264 NEMATOGEN&. 
Plectonema Kirchneri, Cooke, Grevillea xt., p. 75. 
At first attached, but soon floating, and forming subglobose 
se | 
woolly tufts, of a dark blaish green, changing to olivaceous. 
Tufts from half an inch to an inch in diameter. Trichomes 
radiating, with simple (rarely germinate) branches ; joints one 
third or one fourth as long as broad. 
Sizz. Filaments -012--015 mm. diam., with sheath. 
Plectonena mirabile, Kirch. Alg. Schles. p. 229 (scarcely of 
Thuret). 
In ornamental water. Pleasure grounds, Kew. 
The filaments are much thinner than in P. mirabdile and the branches 
issue singly from the sheath, and not in pairs, as in that species. From 
the dimensions given by Kirchner it seems probable that this is the 
species to which he has given the name of P. mirabile. 
Plate CIV. fig. 2. au, tuft, natural size. 6, trichomes & 400. 
FAMILY III. SCYTONEME. 
Filaments with lateral ramifications in which some of the 
cells change into heterocysts. 
This family is divisible into two sections according to the direction 
of the multiplication’ of the cells. 
* Cells only multiplying in the direction of the length of the filament. 
Scytonema, Petalonema, Symphyosiphon, Tolypothrix, 
Cystocoleus. 
** Cells multiplying as well in the direction of the breadth of the 
filament, at least where the branches, which are always produced by 
lateral multiplication, originate. 
Stigonema, Fischera, Haplosiphon. 
The last section includes many very doubtful forms, which will pro- 
bably, as their development becomes better known, be transferred to 
Lichens, of which they are presumably an imperfect condition, 
Genus 107, SCYTONEMA. 4g. (1824.) 
Sheath enclosing a single trichome, ramifications produced 
by the deviation of the trichome, which emerges from the side 
of the sheath. Rawifications usually geminate, produced by a 
fold of the trichome which ruptures outside of the sheath, and 
gives origin to two filaments given off at a right angle. 
Heterocysts scattered here and there in the trichome, without 
any evident relation to the ramifications. 
