BROWN ALGAE 



38i 



produce zoospores are borne both by the simpler and more advanced 

 types but they are absent in Fucvs. Successive steps in differentiation 

 of the sexes may be found within the Brown Algae. The simplest 

 forms produce isogametes which are motile. More complex forms show 

 differentiation of sexes, the small spermatozoids being motile, but the 

 larger ova are non-motile primordial cells. 



Fig. 282. 



Fig. 283. 



Fig. 282. — Mature male conceptacle of Fucus serratus, filled with branched 

 antheridia hairs. (After Thuret.) Fig. 283. — Mature female conceptacle of Fucus 

 serratus, ontaining unbranched hairs, and oogonia. (After Thuret.) Incidentally 

 these drawings show the structure of the mature thallus (p. 358). 



The Phaeophyceae, which comprise eleven Orders, collectively 

 illustrate : (i) an advance in vegetative organisation from simple 

 filamentous plants to plants of large size and elaborate organisation ; 

 (ii) alternation of generations from simpler types in which the gameto- 

 phytic and sporophytic generations are alike in their vegetative 

 development to those in which the sporophyte shows a marked 

 preponderance over the gametophyte ; and (iii) an advance from 

 isogamy to oogamy. 



The propagative cell of the Brown Seaweeds is very constant 

 in its primitive form. It is actively motile in water when set free. 



