THALLOPHYTES 



269 



this type is known as a caenocyte. The sexual reproduction ranges 

 from isogamy to heterogamy with highly developed oogonia and 

 antheridia. Among the most important genera are Botrydium 

 and Vaucheria. 



Botrydium, is a very simple form. It consists of a balloon- 

 shaped body resting on wet soil intO' which it sends numerous 

 processes. It is unicellular, multinuclear and produces numer- 

 ous isogamous gametes. 



Vaucheria (Fig. 122, a) is a simple or branched, filamentous, 

 unicellular but multinuclear plant living on damp soil or in more 

 or less stagnant water. Large multinucleate zoospores are pro- 



FiG. 122. — (a) Vaucheria; (6) Spirogyra; (c) conjugating Spirogyra. 



duced at the tips of the branches ; they are covered with paired 

 cilia, one pair to each nucleus, and are in fact compound zoo- 

 spores. They grow directly into new plants. 



The sexual reproductive organs are highly developed for the 

 production of gametes but show considerable variation. They 

 are produced laterally and may be united or independent. The 

 oogonium is large and contains a single large egg or oosphere. 

 The antheridium is small and curved, with the apical cell pro- 

 ducing the sperms. The sperms escape and swim, one of them 

 iinally penetrating an oogonium through an opening and uniting 

 with the oosphere to form an oospore, which germinates directly 

 into a new plant. 



The Gonjugales range from unicellular plants to multicellu- 



