102 



THE CELL 



new cells are mainly produced by division from pre- 

 existing cells at and near the tips of the branches of 

 the root and shoot. These regions of active cell division 

 are called meristems.^ The cells in this region are all 

 young because they have all recently arisen from cell 

 division. The characteristic features of such a meris- 

 tematic or embryonic cell (Fig. 6, A) are that the 

 cell wall is relatively thin, the cell cavity is completely 

 filled with protoplasm and the nucleus is large, its 

 diameter commonly being as much as two- thirds to three- 

 quarters that of the whole cell. The granular endoplasm * 



vw: 



Fig. 7. — Enlarged diagram of the portion X of Fig. 6. C : c.w., cell wall ; 

 ect., ectoplasm; endo, endoplasm; v.w., vacuole wall; pi., plastid. 

 Supposed to be multiplied 6,600 times. 



contains various granules and droplets (metabolites) 

 of different sizes, It also contains small refringent 

 bodies called plastids, which may under certain cir- 

 cumstances form starch, and if exposed to light develop 

 chlorophyll and become chloroplasts. The endoplasm 

 is bounded on the outside by the very thin layer of 

 ectoplasm, which is, however, only just visible under 

 the highest powers of the microscope and with the 

 best optical definition, since it is only a fraction of 

 I jit in thickness (Fig. 7). 

 The large and conspicuous nucleus (Fig. 6) is a 

 ' Greek fispiari^s, divider. ' Svdov, within. 



