NOSTOCEJ5. 



221 



Certain genera of Nostocliincce have the filaments constantly simple 

 (IVostoc, Analaena), others have the filaments frequently provided with 

 lateral branches (Scytonemece, Galotricliece). Sometimes these rami- 

 fications are normal, and result from the regular division of the 

 trichome (Rivtilaria, &c.), sometimes accidental and irregular. These 

 produce themselves when the trichome has broken, they grow and issue 

 laterally from the sheath, when it is not rare to observe the base of the 

 filaments of certain I/yngbya, or, again, when the hormogones are not able 

 to escape freely, they germinate in the interior of the sheath. Indeed, 

 it is very frequent that the germinations develope themselves in fila- 

 ments of all ages, but in that case the young filaments are not at all 

 enveloped by the general sheath of the filament that bears them. 



Nostocs should be dried as quickly as possible after they are collected, 

 otherwise the trichomes escape and the sheaths are left empty. Thuret, 

 in Ann. des Sci. Nat. 6 ser., Vol. i.,pp. 372-382 (1875). 



The following is a dichotomous key to the classification as proposed by 

 M. Thuret: 



rFilaments tapering at the top into a hyaline 



1 < hair ........ 



( Filaments always destitute of an apical hair . 



("Filaments in which some of the cells change 



2 -< into heterocysts ...... 



(.Filaments without heterocysts 



o j" Filaments with lateral ramifications 

 Filaments without ramifications 



/ 

 \ 



Filaments immersed in a gela- 

 tinous mucilage of a determi- 

 4 NOSTOCE.E < nate shape .... 



/Filaments free or immersed in an 

 v amorphous diffluent mucilage 



C Filaments consisting of a coloured trichome 



enclosed in a hyaline sheath 

 1 Filaments without a sheath distinct from the 

 [_ trichome ....... 



1. Heterocysts scattered in the trichome. 



A. Spores originating in cells not adjoin- 



ing the heterocysts .... 



/3 Trichomes united in small float- 

 ing bundles .... 



B. Spores originating in cells placed on 



each side of the heterocysts 



2. Heterocysts terminal (at both ends of the trichome). 

 C. Spores originating in cells placed just 



12 



2 



3 



7 



10 



4 



Nostoc. 

 . 5 



6 

 A. B. C. 



Anabaena. 

 Aphanizomefion. 



below the heterocysts 



isc-like. Spores very 

 cysts placed at regular 

 .Cells longer than broad. Spores none 



C Cells disc-like. Spores very prominent. He- 

 6 < terocysts placed at regular intervals . 

 (. 



(Filaments spirally twisted . 



< -n i -11 / i 



( -bilauieuts not spirally twisted 



Cij lin drospermum. 



ffbdularia. 



Microohcete-. 



Spirulina. 



... o 



