104 A TEXTBOOK OF THEORETICAL BOTANY 



anterior ends and elongate. A septum appears and gradually the coenocyte 

 structure of the thallus is re-established. 



Sexual Reproduction 



STIGMA 



BEAK 

 NUCLEUS 



The formation of gametes occurs in most 

 species and probably in all. In some species 

 it has been definitely established that the thallii 

 are dioecious. Any segment of the thallus is 

 potentially capable of functioning as a gamet- 

 angium, and its development is similar to that 

 of the zoosporangium. The gametes are iso- 

 gamous and differ in no marked way from the 

 zoospores, except that they are invariably biflagel- 

 late. They are liberated in a similar manner to 

 the zoospores, and fuse externally to give rise 

 to a zygote which germinates immediately to form 

 a fresh Cladophora filament (Fig. 86). 



Alternation of Generations 



It has now been definitely established that in almost all species there is a 

 strict alternation of sexual and asexual generations. These generations are 

 morphologically alike. Meiosis occurs prior to the formation of the zoospores 

 which therefore give rise to male and female monoploid plants, the diploid 

 phase being established after gametic fusion, resulting in the formation of a 

 sporophytic plant, on which the zoospores are produced. A similar type of 

 alternation has been found in a small number of related genera. 



The life-cycle of Cladophora may therefore be represented by the 

 following diagram : — 



Monoploid 



Cladophora- 



Plant 



PYRENOID 



Fig. 86. — Cladophora sp. 

 Mature gamete. 

 {From Fritsch.) 



Antheridlum Oogonium 



Zoospore 



Antherozoid Oosphere 



Zoosporangium 



Oospore 



Diploid 



' Cladophora ' 



Plant 



Fig. 87.— Life-cycle of Cladophora. 



