SECT. I 



CRYPTOGAMS 



371 



Dictyota dicliotvma, with a forked ribbon-sliaped thallus, wliicli 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 (tetra- 

 spores), less commonly eight. They have no cell walls and are unprovided with 

 cilia and must be termed aplanospores (Fig. 296, 1). The oogonia and antheridia 

 in Didijota are grouped in sori (Fig. 296 2, 3), and 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 uninucleate oosphere, the antheridia 



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Fig. 296. — Dictyota dichotoma. Transverse sections of the thallus. 1, With tetrasporangia ; 2, 

 with a group of oogonia ; S, with a group of antheridia (after Thcket). yj, Sperniatozoid 

 (after Williams). (From Oltjiasns' Algae.) 



become septate, resembling the plurilocular gametangia, and each cell gives rise 

 to a sperniatozoid. This, in contrast to the spermatozoids of other Brown Algae, 

 has a single long cilium 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 

 generation (sporophyte) do not, however, show differences in structure as is the case 

 in the Bryopbyta and Pteridophyta. 



Family 2. Fucaceae (^^). — Asexual reproduction is wanting in this order, while 

 sexual rejiroduction is distinctly ooganious. The oogonia and antheridia of Fucus 



