FORMATION AND GROWTH OV CKLLS. 



29 



© 



tion, namely, the successive division of cells into two. This takes 



place only when they are young and active, and mostly before they 

 are full-grown. It is effected by the formation of a 



partition across the cavity of the cell, dividing it into ( &J 



two (Fig. 10-14). In this way, a single cell gives J0 

 rise to a row of connected cells, when the division 



takes place in one direction only ; or to a plane or | <&) 



solid mass of such cells, when it takes place in two K®J 



or more directions, thus producing a tissue. " 



34. In this multiplication of cells by division, as in f^% 

 the original formation of a cell, the contents and the f $p% 

 ■protoplasmic lining play the most im- i 2 

 portant part. The nucleus, when pres- 

 ent, as it commonly is, first divides /^z\ 



into two (Fig. 11) ; then the lining mem- r '" \ 



. . . . fevSJa 



brane, or primordial utricle, is gradu- f ^1 



ally constricted or infolded at the line 13 



of division, which, soon meeting in the 

 centre, separates the whole contents ( j 



into two parts by a delicate partition ; (jljk 



upon this a layer of cellulose is de- LJ^\ 

 posited as a permanent wall, which /-/H"] 

 QO O© completes the transformation of one '{{" 



€$© g ©@ cell into two (Fig. 21, 22). 



3.5. Cells multiplying in this way, and remaining 

 united, build up a row or a surface of cells, or a solid tissue, ac- 

 cording to the mode of division. But in many of the simplest 

 plants, growing in water, the cells separate as they form, and be- 

 come independent. A microscopic plant very common in shallow 

 pools in early spring, forming slimy green masses, well illustrates 

 this, as shown in Figures 15-19. At each step of this multipli- 

 cation new cell-membranes are formed, and the old one, for instance, 

 the wall of Fig. 15 and the common envelope of the two in, Fig. 17, 



O 



US? 



mm 



18 

 QOOO 



OQO© 



FIG. 10 A young cell, — the first cell of an embryo, — with its nucleus in the centre. 

 II The same, with its nucleus divided into two, and a cross-partition beginning to form. 

 12. The partition completed, so converting the first cell into two 13 The lower one again 

 divided into two, making three cells in a row. 14 The fourth cell converted into four by a 

 division in two directions, forming seven cells in all. 



FIG. 15. A single <ell, or plant of a kind of Palmella, magnified 1G. The same dividing, 

 and, 17, completely separated into two. 18. Each of these dividing in the opposite direc- 

 tion, four cells are produced 19. Each of these again dividing into four, they produce a 

 cluster of sixteen cells. 



3* 



