136 A TEXTBOOK OF THEORETICAL BOTANY 



Phaeophyceae. It must not be thought, however, that there is a definite 

 progression within the Phaeophyceae towards a reduction of the gametophyte, 

 for in certain other orders the gametophyte is large and fairly elaborately 

 developed, while the sporophyte is reduced to a filamentous structure bearing 

 the zoosporangia. It might appear therefore that in the Phaeophyceae there 



Laminarii 

 ■ Plant ■ 



Oospore 



Zoosporangium 



Antherozoids Oosphere 



I 



Antberidium Oogonium 



Zoospores ^^'^ 



Gametophyte 



>—^ 



Female 

 Gametophyte 



Fig. 123. — Life-cycle oi Laminaria. 



have been two contrasting tendencies operating, the one to reduce the 

 sporophyte and the other to reduce the gametophyte. 



We may represent the life-cycle of Laminaria in the above diagram, 

 which, it will be seen, does not differ in any essential from that of Dictyota 

 except in the size of the various structures (Fig. 123). 



Fucales 



The Fucales are Phaeophyceae in which the reproductive organs are borne 

 in special cavities in the surface of the thallus, termed conceptacles. Growth 

 is by means of an apical cell ; the thallus is large and composed of an elaborate 

 system of filaments held together by mucilage. There is no morphological 

 alternation of generations, although a cytological one has been shown to be 

 present. The plant itself is diploid. The reduction division occurs prior 

 to the formation of the gametes, which are therefore monoploid, while the 

 fertilized zygote is again diploid. There is no asexual method of reproduction. 

 Sexual reproduction is oogamous. 



We shall consider the life-history oiFucus as an example of this order. 



