58 



Growth 



side or below the apical cell. In ferns, lycopods, and horsetails this con- 

 siderable body of embryonic cells at the apex of the axis somewhat re- 

 sembles the terminal meristems of higher plants. 



Among bryophytes, the origin and arrangement of the leaves and the 

 structure of the various tissues can usually be traced back to precise 

 divisions of the apical cell and its daughter cells so that there is a very 

 definite pattern of cell lineage in the plant body. This is especially 

 diagrammatic in such a form as Sphagnum, where the two markedly 

 different types of cells in the leaves can be seen to originate in differen- 

 tial cell divisions. Such a precise cell lineage is less conspicuous in higher 



Fig. 4-3. Longitudinal section of shoot apex Fig. 4-4. Selaginella wildenovii. Me- 

 of Equisetum, showing apical cell and its dian longitudinal section of young 

 derivatives. ( From Golub and Wetmore. ) shoot, showing apical cell and its de- 



rivatives. ( From Barclay. ) 



forms but often can still be traced even there. In one species of 

 Selaginella (Barclay, 1931), for example, the derivation of the epidermis, 

 cortex, pericycle, endodermis, and vascular cylinder can be traced back 

 to direct descendants of the apical cell (Fig. 4-4). 



Although the distribution of the leaves and the general organization 

 of the shoot are determined in many of the lower forms by the activity 

 of the apical cell and the arrangement of its derivatives, Golub and 

 Wetmore (1948) found that in Equisetum there is no relation be- 

 tween the cellular pattern of the apex and that of the mature axis derived 

 from it. 



