STRATOSE THALLUS 77 



rocks is only a starved condition of Sarcogyne (Biatorelld) simplex, which on 

 calcareous rocks, though with a broader gonidial zone, has, as noted above, 

 a correspondingly much larger hyphal tissue. 



Stahlecker's theory is that the hyphae require more energy to grow in 

 the acid conditions that prevail in siliceous rocks, and therefore they make 

 larger demands on the algal symbionts. It follows that the latter must be 

 stimulated to more abundant growth than in circumstances favourable to 

 the fungus, such as are found in basic (calcareous) rocks; he concludes that 

 on the acid (siliceous) rocks, the epilithic or superficial condition is not only 

 a physical but a biological necessity, to enable the algae to grow and 

 multiply in a zone well exposed to light with full opportunity for active 

 photosynthesis and healthy increase. 



C. CORTICOLOUS LICHENS 



The crustaceous lichens occurring on bark or on dead wood, like those 

 on rocks, are either partly or wholly immersed in the substratum (hypo- 

 phloeodal), or they grow on the surface (epiphloeodal); but even those with 

 a superficial crust are anchored by the lower hyphae which enter any crack 

 or crevice of wood or bark and so securely attach the thallus, that it can 

 only be removed by cutting away the underlying substance. 



a. EPIPHLOEODAL LICHENS. These lichens originate in the same way 

 as the corresponding epilithic series from soredia or from germinating 

 spores, and follow the same stages of growth; first a hypothallus with 

 subsequent colonization of gonidia, the formation of granules, areolae, etc. 

 The small compartments are formed as primary or secondary areolae; the 

 larger spaces are marked out by the encounter of hypothalli starting from 

 different centres. 



The thickness of the thallus varies considerably according to the species. 

 In some Pertusariae with a stoutish irregular crust there is a narrow 

 amorphous cortical layer of almost obliterated cells, a thin gonidial zone 

 about 35 /A in width and a massive rather dense medulla of colourless 

 hyphae. Darbishire 1 has described and figured in Va'ricellaria microsticta, 

 ' one of the Pertusariaceae, single hyphae that extend like beams across the 

 wide medulla and connect the two cortices. In some Lecanorae and Lecideae 

 there is, on the contrary, an extremely thin thallus consisting of groups of 

 algae and loose fungal filaments, which grow over and between the dead 

 cork cells of the outer bark. On palings, there is often a fairly substantial 

 granular crust present, with a- gonidial zone up to about 80 /u, thick, while 

 the underlying or medullary hyphae burrow among the dead wood fibres. 



1 Darbishire 1897. 



