CELL-FORMATION. 29 



by secondary deposit, which, leaves minute canals 

 radiating from a small cavity in the centre to the cir- 

 cumference, as seen in the transverse section of a 

 plum-stone (PL I. fig. 56) ; here the canals appear as 

 dark lines. In others, again, the secondary deposit 

 forms several distinct layers, leaving channels very 

 similar to those of the last ; an example is met with 

 in the gritty tissue of the pulp of a pear. 



The obvious use of the pits and channels in the 

 above tissues is to preserve the permeability of the 

 walls of the elements, which would be destroyed if 

 the walls were equally thickened all over. 



Cell-formation. New cells are formed by the 

 division of old or parent cells. The actual process 

 of division is difficult to observe, as it requires pro- 

 longed observation ; but cells are often met with in 

 all stages of division, of which some instances will 

 be pointed out hereafter. The cell-division takes 

 place in two ways, either according to the endo- 

 genous (evBov, within, ryevvaw, to produce), or the 

 exogenous (efo>, outside, yevvda)) method. The 

 manner in which the division takes place in the 

 former is this -. At first a slight indentation or con- 

 striction of the protoplasm occurs at the line of 

 division ; this deepens until the protoplasm is com- 

 pletely divided. The freshly divided surfaces then 

 become coated with a new portion of cell-wall, so as 

 to make two or more new cells, which either remain 

 in contact or separate from each other. In some 

 cases, the divided portions of protoplasm become 

 coated all over with new cell- walls. 



In the exogenous process, a portion of the proto- 

 plasm protrudes from the surface of the cell, carry- 

 ing the cell-wall before it, so as to form a little bud- 

 like body ; this is next cut off at its point of junction 

 with the parent-cell, and coated, as in the first case, 

 with a new cell-wall, so as to form a new cell. 



Preparation. In examining the vegetable ele- 



D3 



