The upright podetium, as described by Wainio[392] and by Krabbe[393], is a secondary product of the basal granule or squamule. It is developed from the hyphae of the gonidial zone, generally where a crack has occurred in the cortex and rather close to the base or more rarely on or near the edge of the squamule (Cl. verticillata, etc.). At these areas, certain meristematic gonidial hyphae increase and unite to form a strand of filaments below the upper cortex but above the gonidial layer, the latter remaining for a time undisturbed as to the arrangement of the algal cells.
This initial tissue—the primordium of the podetium—continues to grow not only in width but in length: the basal portion grows downwards and at length displaces the gonidial zone, while the upper part as a compact cylinder forces its way through the cortex above, the cortical tissue, however, taking no part in its formation; as it advances, the edges of the gonidial and cortical zones bend upwards and form a sheath distinguishable for some time round the base of the emerging podetium.
Even when the primary horizontal thallus is merely crustaceous, the podetia take origin similarly from a subcortical weft of hyphae in an areola or granule.
B. Structure of the Podetium
a. General structure. In the early stages of development the podetium is solid throughout, two layers of tissue being discernible—the hyphae forming the centre of the cylinder being thick-walled and closely compacted, and the hyphae on the exterior loosely branching with numerous air-spaces between the filaments.
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.
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.