34 ] The Classification of Lower Organisms 



Family Hormogoneae Bomet and Flahault in Ann. Sci. Nat. Bot. ser. 7, 3 : 337 



(1886). 

 Subclass Hormogoneae Engler in Engler and Prantl Nat. Pflanzenfam. I Teil, 



Abt. la: iii(1900). 

 Order Hormogoneae Campbell Univ. Textb. Bot. 84 (1902). 

 Order Hormogonales Atkinson 1905. 

 Blue-green algae whose cells divide predominantly in one plane, so that filaments 

 are produced, together with related colorless organisms. 



So far as cell division is strictly in one plane, any branching of the filaments is of 

 the type called "false": it occurs by breaks in the continuity of the series of cells, 

 followed by the outgrowth, beside the original series, of the newly formed tips. In 

 some members of the group, however, the cells are not strictly confined to division in 

 one plane, with the result that "true" branching is possible. There are a few appar- 

 ently derived examples in which cell division takes place freely in all planes. 



Many of these algae produce spores of the type called arthrospores by the direct 

 conversion of normal cells into thick-walled resting spores. Many (almost but not 

 quite exactly the same ones which produce arthrospores) produce peculiarly differ- 

 entiated cells called heterocysts (the word means "different cells"). These are en- 

 larged thick-walled cells with colorless contents; their most obvious function is to 

 furnish breaking points for the filaments. They are believed to be variants of the 

 arthrospores; they have been seen to germinate and give rise to normal filaments. 

 Ten families may be distinguished as follows: 

 1. Cells dividing strictly in one plane; branch- 

 ing none or of the "false" type. 



2. The filaments not branching nor taper- 

 ing nor producing spores or heterocysts. 

 3. Filaments elongate. 



4. Pigmented, blue-green Family 1. Oscillatoriacea. 



4. Colorless organisms accumu- 

 lating sulfur Family 2. Beggiatoacea. 



3. Filaments reduced to single cells 



which reproduce by budding Family 3. Chamaesiphonacea. 



2. Filaments branching or tapering, or pro- 

 ducing spores or heterocysts, or showing 

 several of these characters. 

 3. Filaments not tapering. 



4. Filaments not branching Family 4. Nostocacea. 



4. Filaments branching. 



5. Blue-green algae mostly 



producing heterocysts Family 5. Scytonematacea. 



5. Minute colorless filaments 



without heterocysts Family 6. Chlamydotrichacea. 



3. Filaments tapering Family 7. Rivulariacea. 



1. Cells dividing in more planes than one, usual- 

 ly after a preliminary filamentous phase. 

 2. Pigmented, blue-green. 



3. Producing extensive filaments with 



heterocysts Family 8. Sirosiphonacea. 



