E. Perforations and Reticulation of the Podetium
Perforations in the podetial wall at the axils of the branches are constant in certain species such as Cl. rangiferina, Cl. uncialis, etc. They are caused by the tension of the branches as they emerge from the main stalk. A tearing of the tissue may also arise in the base of the scyphus, due to its increase in size, which causes the splitting of the diaphragm at the bottom of the cup.
Among the Cladoniae the reticulate condition recurs now and again. In our native Cladonia cariosa the splitting of the podetial wall is a constant character of the species, the carious condition being caused by unequal growth which tears apart the longitudinal fibres that surround the central hollow.
A more advanced type of reticulation arises in the group of the Clathrinae in which there is no inner chondroid cylinder. In Cladonia aggregata, in which the perforations are somewhat irregular, two types of podetia have been described by Lindsay[402] from Falkland Island specimens: those bearing apothecia are short and broad, fastigiately branched upwards and with reticulate perforations, while podetia bearing spermogonia are slender, elongate and branched, with fewer reticulations. An imperfect network is also characteristic of Cl. Sullivani, a Brazilian species. But the most marvellous and regular form of reticulation occurs in Cl. retepora, an Australian lichen ([Fig. 71]): towards the tips of the podetia the ellipsoid meshes are small, but they gradually become larger towards the base. In this species the outer tissue, though of parallel hyphae, is closely interwoven and forms a continuous growth at the edges of the perforations, giving an unbroken smooth surface and checking any irregular tearing. The enlargement of the walls is solely due to intercalary growth. The origin of the reticulate structure in the Clathrinae is unknown, though it is doubtless associated with wide podetia and rendered possible by the absence of an internal chondroid layer. The reticulate structure is marvellously adapted for the absorption of water: Cl. retepora, more especially, imbibes and holds moisture like a sponge.
Fig. 71. Cladonia retepora Fr. From Tasmania.
F. Rooting Structures of Cladoniae
The squamules of the primary thallus are attached, as are most squamules, to the supporting substance by strands of hyphae which may be combined into simple or branching rhizinae and penetrate the soil or the wood on which the lichen grows. There is frequently but one of these rooting structures and it branches repeatedly until the ultimate branchlets end in delicate mycelium. Generally they are grey or brown and are not easily traced, but when they are orange-coloured, as according to Wainio[403] they frequently are in Cladonia miniata and Cl. digitata, they are more readily observed, especially if the habitat be a mossy one.
In Cl. alpicola it has been found that the rooting structure is frequently as thick as the podetium itself. If the podetium originates from the basal portion of the squamule, the hyphae from the chondroid layer, surrounding the hollow centre, take a downward direction and become continuous with the rhizoid. Should the point of insertion be near the apex of the squamule, these hyphae form a nerve within the squamule or along the under surface, and finally also unite with the rhizoid at the base, a form of rooting characteristic of Cl. cartilaginea, Cl. digitata and several other species.
Mycelium may spread from the rhizinae along the surface of the substratum and give rise to new squamules and new tufts of podetia, a method of reproduction that is of considerable importance in species that are generally sterile and that form no soredia.