THE ALGAE 



12- 



oogonial wall. Each female sorus contains twenty-five to fifty oogonia which 

 are not surrounded by paraphyses. 



Oogonium 



,^^^:^Basal cell 



Fig. Ill . — Dictyota dichotoma. Section of gametophytic 

 thallus with sorus of oogonia. 



Both kinds of gametes are set free in large quantities at interv^als of 

 fourteen days, shortly after the highest spring tides. This periodicity is 

 hereditary and is maintained even under culture in aquaria. Fertilization 

 occurs in the sea water and is efi^ected bv the movement of the antherozoids 

 in large numbers to the oosphere, followed by the penetration of the oosphere 

 by one of the antherozoids. Even if an oosphere fails to be fertilized it is 

 capable of development to a limited extent. 



Under normal circumstances the zygote germinates at once to produce 



Tetrasporangium 



Tetraspores 



Tetrasporic 

 Plant 



Oospore 



Oosphere 

 Antherozoid 



Anther 



\ 



Male Plant 



Female Plant 

 Fig. 113. — Life-cycle of Dictyota- 



Oogonium 



