OEDOaONITJM. 273 



ectoplasm. Treat with 2 or 3 per cent, salt solution a 

 zoogonidium which has come to rest ; plasmolysis of the 

 contents reveals the presence of a thin wall. 



389. Sexual Reproductive Organs. In fresh or pre- 

 served material with sexual organs, note that the an- 

 theridium in a branch is continuous with the contents 

 of the ordinary filament, while the upper portion or 

 antheridium proper is cut off by a transverse wall and 

 is usually curved. The mature antheridium is almost 

 colourless and contains numerous nuclei each of which 

 becomes surrounded by protoplasm and forms a biciliate 

 antherozoid, the tip of the antheridium opening by a porr 

 to let the antherozoids escape. The mature oogonium is 

 also separated from the rest of the thallus by a wall ; the 

 single oosphere contains chloroplasts and oil-drops, except 

 at a clear place (" receptive spot ") opposite the pointed 

 beak where a pore is formed to admit the antherozoids. 



OEDOGONIUM. 



390. Oedogonium includes a large number of species, 

 which differ very little in vegetative structure, but show 

 well-marked differences in the distribution of the sexual 

 organs and in the life history. Oedogonium is easily 

 distinguished from other freshwater Algae, even when 

 sterile, by the structure of the cells of the unbranched 

 filaments especially by the peculiar " caps " of the cell- 

 wall. 



In rather less than half the known species, the life 

 history is somewhat complicated by the presence of 

 " dwarf male plants " these species are said to be 

 dioecious and nannandrous; most of the remaining 

 species do not show these dwarf males, and are monoe- 

 cious and macrandrous ; the rest are dioecious but 

 without dwarf males, and are called dioecious and 

 macrandrous. In addition to antheridia and oogonia, 

 Oedogonium shows asexual reproduction by means of 

 zoogonidia. 



p. B. 18 



