Meristems 



57 



Fig. 4-1. Terminal portion of the alga Sphacelaria, 

 showing how thallus is produced by activity of the 

 apical cell and its descendants and how a branch 

 originates. ( From Haberlandt. ) 



Throughout the bryophytes, the ferns, the horsetails, and many of the 

 lycopods, growth of the plant body is governed by the activity of apical 

 cells, one at the apex of the shoot and the other at the apex of the root. 

 These cells are usually pyramidal with the base of the pyramid outward, 

 and division takes place parallel to the three inner sides of the pyramid. 

 Most growth of tissues results from the later division of these daughter 

 cells and their descendants, but growth seems to be initiated and domi- 

 nated by the apical cell (Figs. 4-2, 4-3). It is not clear, however, just 

 what the function of the apical cell is. Wetmore ( communication to the 

 author ) states that he has very rarely seen an apical cell dividing and he 

 suggests that these cells may function as do the groups of large cells just 

 below the apex of root and shoot in angiosperms, which are thought to 

 be centers of metabolic activity. Most of the actual cell division in the 

 meristems of these lower vascular plants takes place in the cells just be- 



Fig. 4-2. Longitudinal section through apex of a fern root, showing origin of tissues 

 from the apical cell. ( From Sacfis. ) 



