DIV. 



THALLOPHYTA 



415 



Order 3. Dictyotaceae ( 45 ) 



Only a small number of forms belong to this family. The fan-shaped Padina 

 pavonia, which occurs in the Mediterranean, and Dictyota dichotoma, with a forked 

 ribbon-shaped thallus, which is widely spread in the European seas (Fig. 83), are 

 examples. The spores are formed as in the Red Algae in sporangia ; usually there 

 are four spores (tetraspores), less commonly eight. They have no cell walls and 

 are unprovided with cilia and must be termed aplanospores (Fig. 357, 1). The 

 tetrasporangia may be derived from the unilocular sporangia of the Phaeosporeae. 

 The oogonia and antheridia in Dictyota are grouped in sori (Fig. 357, 2, 3), and 



D v; F 



FIG. 358. Laminaria digitata. A, Male gametophyte ; a, empty antheridia. B, C, D, Female 

 gametophytes (B is large, C small, while D is reduced to a single oogonium) ; og, oogonium ; 

 o, egg-cell. E, Young sporophyte, still seated on the empty oogonium. F t Further developed 

 sporophyte with the rhizoids. (A x 600 ; B x 292 ; C x 322 ; D x 625 ; E X 322 ; F X 390. 

 After H. KYLIX.) 



arise from adjacent cortical cells, each of which divides into a stalk cell and the 

 oogonium (or antheridium). The peripheral cells of the autheridial group remain 

 sterile and form a kind of indusium. Each oogonium forms a single uninucleate 

 oosphere ; the antheridia become septate, resembling the plurilocular gametangia, 

 and each cell gives rise to a spermatozoid. This, in contrast to the spermatozoids 

 of other Brown Algae, has a single long cilitim, inserted laterally. The zygote 

 germinates without undergoing a period of rest. 



Dictyota is dioecious. The male and female plants arise from the asexually- 

 produced tetraspores ; from the fertilised ovum plants which bear tetraspores are 

 developed. In the tetrad division in the sporangia the number of chromosomes 

 becomes reduced from 32 to 16, and the reduced number is maintained in all the 

 nuclei of the sexual plants, the double number being again attained in fertilisation. 

 There is thus a true alternation of generations. The sexual generation (gametophyte) 



