224 NEMATOGENA, 
trichome * é * % F - Gleotricha. 
{ Spores originating in the lower part of the 
14 : . 
Trichomes never producing any spores . e : . . A. 
A. Frond having a tendency to an 
hemispherical or bladdery form. 
Filaments exhibiting a disposition 
to radiate from the base of the 
frond . : . 2 5 . Rivularia. 
8B Frond flat. Filaments erect, 
parallel : : - . : . Isaetis. 
Sab-tribe I. Pstnonemex. Filaments not attenuated into a 
hair-like extremity. 
Famity I. NOSTOCEA. 
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 
Alg. Fico. p. 279. 
Genus 94. NOSTOC. Vauch. (1803.) 
Thallus gelatinous or membranaceous, 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 ¢richomes 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, lobed, 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 
