no 



THE PLANT CELL 



Sometimes the layers are disposed uniformly all over the inside 

 as in A, Fig. 50, but more frequently the increase in thickness 

 goes on at some points more rapidly than at others. In some 

 cases small areas of the cell-wall are left unaltered ; these thin 

 places appear as bright spots termed /zVj- when a surface view of 



the cell is examined. In sitttpk 

 pits {B) the cavity left unthickened 

 is roughly cylindrical and viewed 

 end on appears as a circle or ellipse. 

 The cavity left unthickened in a 

 bordered pit is funnel-shaped, and 

 in surface view appears as two 

 concentric circles or ellipses (tT). 

 The pits of one cell-wall are gener- 

 ally exactly opposite the pits of an 

 adjoining cell-wall, and serve as a 

 means of communication between 

 the two cells. 



Thickening in the form of spiral 

 and annular or ring-like bands is also very common (Fig. 51). 



5. Cell-division : continuity of protoplasm. — With the extension 

 in length of the stem and root, and the production of new 

 organs at the growing-points of ordinary green plants, a great 

 increase in the number of cells takes place. This cell increase 

 is the result of division of previously existing cells, all of which 

 in any individual plant have originated from the division of a 

 single cell, namely, the fertilised egg-cell of the ovule. 



During the process of division of a cell at the growing-point 

 of a shoot or root, the nucleus first divides into two exactly 

 similar halves in a complicated manner which cannot be dis- 

 cussed here. The two halves or daughter-nuclei then recede from 

 each other a short distance in the dividing cell, and a new cell- 

 wall arises midway between them. The new cell-wall divides 

 the cytoplasm into two distinct parts, and is always placed at 



Fig. 5t. — Portions of vessels show- 

 ing (i) annular, (2) spiral thickening 

 of their walls. 



