i62 A TEXTBOOK OF THEORETICAL BOTANY 



The main branches of the thalkis are initiated by oblique divisions of the 

 apical cell. Smaller monosiphonous branches of limited growth, called 



Divided apical cell 



Apical cell 



Fig. 151. — Polysiphoriia sp. Apex of frond 

 showing dichotomy of apical cell. 



A 



PERICENTRAL 

 SIPHON 



CORTICAL 

 CELLS 



CENTRAL 

 SIPHON 



B 





*}'.':'■ :\\:X 



m 



D 



""^ 



Fig. 152. — Polysiphonia fastigiata. A, Formation of pericentral siphons in 

 transverse sequence, cut off from cell of central siphon. B, Transverse 

 section of completed structure of branch. C to F, Development of the 

 connection between two cells of a pericentral siphon. 



trichoblasts, are cut off from the upper ends of certain of the pericentral 

 cells. 



