STRATOSE THALLUS 75 



in the rock. It has now been demonstrated that a number of h'chens, 

 formerly described as athalline, possess a considerable vegetative body 

 which cannot be examined until the limestone in which they are embedded 

 is dissolved by acids. One such species, Petractis {Gyalectd) exanthematica, 

 studied by Steiner^ and later by Fiinfstiick^, is associated with the blue- 

 green filamentous alga, Scytonema, and is homoiomerous in structure, the 

 alga growing through and permeating the whole of the embedded thallus. 

 A partly homoiomerous thallus, associated with Trentepohlia, has been 

 described by Bachmann'' He found the bright-yellow filaments of the 

 alga, covering the surface of a calcareous rock. By reason of their apical 

 growth, they pierced the rock and dissolved a way for themselves, not only 

 among the loose particles, but right through a clear calcium crystal reaching 

 generally to a depth of about 200 /*, though isolated threads had gone 3SO/a 

 below the surface. Near the outside the tendency was for the algae to 

 become stouter and to increase by intercalary growth and by budded yeast- 

 like outgrowths; lower down they were somewhat smaller. The hyphae 

 that became united with the algae were unusually slender and were charac- 

 terized by frequent anastomoses. They closely surrounded the gonidia 

 and also filled the loose spaces of the limestone with their fine thread-like 

 strands. Though oil was undoubtedly present in the lower hyphae there 

 were no swollen nor sphaeroid cells*. Some interesting experiments with 

 moisture proved that the part of the rock permeated with the lichen 

 absorbed much more water and retained it longer than the part that was 

 lichen-free. 



Generally the embedded tissues follow the same order as in other 

 crustaceous lichens : an upper layec of cortical hyphae, next a gonidial 

 zone, and beneath that an interlaced tissue of medullary or rhizoidal hyphae 

 which often form fat-cells'. Friedrich^ has given measurements of the 

 immersed thallus oi Lecanora {Biatorelld) simplex: under a cortical layer of 

 hyphae there was a gonidial zone 600-700/a thick, while the lower hyphae 

 reached a depth of 12 mm.; he has also recorded an instance of a thallus 

 reaching a depth of 30 mm. 



On siliceous rocks such as granite, rhizoidal hyphae penetrate the rock 

 chiefly between the thin separable flakes of mica. Bachmann"= has recog- 

 nized in these conditions three distinct series of cell-formations: (i) slender 

 long-celled sparsely branched hyphae which form a network- by frequent 

 anastomoses; (2) further down, though only occasionally, hyphae with 

 short thick-walled bead-like cells; and (3) beneath these, but only in or 

 near mica crystals, spherical cells containing oil or some albuminous 

 substance. 



^ Steiner 1881. ^ Funfstuck 1899. " Bachmann 1913. 



* See p. IIS. " Friedrich 1906. * Bachmann 1907. 



