Although certain amounts of cellulose may be present, the walls 

 are of pectose or pectic acid to which silicious substances may be 

 added. The duplex character of the wall is especially common in 

 this class, there being two halves, one of which overlaps the other 

 in the midregion of the cell or toward one end (rarely sections 

 appearing at both ends). In filamentous forms tliis results in the 

 formation of characteristic H -shaped pieces which appear when cells 

 dissociate, because they separate at the points of juncture of the wall 

 pieces and not at the cross walls. The cell at the end of a broken 

 filament has adjoined to it half the wall of its previous neighboring 

 cell. (For a discussion of wall structure and other special features 

 of this class, see Fritscji, 1934; Pascher, 1937-1939; Poulton, 1925; 

 Smith, 1933 and 1938. ) 



Reproduction in this class is by aplanospores, zoospores, and in 

 a few forms by isogametes. 



Key to the Orders 



1. Plants filamentous; cells adjoined end to end 



to form a uniseriate strand ...Heterotrichales 



1. Plants not filamentous 2 



2. Plants macroscopic (2-3 mm. in diameter); coenocytic 



vesicles; terrestrial — - — Heterosiphonales 



2. Plants microscopic; not coenocytic vesicles 3 



3. Cells motile by means of 2 flagella of unequal length ... Heterochloridales 



3. Cells not flagellated 4 



4. Cells amoeboid, or epiphytic, -wUh rhizopodal processes (some- 

 times not clearly evident ) ; inclosed by a lorica ( outer 



shell) with a stipe-like attaching organ Rhizochloridales 



4. Cells not rhizopodal; without a lorica 5 



5. Plants colonial, usually with cells embedded in mucilage and 

 adjoined by mucilaginous stalks; non-motile but capable 



of returning directly to a motile condition Heterocapsales 



5. Plants unicellular ( rarely colonial as in Gloeobotrydaceae, or forming 

 clusters only incidentally), non-motile in the vegetative condition 

 and incapable of returning to motility directly Heterococcales 



ORDER HETEROCHLORIDALES 



In this small order the plants are unicellular and are motile by 

 means of 2 flagella ( in our specimens ) of unequal length. Ordinarily 

 there are 2 elongate, plate-like chromatophores; food reserve in the 

 form of fats, leucosin, and, in some forms, glycogen. The primitive 

 type of cell in this order may assume a rhizopodal state during 

 which food particles are ingested in an amoeboid fashion. Repro- 

 ductive methods other than simple cell division are unknown. There 

 is 1 family and 1 genus in our collections. 



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