PDF - CES (IISc)

RADIATE THALLUS 105
In Letharia (L. vulpina, etc.) the structure is midway between Ramalina
and Usnea : the central axis is either a solid strand of chondroid hyphae or
several separate strands.
Fig. 63 B. Usnea lo ngissima Ach. A, transverse section of fibril x 85. B, a, chondroid axis;
b, gonidial tissue; c, cortex x 525 (after Schulte).
In three other genera with upright fruticose thalli, Sphaerophorus, Ar-
gopsis 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 Ker-
guelen, 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 Rama-
lina and Cladonia; or the same end is achieved by a strongly developed