THE THALLUS 285 



phylum, there is but little difference in form between the thalline warts of 

 the lichen crust and the original small Nostoc colonies such as are to be 

 found on damp mosses, etc. 



In Collema itself, the less advanced species are scarcely more than crusts, 

 though the more developed show considerable diversity of lobes, either short 

 and pulpy, or spreading out in a thin membrane. The Nostoc chains pervade 

 the homoiomerous thallus, but in some species they lie more towards the 

 upper surface. There is no cortex, though once and again plectenchyma 

 appears in the apothecial margin, both in this genus and in Leprocollema 

 which is purely crustaceous. 



Leptogium is a higher type than Collema, the thallus being distinguished 

 by its cellular cortex. The tips of the hyphae, lying close together at the 

 surface, are cut off by one or more septa, giving a one- or several-celled 

 cortical layer. The species though generally homoiomerous are of thinner 

 texture and are less gelatinous than those of Collema. 



c. PYRENIDIACEAE. This small family of pyrenocarpous Phycolichens 

 may be considered here though its affinity, through the form of the fruiting 

 body, is with Archilichens. The gonidia are species of Nostoc, Scytonema 

 and Stigonema. There are only five genera; one of these, Eolichen, contains 

 three species, the others are monotypic. 



The crustaceous genera have a non-corticate thallus, but an advance to 

 lobate form takes place in Placothelium, an African genus. The two genera 

 that show most development are both British: Coriscium (Normandina), 

 which is lobate, heteromerous and corticate though always sterile and 

 Pyrenidium which is fruticose in habit-; the latter is associated with Nostoc 

 and forms a minute sward of upright fronds, corticate all round ; the peri- 

 thecium is provided with an entire wall and is immersed in the thallus. 



If the thallus alone were under consideration these lichens would rank 

 with Pannariaceae. 



d. HEPPIACEAE AND PANNARIACEAE. The next stage in the develop- 

 ment of Phycolichens takes place through the algae, Scytonema and Nostoc, 

 losing not only their gelatinous sheaths, but also, to a large extent, their 

 characteristic forms. Chains of cells can frequently be observed, but accurate 

 and certain identification of the algal genus is only possible by making 

 separate cultures of the gonidia. 



Scytonema forms the gonidium of the squamulose Heppiaceae consisting 

 of the single genus Heppia. The ground tissue of the species is either 

 wholly of plectenchyma with algae in the interstices, or the centre is occupied 

 by a narrow medulla of loose filaments. 



In the allied family Pannariaceae, a number of genera contain Scytonema 

 or Nostoc, while two, Psoroma and Psoromaria, have bright-green gonidia. 



