THE GREEN ALGAE OF NORTH AMERICA 225 



duce one or two spermatozoids ; in the latter case the sper- 

 matozoids may be formed side by side, with a vertical par- 

 tition (division vertical), or one above the other (division 

 horizontal). The oogonium receives the spermatozoid either 

 by a round opening in the wall (poriferous oogonium), or by 

 the upper part of the oogonium separating from the lower 

 part by a circular division all around the wall (operculate). 

 The pore may be opposite the middle of the spore (pore median), 

 higher up (pore superior), or lower down (pore inferior). In 

 the operculate species the same distinction is indicated by 

 division median, superior or inferior. The membrane of the 

 oospore may consist of one to three layers ; in the latter case 

 these are known as the outer membrane or exospore, median 

 membrane or mesospore, and inner membrane orendospore. In 

 the monoecious species the antheridia occupy positions similar 

 to those of the androsporangia in the nannandrous gynandro- 

 sporous species, and are similarly known as epigynous, etc. 

 The formation of the spermatozoids is the same as in the nan- 

 nandrous species. 



The subdivision of the genus is largely artificial ; the usual 

 division is into monoecious, dioecious macrandrous, and dioe- 

 cious nannandrous species ; but interesting parallelisms may be 

 found between the first and third of these series. There seem 

 to be more species of limited distribution in this genus than in 

 most genera of green algae, but it may be due to imperfect 

 knowledge ; certainly some species occur at widely distant sta- 

 tions. Undoubtedly many species not yet recorded for North 

 America will be found here when more attention is paid to them. 

 They are all species of quiet or slowly flowing water, and may- 

 be found at any time, except when the water is frozen over. 



KEY TO THE SPECIES OF OEDOGONIUM. 



i. , Macrandrous. 2. 



i. Nannandrous. 45- 



2. Oogonium opening by a pore. 3. 



2. Oogonium opening by a lid. 33. 



3. Spores globose. 4- 



3. Spores ellipsoid. 17. 



4. Pore median. 5- 



4. Pore supramedian. 7- 



4. Pore superior. n. 



