NOSTOCE.K. 



225 



fragments of the trichome which he terms hormogones. The mucilage of 

 old plants being softened, portions of the threads which lie between the 

 heterocysts are detached and escape from the mucilage, whilst the hetero- 

 cysts remain behind. These escaped fragments become endowed with 

 motion, similar to that observed in Osdllaria. The cells of the horrno- 

 gone increase by division at right angles to the filament, and ultimately 

 separate longitudinally, becoming the centres of new plants. 



Besides the reproduction by hormogones, certain special cells of the 

 trichome enlarge and become converted into spores, which germinate and 

 produce new plants, but no evidence of sexual reproduction has yet been 

 adduced. 



Messrs Bornet and Thuret have subdivided the genus Nostoc into 

 eight groups, of which two are not represented in Britain. The 

 following is their synopsis, with the British species printed in small 

 capitals. 



I. INTKICATA. Aquatic species. Fronds soft, gelatinous without deter- 

 minate form, often floating. 



A. Trichomes forming irregular masses, de- 



prived of the general mucilage. 

 Circumvolutions of trichome compact and 



indistinct ....... 



Circumvolutions of trichome distinct . 



B. Trichomes involved in mucilage more or 

 less abundant. 



a Trichomes flexuous, aggregated ; joints 

 short and close together; sheaths uc- 

 coloured, very refractive 



/3 Trichomes loosely interwoven, joints 

 of equal diameter, rather distant. 



1. Spores subglobose. 



2. Spores oval. 



* Mucilage soft ; sheath none, 



indistinct, or uncoloured 

 ** Mucilage firm ; sheaths and 



1. hederula. 



2. tenuissimum. 



3. LINCKIA. 



4. PISCINALE. 



5. CARNEUM. 



mucilage tinged with yellow at 



the periphery . . . .6. nvulare. 



II. GELATINOSA. Fronds soft and gelatinous ; adherent ; joints of 

 tiichotnes cylindrically elongated in the young filaments. Spores 

 oblong, large. 



A. Growing in watery or inundated places; 

 fronds thick, deformed. 



a Trichomes heterogenous, composed of 

 two sorts of joints, one cylindrical, the 

 other cask-shaped or compressed spheri- 

 cal ..... 



)8 Trichomes homogenous 



B. Plant terrestrial. Frond plane, applied to 

 the ground by the inferior surface 



7. SPONGIJEFORME. 



8. yelatinosum. 



9. ELLIPSOSPORUM. 



III. HUMIFUSA. Terrestrial species. Fronds at first globose, then con- 

 fluent, and forming gelatinous cushions adhering to the substratum by 

 their lower face. Spores smooth. 



A. Fronds in orbicular discs, or indefinite and 

 continuous. 



