224 NEMATOGENJ3. 



{ 



Spores originating in the lower part of the 



trichome ........ GlccotricJia. 



Trichomes never producing any spores . . . . .A. 



A. Frond having a tendency to an 

 hemispherical or bladdery form. 

 Filaments exhibiting a disposition 

 to radiate from the base of the 

 frond Rivularia. 



/3 Frond flat. Filaments erect, 



parallel Isaetis. 



Sab-tribe I. PSILONEMEJE. Filaments not attenuated into a 



hair-like extremity. 



FAMILY I. NOSTOCB^E. 



Trichomes furnished with heterocysts, involved in a very 

 copious gelatin, more or less firm or diffluent, which is collected 

 into a variously expanded, or very often indefinite thallus, or 

 rarely with the mucilage quickly dissolved, subsolitary. Borzi 

 A!g. Fico. p. 279. 



GEXUS 94. NOSTOC. PaucJi. (1803.) 



Thallus gelatinous or membranaceons, girt by a more or less 

 firm periderm, definite, globose, or variously expanded. Tri- 

 chomes flexuously curved, irregularly interlaced, now and then 

 vaginate, joints globose or elliptical, distinct, or more or less 

 closely connected. Heterocysts terminal or intercalated, larger 

 or equal to the other cells. Spores equal to the heterocysts, or 

 a little larger, green, becoming bluish, olivaceous, or yellowish 

 brown. 



The Nostocs consist of a more or less firm jelly, in which beaded fila- 

 ments are imbedded, consisting of chains of small, somewhat globose 

 simple cells. These filaments or trichomes are usually surrounded by a 

 sheath, which is often so delicate as scarcely to be visible, or it is almost 

 obsolete. The frond or thallus may be globose, discoid, lotted, or 

 irregular, with a more or less distinct outer layer forming a, kind of 

 epidermis. 



At irregular distances in the trichomes are larger cells, or heterocysts, 

 formerly regarded as spermatia, which differ in colour from the other 

 cells of the trichome. Individual cells become heterocysts uninfluenced 

 by any definite law at present demonstrated. 



Increase in the filaments is caused by division of the cells in the longi- 

 tudinal direction, whereby the trichome is constantly being lengthened, 

 and new cells added, which lie in the mucilage. 



Thuret has explained the process by which new plants originate from 



