CHAPTER VII 



PHAEOPHYCEAE (cont.) 



DICTYOTALES, LAMINARIALES AND FUCALES 

 DICTYOTALES (ISOGENERATAE) 



*Dictyotaceae: Dictyota (like a mat). Fig. 112. 



This genus is representative of the Dictyotales, an order character- 

 ized by a well-marked regular alternation of two identical genera- 



Fig. 112. Dictyota dichotoma. A, portion of plant showing regular dichotomy. 

 B, apical cell. C, apical cell divided. D, group of antheridia surrounded by 

 sterile cells. E, single antheridial cell and a sterile cell. F, sorus of oogonia. 

 G, tetrasporangium. (A-D, F, G, after Oltmanns; E, after Williams.) 



tions. Asexual reproduction is brought about by means of tetra- 

 spores produced in superficial tetrasporangia, whilst the sex 

 organs, which are heteromorphic, are always borne in sori. The 

 thallus possesses a specialized bilaterality with well-marked apical 

 growth. 



In Dictyota, as represented by the cosmopolitan species D. dicho- 

 toma, the flattened thallus exhibits what is practically a perfect 

 dichotomy because there is always a median septation of the apical 

 cell. Viewed in transverse section the thallus is seen to be composed 

 of three layers, a central one of large cells and an upper and lower 



11-2 



