154 Chlorophycece 



Tribe 2. Spirotceniecv. 



In the British genera of this tribe the cells are solitary, 

 relatively short and unconstricted (with the exception of a few 

 species of Cylindrocystis). The cell-wall has no differentiated 

 outer layer and is quite smooth. The individuals reach the adult 

 condition by periodical growth, chiefly in length. 



Genus Spirotaenia Breb., 1848. The cells are straight or very 

 slightly curved, oblong-cylindrical or fusiform, and frequently 

 enveloped in mucus. There is no median constriction and the 

 apices of the cells may be rounded, truncate, subacute or very 

 acute. There is only one chloroplast in each cell, which may be 

 band-like and parietal, or axile and cristate, and is always spirally 

 twisted to the left. The nucleus is generally asymmetrical, and 

 the cell-wall is smooth and structureless. The genus is divided 

 into two sections; in sect. 1, Monotceniece Rabenh., the chloroplast 

 is a parietal band spirally arranged round the inside of the cell- 

 wall ; in sect. 2, Polytceniece Rabenh., the chloroplast is axile with 

 a variable number of spirally twisted ridges. There may be one 

 or many pyrenoids in the chloroplast. 



There are fourteen British species of the genus, none of which is common. 

 S. condensata Breb. (fig. 54 A ; length 150270 ^, breadth 1827 /*) is the 

 largest and most widely distributed, and S. closteridia (Breb.) Arch, is the 

 smallest (length 13'5 /*, breadth 4'5 4 -6 /*). They are all very delicate, with 

 thin cell-walls, and most of them can only be identified with certainty from 

 living specimens. The zygospores of few of them are known and they are 

 rarely met with ; the outer layer of the wall of the zygospore is usually 

 ornamented. Most of the species occur in peat-bogs. 



Genus Mesotsenium Nag., 1849. The cells are cylindrical or 

 subcylindrical, generally straight or slightly curved, and are with- 

 out any trace of a median constriction. The apices are, as a rule, 

 broadly rounded. The chloroplast, of which there is usually only 

 one in a cell, is a flattened, axile plate extending from end to end 

 of the cell, and there may be one or several pyrenoids. Sometimes 

 there are two flattened chloroplasts. The cells often contain 

 numerous globules of an oily nature. 



There are ten British species of the genus, seven of which occur as 

 mucilaginous masses amongst mosses and hepatics, generally on wet rocks. 

 The largest of these is M. De Greyi Turn. (fig. 54 D; length 74125 /*; 

 breadth 15-5 30 /*) and the smallest is M. macrococcum (Kiitz.) Roy & Biss. 

 var. micrococcum (Kiitz.) W. & G. S. West (length 13'5 15'3 ^ ; breadth 



