Nostoc Vauch. Composed of flexuous trichomes, with intercalary heterocysts (colourless cells) ([Fig. 18]). Dense gelatinous colonies of definite form are built up by cohesion. In some lichens the trichomes retain their chain-like appearance, in others they are more or less broken up and massed together, with disappearance of the gelatinous sheath (as in Peltigera); colour mostly dark blue-green.
Fig. 18. Examples of Nostoc. N. Linckia Born. A, nat. size; B, small portion × 340; C, N. coerulescens Lyngbye, nat. size (after West).
Fig. 19. Example of Scytonema alga. S. mirabile Thur. C, apex of a branch; D, organ of attachment at base of filament. × 440 (after West).
Nostoc occurs in a few or all of the genera of Pyrenidiaceae, Collemaceae, Pannariaceae, Peltigeraceae and Stictaceae, and N. sphaericum Vauch. (N. lichenoides Kütz.) has been determined as the lichen gonidium. When the chains are broken up it has been wrongly classified as another alga, Polycoccus punctiformis.
Fam. Scytonemaceae. Trichomes of single-cell rows, differentiated into base and apex. Pseudo-branching arises at right angles to the main filament.
Scytonema Ag. Pseudo-branches piercing the sheath and passing out as twin filaments ([Fig. 19]); colour, golden-brown. This alga occurs in genera of Pyrenidiaceae, Ephebaceae, Pannariaceae, Heppiaceae, in Petractis a genus of Gyalectaceae, and in Dictyonema one of the Hymenolichens.
Fam. Stigonemaceae. Trichomes of several-cell rows with base and apex; colour, golden-brown.
Stigonema Ag. Stouter than Scytonema, with transverse and vertical division of the cells, and generally copious branching ([Fig. 20]). This alga occurs only in a few genera of Ephebaceae. S. panniforme Kirchn. (Sirosiphon pulvinatus Bréb.) has been determined as forming the gonidium.