In three other genera with upright fruticose thalli, Sphaerophorus, Argopsis and Stereocaulon, rigidity is maintained by a medulla approaching the chondroid type. In Sphaerophorus the species may have either flattened or cylindrical branching stalks, but in all of them, the centre is occupied by longitudinal strands of hyphae. Argopsis, a monotypic genus from Kerguelen, has a cylindrical branching thallus with a strong solid axis; it is closely allied to Stereocaulon, a genus of familiar moorland lichens. The central tissue of the stalks in Stereocaulon is also composed of elongate, thick-walled conglutinate hyphae, formed into a strand which is, however, not entirely solid.
C. Survey of Mechanical Tissues
Mechanical tissues scarcely appear among fungi, except perhaps as stoutish cartilaginous hyphae in the stalks of some Agarics (Collybiae, etc.), or as a ring of more compact consistency round the central hyphae of rhizomorphic strands. It is practically a new adaptation of hyphal structure confined to lichens of the fruticose group, where there is the same requirement as in the higher plants for rigidity, flexure and tenacity.
Rigidity is attained as in other plants by groups or strands of mechanical tissue situated close to the periphery, as they are so arranged in Ramalina and Cladonia; or the same end is achieved by a strongly developed fastigiate cortex as in Roccella. Bending strains to which the same lichens are subjected, are equally well met by the peripheral disposition of the mechanical elements.
Tenacity and elasticity are provided for in the pendulous forms either by a fibrous cortex as in Alectoria, or by the chondroid axis in Usnea. Haberlandt[379] has recorded some interesting results of tests made by him as to the stretching capacity of a freshly gathered pendulous species in which the central strand was from ·5 to 1 mm. thick. He found he could draw it out 100 to 110 per cent. of its normal length before it gave way. In an upright species the frond broke when stretched 60 to 70 per cent. In both of the plants tested, the central strand retained its elasticity up to 20 per cent. of stretching. The outer cortical tissue was cracked and broken in the experiments. Schulte[380] calculated somewhat roughly the tenacity of Usnea longissima and found that a piece of the main axis 8 cm. long carried up to 300 grms. without breaking.
D. Reticulate Fronds
In the upright radiate thallus, more especially among the Ramalinae, though also among Cladoniae[381], there has appeared a reticulate thallus resulting from the elongate splitting of the tissues, and due to unequal growth tension and straining of the gelatinous cortex when swollen with moisture. In several species of Ramalina, the strap-shaped frond is hollow in the centre; and strands of strengthening fibres give rise to a series of cortical ridges. The thinner tissue between is frequently torn apart and ellipsoid openings appear which do not however pierce beyond the central hollow. Such breaks are irregular and accidental though occurring constantly in Ramalina fraxinea, R. dilacerata, etc.
A more complete type of reticulation is always present in a Californian lichen, Ramalina reticulata, in which the large flat frond is a delicate open network from tip to base ([Fig. 64]). It grows on the branches of deciduous trees and hangs in crowded tufts up to 30 cm. or more in length. Usually it is so torn, that the real size attainable can only be guessed at. It is attached at the base by a spreading discoid hold-fast, and, in mature plants, consists of a stoutish main axis from which side branches are irregularly given off. These latter are firm at the base like the parent stalk, but soon they broaden out into very wide fronds. Splitting begins at the tips of the branches while still young; they are then spathulate in form with a slightly narrower recurved tip, below which the first perforations are visible, small at first, but gradually enlarging with the growth of the frond.
