). The formation of the 

 partition-wall may occur simultaneously in all parts, or it may 

 be formed on one side first and the nuclei move across the 

 cell until it joins the lateral walls (fig. 22B). In this way an 

 isolated unicellular plant of Pleurococcus (A, fig. 18) may 

 divide into two cells so that it consists of two hemispherical 

 cells, each capable of independent growth (fig. 23, A). 

 After a time these cells may separate from each other by the 

 cracking of the original wall at the line of juncture with the 

 new partition and the cleaving of this partition parallel to its 

 surfaces into two layers, one of which covers a portion of 

 each of the thus disconnected cells (fig. 18, C). If this 

 process of division and separation goes on, the result will be 

 the production of a number of independent cells more or less 

 closely associated but not connected. 



24. Cell-rows, surfaces, and masses. — In many cases, 

 however, a second division occurs in one or both cells before 



Fig. 23. — Diagrams of cell division. A, division of a spherical cell into two hemi- 

 spherical cells, a, b, by the wall i. B, the same after further division in planes 2, 

 2, 3, parallel to 1. a has divided by wall 2 into a' and another cell which has again 

 divided by wall 3 into a", a", b has divided into b', b\ the inner of which has 

 elongated preparatory to a division into b", b", as by wall 3. C, fig. A after a 

 second division, by wall 2, at right angles to 1. 



separation; and sometimes even a third division takes place. 

 It is evident that the position of the later partitions deter- 

 mines the form of this temporary aggregate of cells, (a) If 

 each of the two divides in a plane parallel to the first parti- 



