370 



HOT A N V 



Order 2. Cyelosporeae 



Family 1. Dictyotaceae ( 4:! ' "). Only a small number of forms belong to this 

 family. Padina pavonia, which occurs in the Mediterranean, and Dictyota dichoi'niiu. 

 which is widely spread in the European seas (Fig. 8), are examples. They are 

 distinguished from the Fucaceae by bearing asexual and sexual organs on distinct 

 individuals. 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. 306, 1). 

 The oogonia and antheridia in Dictyota are grouped in son (Fig. 306, 2, 3) and 



Fio. 306. Dictyota dichotoma. Tran>vi ]> sections of tin- thallns. 7. WHhtetruparaagim; .'.with 

 a group of oogonia ; 3, with a group of antheridia (after THUKET). I,, Si>ermatozoiil (;iftt-r 

 WILLIAMS). (From OLTMANNS' Algae.) 



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

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

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

 oosphere, the antheridia become septate, resembling the plurilocular gametaujjia. 

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

 of other Brown Algae, has a single long ciliuni inserted laterally. 



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 (in contrast to the condition of things in the Phaeosporeae) a true 

 alternation of generations. The sexual generation (gametophyte) and the asexual 



