No. 10.] FRESH-WATER ALG.E. . 25 



Botrydium Wall. Small, non-cellular, green, globose 

 plants, with colorless, much divided roots, descending into the 

 moist earth, upon the surface of which this Alga lives. The 

 zoogonidia are small, ovoid, and provided with a long cilium. 

 If the plant becomes submerged, the whole may turn into a 

 zoogonidiangium, and the zoogonidia escape through an open- 

 ing in the apex. Non-motile spores are often produced in 

 great numbers in the rhizoids. If the plant becomes too dry, 

 the green portion migrates into the rhizoids, and a number of 

 spores are produced. 



FAMILY II. TRIB0NEMACE.E. 



Plants unicellular or filamentous ; cells spherical, cylindri- 

 cal, or elongated, often united to form filaments, and spirally 

 coiled. The cell wall is always firm, and usually thick. 

 Asexual reproduction by zoogonidia. Aplanospores occur in 

 Tribonema. Sexual reproduction by isogamous (i. e., similar) 

 gametes. 



Key to Genera. 



1. Plants unicellular 2 



Plants filamentous, cell wall firm, splitting into H- 



shaped pieces Tribonema* 



2. Cells globose, aggregated in mucilaginous colonies 



Chlorobotrys 

 Cells elongate, usually shortly stipitate and often 

 spirally coiled Ophiocytium* 



Description of Genera. 



Tribonema Derbes and Solier (Conferva, as used by 

 Lagerheim). Filaments composed of cylindrical cells, covered 

 with a thick cell wall which frequently breaks up into H-shaped 

 pieces. The cells each contain one or two nuclei and several 

 chromatophores. Asexual reproduction by zoogonidia with 

 two unequal cilia, and by non-motile spores which escape from 

 the broken filaments. Sexual reproduction by isogamous 

 gametes, one of which comes to rest and rounds off before 

 another conjugates with it. 



This genus covers many of those forms previously called 

 Conferva, a name that is now given up. Hazen places it with 



