THE ALGAE 



125 



female organs are borne on separate plants, and the asexual reproductive 

 organs are produced on different plants from the sexual ones ; but, as far as 



Fig. 108. — Dictyota dicliotoma. Thallus of plant 

 about half natural size. 



their general appearance is concerned, all three kinds of plants are exactly 

 the same. 



Asexual Reproduction 



The sporangium arises from a superficial cell of the thallus, which grows 

 into a small spherical projection from the surface (Fig. 109). Its nucleus 



f^^^^s.ssps^'^^ "W 



«*H '••/ 



Tetrasporangium 





Fig. 109. — Dictyota dicliotoma. Section of sporophytic 

 thallus bearing a tetrasporangium. 



enlarges and then divides meiotically, so that four monoploid nuclei are 

 produced and the cytoplasm divides into four portions. No separating walls 

 are formed, but the four monoploid spores are eventually liberated by a 

 breakdown of the wall of the sporangium. These spores have no flagella. 



