PDF - CES (IISc)

MORPHOLOGY
The development of the isidial outgrowth has been described by Rosen-
dahl 1 in several species of Parmelia. In one of them, P. papulosa, which has
a cortical layer one cell thick, the isidium begins as a small swelling or wart
on the upper surface of the thallus. At that stage the cells of the cortex
have already lost their normal arrangement and show irregular division.
They divide still further, as gonidia and hyphae push their way up. The
full-grown isidia in this species are cylindrical or clavate, simple or branched.
They are peculiar in that they bear laterally
here and there minute rhizoids, a development
not recorded in any other isidia. The inner
tissue accords with that of the normal thallus
and there is a clearly marked cortex, gonidial
zone and pith. A somewhat analogous development
takes place in the isidia of Parmelia pro-
boscidea; in that lichen they are mostly prolonged
into a dark-coloured cilium.
In Parmelia scortea the cortex is several
cells thick, and the outermost rows are com-
pressed and dead in the older parts of the
thallus; but here also the first appearance of
the isidium is in the form of a minute wart.
The lower layers (4 to 6) of living cortical cells
divide actively; the gonidia also share in the
new growth, and the protuberance thus formed
pushes off the outer dead cortex and Fig. 85. Vertical section of isidia of
Parmelia scortea Ach. A, early
:
stage; 60 (after
emerges
as an isidium (Fig. 85). They are always rather
stouter in form than those of P. papulosa and
B. later stage,
Rosendahl).
may be simple or branched. The gonidia in this case do not form a
definite zone, but are scattered through the pith of the isidium.
Here also should be included the coralloid branching isidia that adorn
the upper surface and margins of the thallus of Umbilicaria pusttilata.
They begin as small tufts of somewhat cylindrical bodies, but they sometimes
broaden out to almost leafy expansions with crisp edges. Most
frequently they are situated on the bulging pustules where intercalary
growth is active. Owing to their continued development on these areas,
the tissue becomes slack, and the centre of the isidial tuft may fall out,
leaving a hole in the thallus which becomes still more open by the tension
of thalline expansion. New isidia sprout from the edges of the wound and
the process may again be repeated. It has been asserted that these structures
are only formed on injured parts of the thallus something like gallformations
but Bitter 2 has proved that the wound is first occasioned by
the isidial growth weakening the thallus.
1 Rosendahl 1907.
2 Bitter 1899.