HOW TO KNOW THE FRESH-WATER ALGAE 



255a Cells with several chloroplasts; thallus inclosed in a mucilaginous 

 sheath. Fig. 181 PSEUDOULVELLA 



Fig. 181. Pseudoulvella americana 

 (Snow) Wille. 



The prostrate disc-like thalli of 

 this plant are relatively large, 1 

 cell in thickness at the margin, 

 several cells thick near the center. 

 The entire plant is covered by a 

 gelatinous film through which an 

 occasional seta projects from the 

 cell walls, but these are rarely 

 found. 



Figure 181 



255b Cells with 1 reticulate chloroplast; thallus not inclosed in a sheath. 

 Fig. 182 ULVELLA 



Fig. 182. Ulvella involens (Savi) Schmi- 

 dle (Dermatophyton) , diagram of at- 

 tached colony showing arrangement 

 of cells. 



This plant forms irregular disc-like 

 or cushion-like growths which are sev- 

 eral cells in thickness when mature. 

 Figure 1 82 They grow on submerged aquatic plants, 



sometimes on animals; do not possess setae. 



256a (243) Plants parasitizing leaves of terrestrial Angiosperms (mag- 

 nolia, tea, etc.). Fig. 183 CEPHALEUROS 



Fig. 183. Cephaleuros virescens Kunze, 

 diagram of thallus as it grows under 

 leaf epidermis of host plant, with erect 

 branches bearing sporangia. 



This species occurs in tropical and sub- 

 tropical parts of the world, especially in 

 areas where there is an optimum humidity 

 so that the leaves of the plants on which 

 the alga is parasitic are moist. The host 

 may be Magnolia, tea (Thea), citrus trees, 

 or Rhododendron. Because of the discol- 

 oration and degeneration of host tissue 

 in the vicinity of the parasite some damage is caused by this alga and 

 a certain amount of economic loss results, especially in tea plantations. 

 Although the parasitized areas appear gray-green in color, the indi- 

 vidual filaments of the cushion-like thallus of the alga are usually 

 reddish because of the contained haematochrome pigment. 



Figure 1 83 



115 



