Key to the Families 



1. Heterocysts lacking; trichomes with or witliout a sheath; sometimes 



with several trichomes within a single sheath oscillatoriaceae 



1. Heterocysts present (rarely absent in some genera which are not 

 represented in om' collections; see Anipliithrix) 2 



2. Trichomes exhibiting true branching (branches arising as a 



result of cell division in 2 planes) - 3 



2. Trichomes exhibiting false branching, or without branches 4 



3. Trichomes and lateral branches uniseriate; heterocysts lateral, 



usually at the ends of short branches; sheaths confluent ^'nostochopsaceae 



3. Trichomes and lateral branches multiseriate, or if uniseriate with 



thick, firm sheaths which are not confluent stigonemataceae 



4. Trichomes decidedly tapering toward the apex, usually from a 



basal heterocyst rivulariaceae 



4. Trichomes not tapering toward the apex 5 



5. Trichomes with a definite, firm sheath; false branching 



frequent - scytonemataceae 



5. Trichomes without a sheath, or with only a thin one; no 



branching - nostocaceae 



Suborder Homocystineae 



Simple trichomes, without cellular differentiations, with dissepi- 

 ments (sometimes granular) except in the unicellular genus Spiru- 

 lina, and usually sheathed. The sliding, spiral, or oscillating move- 

 ment shown by most members is characteristic of this suborder but 

 not confined to it. Trichomes reproduce by fission or by fragmenta- 

 tion (to form hormogonia), sometimes breaking by means of dead 

 cells ( necridia ) , into sections that then increase in length by fission. 

 The suborder comprises 1 family. 



FAMILY OSCILLATORIACEAE 



Key to the Genera 



1. Trichomes spiralled or regularly coiled 2 



1. Trichomes straight, bent, or entangled, but not regularly coiled 4 



2. Trichomes vidth a sheath Lyngbya (in part) 



2. Trichomes without a sheath — 3 



3. Trichomes 1-celled, usually relatively short - Spirulina 



3. Trichomes much longer, many-celled, with the 



dissepiments sometimes indistinct .— Arthrospira 



4. Trichomes without an evident sheath (though hormogonia may 

 sometimes show a thin sheath) Oscillatoria 



4. Trichomes with an evident sheath 5 



5. Sheath containing a single trichome 6 



5. Sheath containing several to many trichomes - 9 



6. Sheaths firm and definite, not sticking to or confluent 



with sheaths of other plants Lyngbya (in part) 



6. Sheaths mucous and sticky, confluent with those 



of other plants, often indefinite ' 



[478] 



