THE THALLOPHYTA 



273 



cases where sexuality has been proven are always isogamous 

 except in the genus Vauchcria, which is such a familiar and dis- 

 tinctive type that we shall describe it more fully. This alga 

 forms the common "green felt" so often found on damp soil or 

 in muddy pools, and consists of a tangled mass of coarse, branch- 



. Fig. 151. — Vaucheria. Asexual reproduction. The tip of a filament is cut 

 off by a wall and its contents becomes a large zoospore, with many nuclei and 

 many groups of paired cilia. The zoospore breaks through the wall and escapes. 

 {After Gotz.) 



ing, tubular filaments. Large zoospores are produced, each of 

 which is merely the contents of the tip of a filament which has 

 been cut off by a wall and has escaped (Fig. 151). The sexual 

 organs are not simply modified vegetative cells, as in the plants 



Fig. 152. — Vaucheria. Sexual reproduction. Oogonia, o, each contain a 

 single egg. Antheridia, a, have discharged all their sperms and are empty. A, 

 V. terrestris. B, V. sessilis X 130. 



previously studied, but arc specialized for gamete production 

 (Fig. 152). A cell partitioned off by a wall from the main fila- 

 ment or from a small lateral branch becomes the oogonium, within 

 which a single, large egg is formed. From the tip of another 



