ORDER CHAETOPHORALES 



This order includes branched filamentous plants which are either 

 entirely prostrate or which have an erect system of branches that 

 arise from a horizontal portion of the thallus. In many members there 

 is a basal-distal differentiation. Exceptions are unicellular genera, 

 Protococcus and CJmetosphaeridium. The cells are for the most part 

 cylindrical, although in a few genera they are globose. A common, 

 but not universal, character is the seta or hair, which has 2 expres- 

 sions in the order; in some, the seta is a hair-like outgrowth of the 

 cell wall, but in the second type it is either a lateral or terminal 

 attenuated cell or series of cells. The cells forming the branches may 

 be about the same size as those of the main axis, or they may be dis- 

 tinctly smaller. 



The cell wall ordinarily is thin and sometimes mucilaginous, some 

 forms being inclosed by a copious mucilage. The chloroplasts are 

 ulob-ichaceous parietal bands or plates which sometimes completely 

 encircle the wall. There may be from 1 to several pyrenoids. 



Variations from the usual form of the chloroplast are found in the 

 Trentepohliaceae, a group which well might be interpreted as con- 

 stituting a separate order because the species have features not 

 shared by other members of the Chaetophorales. 



Asexual reproduction is by zoospores produced in the upper or 

 outer cells of the thallus, as well as in special sporangia. Isogamous 

 sexual reproduction is the rule, but in Chaetonema and Coleochaete 

 it is oogamous. Although these genera do not conform in their 

 method of sexual reproduction, they have such vegetative charac- 

 ters as setae, habit of growth, and form of chloroplast in agreement 

 with other members of this order. In Aphanochaete there is anis- 

 ogamy. See Fritsch (1916, 1935) and West and Fritsch (1927) on the 

 phylogenetic position and characteristics of the Chaetophorales. 



Key to the Families 



1. Plants unicellular or forming loose aggregates of cells 



without definite filamentous order 2 



1. Plants definitely filamentous or disc-like, or 

 pseudoparenchymatous thalli (cushion-like expanses 



of densely compacted filaments) - - 3 



2. Cells solitary or in clumps, occasionally forming false 



filaments; setae lacking; plants mostly subaerial protococcaceae 



2. Cells solitary or gregarious, globose, each bearing 



a long seta which is sheathed at the base coleochaetaceae (in part) 



3. Filaments little branched, in our specimens without setae; 

 walls thick; zoospores formed in swollen cell at the tips of 

 the branches which arise from a prostrate portion of the 

 thallus; plants growing on shells and wood, or aerial on 



tree trunks and rocks...... - - trentepohliaceae 



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