Algal cells have been found to be common to different lichens, but in Cladoniae Chodat[397] claims to have proved by cultures that each species tested has a special gonidium, determined by him as a species of Cystococcus, which would render colonization by algae from the open much less probable. In addition, the fungal hyphae are specific, and any soredia (with their combined symbionts) that alighted on the podetium could only be utilized if they originated from the same species; or, if they were incorporated, the hyphae belonging to any other species would of necessity die off and be replaced by those of the podetium.
c. Cortical tissue. In some species a cortex of the decomposed type of thick-walled conglutinate hyphae is present, either continuous over the whole surface of the podetium, as in Cl. gracilis ([Fig. 68]), or in interrupted areas or granules as in Cl. pyxidata ([Fig. 69]) and others. In Cl. degenerans, the spaces between the corticated areolae are filled in by loose filaments without any green cells. Cl. rangiferina, Cl. sylvatica, etc. are non-corticate, being covered all over with a loose felt of intricate hyphae.
Fig. 68. Cladonia gracilis Hoffm. (S. H., Photo.).
Fig. 69. Cladonia pyxidata Hoffm. (S. H., Photo.).
In the section Clathrinae (Cl. retepora, etc.) the cortex is formed of longitudinal hyphae with thick gelatinous walls.
d. Soredia. Frequently the podetium is coated in whole or in part by granules of a sorediate character—coarsely granular in Cl. pyxidata, finely pulverulent in Cl. fimbriata. Though fairly constant to type in the different species, they are subject to climatic influences, and, when there is abundant moisture, both soredia and areolae develop into squamules on the podetium. A considerable number of species have thus a more or less densely squamulose “form” or “variety.”