8. Filaments of definite length, short and frequently interrupted; 



basal differentiation lacking Stichococcus (in part) 



9. Filaments often showing basal differentiation; chloroplast a complete 



or nearly complete parietal band Ulothrix (in part) 



9. Filaments without basal differentiation; chloroplast a folded plate 



along one side of the cell and not nearly encircling it 10 



10. Chloroplast as long as the cell or nearly so 



( See Ulothrix cylindricum, however ) Stichococcus ( in part ) 



10. Chloroplast half the length of the cell Hormidium 



ULOTHRIX Kuetzing 1833, p. 517 



Simple, unbranched filaments of cylindrical cells, often showing 

 basal differentiation and arising from a special holdfast cell; becom- 

 ing free-floating in some species. Chloroplast a parietal band which 

 extends % to % of the way around the cell ( forming a complete ring 

 in 1 species ) , and sometimes extending the entire length of the cell. 

 Asexual reproduction by 4-8 quadriflagellate zoospores cut out from 

 the protoplast of unspecialized cells; sexual reproduction by iso- 

 gametes formed 8-16-64 in a cell, smaller than the zoospores and 

 biflagellate. Palmella stages not uncommon. 



This genus is distinguished from others in the family principally 

 by the approximate completeness with which the chloroplast covers 

 the wall, and also by the attached habit of growth, which involves a 

 basal holdfast cell in some species. 



Key to the Species 



1. Filaments (20)-25-45-(60)m in diameter; wall thick; 



chloroplast a complete parietal band U. zonata 



1. Filaments 20/i or less in diameter; wall thin 2 



2. Filaments ll-20;u, in diameter 3 



2. Filaments smaller, 5-1 Ijw in diameter 5 



3. Filaments composed of long, cylindrical cells, 11-12.5^ in 



diameter, the length 2^-3 times the width U. cylindricum 



3. Filaments composed of shorter cells, 13-20At in diameter 4 



4. Cells up to 20yot in diameter, shorter than wide U. tenuissima 



4. Cells 13-16m in diameter, 1-2 times longer than wide U. aequalis 



5. Cells 4-5/u, in diameter; chloroplast extending nearly the entire 



length of the cell — ^ - U. subtilissima 



5. Cells wider; chloroplast extending % or less the length of the cell 6 



6. Cells 5-6m in diameter, short-cylindric; chloroplast covering 



^^2 or less of the cell wall ....U. variabilis 



6. Cells larger, 6-9,ct in diameter , 7 



7. Filaments constricted at the cross walls, 5.7-9/u in diameter; 



cells 2—4 times diam. long U. subconstricta 



7. Filaments not or slightly constricted at the cross walls; cells 

 rectangular, 7.5-10m in diameter, the length 2/3 to 1 1/3 

 the width U. tenerrima 



[95] 



