Protocoecacece 



231 



Sub-family V. TETRAEDRIE^. 



The plants of this sub-family are always solitary unicells. 

 Each cell is flattened and angular, usually with a definite number 

 of angles, or it is tetrahedral, octahedral, or polyhedral. The 

 angles may be rounded, emarginate, or furnished with spines. 



Genus Tetraedron Kiitz., 1845. [Polyedrium Nag., 1849.] 

 The cells of this genus, which occur as solitary individuals or 

 rarely collected into temporary aggregates, are flattened and 

 angular (triangular, quadrangular,, or polygonal) or polyhedric. 

 The angles are obtuse, acuminate, or furnished with one or more 

 simple or furcate spines. There is a single large chloroplast, 

 parietally disposed and containing one pyrenoid. Multiplication 

 takes place by the formation of four or eight autospores, which are 

 set free by the rupture of the wall of the mother-cell. Sometimes 

 they are expelled into a delicate vesicle, which, however, soon 

 disappears. 



Fig. 101. A, Tetrac.dron minimum (A. Br.) Hansg., from Keston Common, Kent. 

 B, T. caudatum (Corda) Hausg., from Pilmoor, N. Yorks. C, T. reyulare 

 Kiitz., from near Bowness, Westmoreland. I), T. enorme (Ealfs) Hansg., from 

 Mickle Fell, N. Yorks. E G, T. Iwrrlditni W. & G. S. West, from Putney 

 Heath, Surrey. (All x 450.) ants, autospores. 



There are about 14 species of the genus known to occur in the British 

 Islands. T. minimum (A. Br.) Hansg. is the most abundant of the flattened 

 species; diam. of cells 6'5 16 /^; thickness of cells 5 7 p.; fig. 101 A. 

 T. regulare Kiitz. ( = Polyedrium tetraedricum Nag.) is the commonest of the 

 polyhedric species; diam. of cells 13'5 40 p. ; fig. 101 C. T. enorme (Ealfs) 

 Hansg. was originally described as a Desrnid ; tig. 101 D. 



The genus Cerasterias Reinsch (1867) is sometimes separated 

 from Tetraedron owing to the depth of the lobulation. The cells 

 are solitary, tetrahedric in character, and the divisions into lobes 



