PDF - CES (IISc)

9 2
MORPHOLOGY
retain the colour of the cortex and are greyish or whitish-grey, as in Physcia
ciliaris or in Physcia hispida (Fig. 1 10). They provide a yellow fringe to
the apothecia of Physcia chrysophthalma and a green fringe to those of
Usnea fiorida. They are dark-brown or almost black in Parmelia perlata
var. ciliata and in P. cetrata, etc. as also in Gyrophora cylindrica. The fronds
of Cetraria islandica and other species of the genus are bordered with short
spinulose brown hairs whose main function seems to be the bearing of
"pycnidia" though in many cases they are barren (Fig. 128).
Superficial cilia are more rarely formed than marginal ones, but they are
characteristic of one not uncommon British species, Parmelia proboscidea
(P. pilosella Hue). Scattered over the surface of that lichen are numerous
crowded groups of isidia which, frequently, are prolonged upwards as darkbrown
or blackish cilia. Nearly every isidium bears a small brown spot on
the apex at an early stage of growth. Similar cilia are sparsely scattered
over the thallus, but their base is always a rather stouter grey structure,
which suggests an isidial origin. Cilia also occur on the margin of the lobes.
As lichens are a favourite food of snails, insects, etc., it is considered
that these structures are protective in function, and that they impede, if
they do not entirely prevent, the larger marauders in their work of destruction.
b. RHIZINAE. Lichen rootlets are mainly for the purpose of attachment
and have little significance as organs of absorption. They have been noted
in only one crustaceous lichen, Varicellaria microsticta 1 an , alpine species
that spreads over bark or soil, and which is further distinguished by being
Fig. 53. Rhizoid of Parmelia exasferata Carroll (P. aspidota Rosend.). A, hyphae growing out
from lower cortex x 450. B, tip of rhizoid with gelatinous sheath x 335 (after Rosendahl).
provided with a lower cortex of plectenchyma. In foliose lichens they are
frequently abundant, though by no means universal, and attach the spreading
fronds to the support. They originate, as Schwendener 2
pointed out, from
the outer cortical cells, exactly as do the cilia, and are scattered over the
1 Darbishire 1897.
2 Schwendener 1860.