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104
MORPHOLOGY
sometimes as a continuous ring which may project irregularly into the pith
it is frequently in the form of strands or bundles which
(R. calicaris) ; more
alternate with the groups of gonidia (R. siliquosa, R. Curnozvii, etc.). In
R. fraxinea these strands may be scarcely discernible in young fronds, though
sometimes already well developed near the tips. Occasionally isolated strands
of fibres appear in the pith (R. Curnowii\ or the sclerotic projections may
even stretch across the pith to the other side (R. strepsilis} (Fig. 75 B).
In the Cladoniae support along with flexibility is secured to the upright
that form round the
podetium by the parallel closely packed hyphae
hollow cylinder a band called the "chondroid" layer from its cartilage-like
consistency.
b. CHONDROID AXIS. The central medullary tissue in Ramalina is, with
few exceptions, a loose arachnoid structure ; often the fronds are almost
hollow. In one species of Usnea, U. Taylori, found in polar regions, there
is a similar loose though very circumscribed medullary and gonidial tissue
in the centre of the somewhat cylindrical thallus, and a wide band of sclerotic
fibres towards the cortex.
Fig. 63 A.
branch.
longissi
A, (Jinea barbata Web. Longitudinal section of filament with young adventitious
, chondroid axis; /;, gonidial tissue; c, cortex, x too (after Schwendener). B, U.