LICHENS. 313 



the thallus, the Alga not being restricted to a definite zone 

 as in the heteromerous Lichens ; (4) the thallus is much 

 folded, owing to inequalities in growth and to the inter- 

 relation between the Fungus and the Alga ; (5) the thallus 

 contains Nostoc as the Alga constituent, and therefore has 

 a bluish tint, varying from greyish blue to almost black ; 

 (6) soredia are rarely produced owing doubtless to the 

 difficulty of enclosing the Algal jelly by the hyphae. 



455. Structure of Nostoc. In order to understand 

 the structure of Collema, that of Nostoc should be exam- 

 ined. The species of Nostoc grow in rounded, or flattened 

 and often lobed, olive-green gelatinous masses, either on 

 wet ground, among mosses, on stones in streams, or floating 

 in the water of streams and ditches. The plant is soft 

 and gelatinous when wet, brittle when dry. Mount a small 

 specimen of Nostoc, or a section of a large one, in water, 

 and note the irregularly contorted filaments which are 

 embedded in the gelatinous matrix ; in each filament note 

 (1) the single row of rounded cells with bluish-green 

 contents, interrupted at intervals by (2) larger cells, the 

 heterocysts, with thicker walls and transparent colourless 

 contents. 



456. Structure of Collema. Now examine some 

 species of Collema, of which C. pulposum is one of the 

 commonest, growing on moist soil, stones, old walls, and 

 among Mosses ; another species grows on tree trunks in 

 damp woods, with a thin dark-coloured thallus. Like 

 Nostoc itself, Collema is thin and brittle when dry, soft 

 and pulpy and gelatinous when wet. If fertile, it is easily 

 distinguished from Nostoc by the apothecia. 



Cut sections, and note (1) the chains of blue-green 

 Nostoc cells, with heterocysts at intervals ; (2) the trans- 

 parent gelatinous matrix ; (3) the branching colourless 

 septate narrow Fungus hyphae ; (4) that though the thal- 

 lus is more nearly " homoiomerous " than in any other 

 Lichens, the Nostoc chains are more abundant towards the 

 upper surface ; (5) there is no definite cortex on the surface 

 cortex is present, however, in other Collemaceae, which 



