FOEIMATION OF CELLS— TISSUES 



313 



original size after each division. In stems there is no segment 

 out off from the external face, but this takes place in roots, so that 

 the apical cell is never exposed in the latter case (fig. 769). The 

 segments when out off divide usually periclinally each into two ; 

 further divisions of each of the latter give rise to the mass of the 



Fig. 669. 



Fig. 670. 



Fig. 669. Growing point of stem of Eqai- 

 se/uni. a. Apical cell. &, c. Segments 



cut off from it. Fig. 670. Growing 



point of stem of Flodea. a. Apical 



meristem. After Kny. Fig. 671. 



Growing point of Pelvetia. a. Apical 

 cell. After Kny. 



Fig. 671. 



tissue. Such a growing point 

 may be conical or depressed, ac- 

 cording to the amount of growth 

 taking place in the inner or 

 outer segments and the cells 

 which arise from them (figs. 669 ; 

 and 671). This form of primary 

 meristem, in which an apical cell 

 is so prominent a feature,is chiefly 

 characteristic of Cryptogams. 



An intermediate form is met 

 with in some of the Vascular 

 Cryptogams in which a group of 

 initial cells, frequently four in 

 numiber, replaces the apical cell. 



Besides these apical meristems, 

 other parts of the mass of the plant. These collections are chiefly 

 connected with growth of the axis in thickness rather than in 

 length, and will be treated of later in detail. The cells are either 

 much like those of the small-celled meristem or are longer 



similar tissue occurs in 



