THE ALGAE 



85 



Sexual Reproduction 



The sexual reproduction of Oedogoniiim is of an advanced and com- 

 plicated type. The distribution of the sexes varies in the different species. 

 In some both sexes occur in the same filament, when it is said to be 

 monoecious, while in others the male and female organs are borne on 

 different filaments, and the plants are spoken of as dioecious. The male 

 cells are formed by repeated transverse divisions of a vegetative cell, which 

 in monoecious species is close to the female organ. These divisions continue 

 until a row of about a dozen small cells are produced, each of which is 

 termed an antheridium. The contents of each antheridium divide into two, 

 and each portion becomes an antherozoid. These antherozoids resemble 

 the zoospores, but are much smaller. 



The female organ, or oogonium, is formed from a single vegetative cell 

 which divides into two, the upper cell being much richer in protoplasm than 

 the lower one. It has a large nucleus and forms the oogonium, while the 

 lower cell is termed the supporting cell. The oogonium swells out to about 

 twice its original diameter and becomes rounded or oval in outline. The 

 cell contents are withdrawn from the wall and form a free, rounded, proto- 

 plasmic body or oosphere. A small clear patch appears on one side, which 

 is called the receptive spot. The wall of the oogonium now opens either 

 by a split or by a thinning of the wall above the receptive spot, and is thereupon 

 ready for the entry of the antherozoid. 



In addition to the normal monoecious 

 and dioecious types, certain dioecious 

 forms of Oedogoniiim are known in which 

 the male filaments are very much smaller 

 than the female. Such plants are spoken 

 of as dwarf male or nannandrous 

 forms. Cells of the parent filament 

 divide by repeated transverse division, 

 and from each of the cells formed a 

 single zoospore escapes. It is inter- 

 mediate in size between the asexual 

 zoospores and the antherozoids and is 

 termed an androspore. The andro- 

 spores swim till they reach the female 

 filaments, attaching themselves either to 

 the oogonium itself or to a cell close 

 to it. Here they germinate and produce 

 a filament of two or three cells. With 

 the exception of the lowest, which forms 

 a holdfast, each becomes an antheridium 

 liberating two antherozoids (Fig. 66). 



Fertilization is efl^ected by the entrance of the antherozoid through the 

 opening in the wall of the oogonium, after which it unites with the oosphere. 



Ce// caps 



Oosphere 



Antheridium 



Basal cell 



Fig. 66. — Oedogonium sp. Reproductive 

 organs of a nannandrous species 

 showing a dwarf male plant attached 

 below the oogonium. 



