UlotrichaeeaB 



283 



Family Ulotrichacese. 



The Algas of this family are unbranched and readily recognizable by their 

 cytological characters. The lowest types are those in which the cells of the 

 filaments are largely held in place by a conspicuous gelatinous envelope 

 (Geminella, Radio/Hum). More advanced are those genera in which there 

 are short filaments of loosely connected cells without any mucous investment 

 (Stichococcus, Catena). The highest types are those forms with cylindrical 

 filaments of firmly united cells (Ulothrix, Binuclearia). 



The cell-wall is always hyaline and colourless, but varies much in 

 thickness. It .is sometimes delicate (Catena, Stichococcus, Rhaphidonema, 

 some species of Ulothrix), and sometimes thick and lamellose (Psephotaxus, 

 Binuclearia, Ulothrix zonata). In the lower types (Geminella, Radiofilum) 

 the outer layers become converted into a wide mucilaginous envelope. In 

 Binuclearia tatrana and Ulothrix zonata the degree of lamellation of the 

 cell-wall depends upon the state of growth. 



There is a single parietal plate-like chloroplast in each cell, with an 

 entire or variously lobed margin, usually 

 occupying only a portion of the cell-wall. 

 It generally possesses one or more pyrenoids, 

 but may be entirely destitute of them. 

 There is a small inconspicuous nucleus, 

 usually placed internally to the chloroplast. 



In the genus Geminella the end-walls 

 of the cells become hemispherical soon 

 after cell-division and the cells, although 

 remaining as a filament, are only held 

 in position by the mucilaginous sheath 

 (fig. 180). In Stichococcus the external 

 characters are intermediate between those 

 of Geminella and Ulothrix. The cells divide 

 to form cylindrical filaments which under 

 normal conditions undergo fragmentation 

 into short pieces consisting of only a few 

 cells. This is caused by the apposed ends 

 of certain pairs of adjacent cells becoming 

 hemispherical, with the result that the 

 filament becomes dislocated at these points. 



In some species the fragmentation is so complete that the plants are almost 

 unicellular (fig. 183 D F). 



Dismemberment of the filaments of Ulothrix often occurs by the death of 

 some of the cells, each dismembered piece growing into a new filament. 



Fig. 180. A, Geminella mutabilis 

 (Breb.) Wille. B, G. ordinata G. S. 

 West. G E, G. protofjenita (Kiitz.) 

 G. S. West. All x 440. 



