PDF - CES (IISc)

REPRODUCTION IN DISCOLICHENS 169
rufescens 1 and P. malacea* the two systems paraphyses and ascogonium
grow simultaneously, though in P. horizontalis the ascogonium has disappeared
by the time the paraphyses are formed. In the genus Nephroma,
in Physcia stellaris and in Xanthorina parietina the paraphyses are also late
in making their appearance.
In most instances, the paraphyses push their way up between the cortical
cells which gradually become absorbed, or they may stop short of the sur-
face as in Nephromium tomentosum*. The overlying layer of cortical cells in
that case dies off gradually and in time disappears. Such an apothecium is
said to be " at first veiled." Later formed paraphyses at the circumference
of the apothecium form the parathecium, which is thus continuous with the
hypothecium.
e. VARIATIONS IN APOTHECIAL DEVELOPMENT. Lichens are among
the least stereotyped of plants : instances of variation have been noted in
several genera.
aa. PARMELIAE. A somewhat complicated course of development has
been traced by Baur 2 in Parmelia acetabulum. In that lichen the group of
three to six carpogonia do not lie free in
the gonidial tissue, but originate nearer
the surface (Fig. 96) and are surrounded
from the first by a tissue connected with,
and resembling the tissue of the cortex.
In the several ascogonia, there are more
cells and more spirals than in Collema
or in Physcia, and all of them are some-
what confusedly intertwined. The tri-
chogynes are composed of three to five
cells and project 10 to I5ytt above the
surface. When further development be-
gins, the ascogonial cells branch out and
form a primary darker layer or hypox
55 (after Baur).
thecium above which extends the subhymenium, a light-coloured band of
loosely woven hyphae. Branches from the ascogonial hyphae at a later stage
push their way up through this tissue and form above it a second plexus of
hyphae the base of the hymenium. Baur considers this a very advanced
type of apothecium; he found it also present in Parmelia saxatilis, though,
in that species, the further growth of the first ascogonial layer was more
rapid and the secondary plexus and hymenium were formed earlier in the
life of the apothecium. He has also stated that a similar development occurs
in other genera such as Usnea, though Nienburg's 3 work scarcely confirms
that view.
1 Funfsttick 1884.
* Baur 1904.
3 Nienburg 1908.