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