CHAPTER V. THE THALLOPHYTA. 



29I 



tinction in form and size between the spores which conjugate 

 and those of the asexual generation. Fig. 486 represents one of 

 these plants. 



Fig. 486 — a, Ulva Lactuca, frond about natural size ; b, portion of frond magnified, 

 showing arrangement of cells ; the unshaded ones are those from which zoospores have 

 ^scaped ; c, biciliated zoospores. 



The Oedogotiiums are filamentous, 

 mostly unbranching fresh-water algse, 

 whose cells are densely packed with 

 chlorophyll-bodies. They are not 

 uncommon in ponds, ditches and 

 slow streams, where they occur in 

 patches, attached by means of root- 

 like processes to sticks, stones, the 

 stems of aquatic plants, etc. There 

 are a large number of species, some 

 of them monocarpous, others dioe- 

 cious. They reproduce asexually, 

 not only by the transverse fission of 

 their cells, but by the production of 

 zoospores. In the latter process, the 

 protoplasm of the cell becomes ag- 

 gregated into a rounded or oblong 

 mass, acquires a fringe of cilia at one 

 end, escapes from the cell-wall, and 

 moves through the water for a time, 

 but finally comes to rest, sends out 

 from one end root-like processes, attaches itself to some object 

 in the water, and develops into a filament. See Fig. 487. 



Fig. 487. — Oedogonium, showing 

 asexual modes of reproduction, a, 

 portion of filament with the proto- 

 plasm escaping to become a free, 

 ciliated cell, b ,- c, the same cell 

 throwing out rhizoids. Magnified 

 about 250 diameters. 



