Tribommacece 253 



Family 2. TRIBONEMACEJE. 



The plants of this family are unicellular or filamentous. The 

 cells are globose, cylindrical, elongate, often spirally coiled or 

 united to form long flexuose filaments. There is generally a single 

 nucleus in each cell, but sometimes two or more are present, 

 giving the cell a more or less crenocytic character. The cell-walls 

 are always firm and usually of some thickness, except in the genus 

 Buinilleria, in which the walls sometimes become swollen and 

 hyaline. Asexual reproduction takes place by zoogonidia with one 

 long and one short cilium, and two or several parietal chromato- 

 phores. Aplanospores are also of frequent occurrence in the genus 

 Tribonema. Sexual reproduction occurs by isogamous piano- 

 gametes which have been described, but perhaps erroneously so, 

 as having two equal cilia. 



There are four British genera, three of which are abundant. 



A. Plants unicellular. 



* Cells globose, aggregated in mucilaginous colonies . . . Chlorobotrys. 

 ** Cells elongate, usually shortly stipitate and often 



spirally coiled Ophiocytium. 



B. Plants filamentous. 



* Cell- walls firm, splitting into H -pieces Tribonema. 



** Cell- walls hyaline, H -pieces not very evident; fila- 

 ments small Bumilleria. 



Genus Chlorobotrys Bohlin, 1902 1 . The cells are globose or 

 subglobose, solitary, or more commonly aggregated in families of 

 2, 4, 8, or 16. Each family is surrounded by an ample mucous 

 integument, very hyaline and quite homogeneous. The cell-walls 

 are firm, smooth, of some considerable thickness, and they contain 

 a certain proportion of silica. The chromatophores are parietal, 

 yellow-green discs, from 6 to 30 of which are disposed on the walls 

 of each cell. Sometimes the pigment becomes more or less diffuse. 

 There is frequently a prominent red pigment-spot in each cell of 

 the family, but sometimes this is entirely absent. Multiplication 

 takes place by the division of the cells primarily in two directions, 

 but afterwards in three directions 2 . Families of 4, 8, or 16 cells 

 are therefore frequently very symmetrical, but beyond this number 

 they become irregular. During cell-division the contiguous walls 



1 Bohlin in Bihang till K. Sv. Vet.-Akad. Handl. Bd 27, no. 4, 1902, p. 34, 

 t. i, f. 9. 



- West & G. S. West in Journ. Bot. April, 1903. 



