In all species, with the exception of _Cl. solida_, which remains solid during the life of the plant, a central cavity arises while the podetium is still quite short (about 1 to 1·5 mm. in _Cl. pyxidata_ and _Cl. degenerans_). The first indication of the opening is a narrow split in the internal cylinder, due to the difference in growth tension between the more free and rapid increase of the external medullary layer and the slower elongation of the chondroid tissue at the centre. The cavity gradually widens and becomes more completely tubular with the upward growth of the podetium; it is lined by the chondroid sclerotic band which supports the whole structure (Fig. 67). _b._ GONIDIAL TISSUE. In most species of Cladoniaceae, a layer of gonidial tissue forms a more or less continuous outer covering of the podetium, thus distinguishing it from the purely hyphal stalks of the apothecia in Caliciaceae. Even in the genus _Baeomyces_, while the podetia of some of the species are without gonidia, neighbouring species are provided with green cells on the upright stalks clearly showing their true affinity with the _Cladoniae_. In one British species of _Cladonia_ (_Cl. caespiticia_) the short podetium consists only of the fibrous chondroid cylinder, and thus resembles the apothecial stalk of _Baeomyces rufus_, but in that species also there are occasional surface gonidia that may give rise to squamules. [Illustration: Fig. 67. _Cladonia squamosa_ Hoffm. Vertical section of podetium with early stage of central tube and of podetial squamules × 100 (after Krabbe).] Krabbe[394] concluded from his observations that the podetial gonidia of _Cladonia_ arrived from the open, conveyed by wind, water or insects from the loose soredia that are generally so plentiful in any _Cladonia_ colony. They alighted, he held, on the growing stalks and, being secured by the free-growing ends of the exterior hyphae, they increased and became an integral part of the podetium. In more recent times Baur[395] has recalled and supported Krabbe’s view, but Wainio[396], on the contrary, claims to have proved that in the earliest stages of the podetium the gonidia were already present, having been carried up from the gonidial zone of the primary thallus by the primordial hyphae. Increase of these green cells follows normally by cell-division or sporulation. 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.