lyo A TEXTBOOK OF THEORETICAL BOTANY 



crowned with a pair of " horns," which are the beginnings of the next 



youngest branches (Fig. i6i). 



Fig. i6i. — Ccra»iiuni nihnnn. 

 apical dichotomy and 

 internodes. 



Apices of branches, showing 

 appearance of nodes and 



The segments cut off basally from the apical cell become the central 

 cells of the axis. Each of these very soon cuts off a single layer of small 



pericentral cells, usually seven in num- 

 l3er, beginning on the outer (convex) side 

 of the curved filament and proceeding 

 regularly round to the opposite side. 

 These cells are cut off at the upper end 

 of each central cell so that they form 

 nodal rings (Fig. 162). Each pericentral 

 cell now produces four cortical cells 

 which develop and divide, forming a 

 close external layer of irregularly shaped 

 cells around the node. As the cortex is 

 forming the central cells elongate and 

 increase in thickness. In C. rubrum the 

 cortical cells divide rapidly enough to keep 

 pace with this growth, so that the filament 

 retains a continuous cortex when mature. 

 In some other species, however, the central 

 cells elongate to such an extent that they 

 separate the nodal bands of cortex from 

 each other, and the equatorial belt of each 

 central cell is left uncovered and exposed. 

 The outer surface of the cortex is 

 covered by gelatinous material, and at certain seasons there are produced, 

 from the cortical cells, large numbers of long unicellular hairs. In some 



NODAL 

 CELLS 



CENTRAL 

 SIPHON 



CORTICAL 

 CELLS 



Fig. 162. — Ceramium areschougii. A, 

 Development of cortical cells from 

 the nodes of the axis. B, Diagram 

 of apical growth. C, Scheme of 

 the successive development of the 

 pericentral cells. (From Oltuiatms.) 



