Heterotrichales 



411 



It seems probable that the genus Actidesmium Reinsch (1891) should be included 

 in the Ophiocytiaceae. This Alga forms free-floating colonies in which the primary 

 mother-cells radiate from a central point. These cells are elongate-fusiform and each sets 

 free 16 zoogonidia from its distal end. The latter come to rest on the open end of the 

 mother-cell and there grow into new cells, the whole colony being reminiscent of 

 Ophiocytium Arbuscula. The cells possess a parietal chromatophore and the food-reserve 

 is oil. 



Fig. 262. Diagrams to illustrate the structure of the cell-wall in Tribonema and Ophiocytium. 

 The series on the left represents cell-division and the growth of the cell-wall in Tribonema. 

 On the right the growth of the wall in Ophiocytium is depicted. (After Bohlin.) 



Order 2. HETEROTRICHALES. 



All the Algae of this order are filamentous, with or without a little 

 enveloping mucus. Almost all forms of asexual reproduction occur within 

 the order and gamogenesis has also been observed. 



There is only one established family the Tribonemacese which is more 

 or less equivalent to the Ulotrichacese among the Isokontse. 



Family Tribonemacese. 



First established in 1904 (G. S. W., '04) this family is here limited to 

 filamentous unbranched types of the Heterokontse. Tribonema is one of the 

 most abundant of the genera of Green Algae, T. bombycina having a world- 

 wide distribution. In T. affinis and other species the filaments are exactly 

 cylindrical, but in T. bombycina the cells are normally a little barrel-shaped 

 and not infrequently a trifle irregular in their growth (fig. 263 A). The 



