c, An isolated mature soredium, with an algal cell (_Pleurococcus_) in
the envelope or hyphae.
d, Another with several algal cells in optical longitudinal section.
e, f, Two soredia in the act of germinating; the hyphal envelope has
grown out below into rhizoid branches, and above shows already the
structure of the apex of the thallus (see fig 9).]
They present various shapes, of which the following are the principal:
(a) _peltate_, which are large, rounded, without any distinct thalline
margin[1] (e.g. _Usnea_, _Peltigera_); (b) _lecanorine_, or
scutelliform, which are orbicular and surrounded by a distinct, more
or less prominent thalline margin (e.g. _Parmelia_, _Lecanora_),
having sometimes also in addition a proper one¹ (e.g. _Thelotrema_,
_Urceolaria_); (c) _lecideine_, or patelliform, which are typically
orbicular, with only a proper margin (e.g. _Lecidea_), sometimes
obsolete, and which are occasionally irregular in shape, angular or
flexuose (e.g. _Lecidea jurana_, _L. myrmecina_), or complicated and
gyrose (e.g. _Gyrophora_), and even stipitate (e.g. _Baeomyces_); (d)
_lirelliform_, which are of very irregular figure, elongated, branched
or flexuose, with only a proper margin (e.g. _Xylographa_, _Graphis_,
&c.) or none (e.g. some _Arthoniae_), and often very variable even in
the same species. In colour the apothecia are extremely variable, and
it is but rarely that they are the same colour as the thallus (e.g.
_Usnea_, _Ramalina_). Usually they are of a different colour, and may
be black, brown, yellowish, or also less frequently rose-coloured,
rusty-red, orange-reddish, saffron, or of various intermediate shades.
Occasionally in the same species their colour is very variable (e.g.
_Lecanora metaboloides_, _Lecidea decolorans_), while sometimes they
are white or glaucous, rarely greenish, pruinose. Lecideine apothecia,
which are not black, but otherwise variously coloured, are termed
_biatorine_.
[Illustration: After Tulasne, from De Bary's _Vergleichende
Morphologie und Biologie der Pilze, Mycetozoen und Bacterien_, by
permission of Wilhelm Engelmann.
FIG. 13.--A, B, _Gyrophora cylindrica._ (A mag. 90, B 390 times, C
highly magnified.)
A, A vertical median section through a spermogonium imbedded in the
thallus.
o, Upper rind.
u, Under rind.
m, Medullary layer of the thallus.
B, Portion of a very thin section from the base of the spermogonium.
w, Its wall from which proceed sterigmata with rod-like spermatia
(s).
m, Medullary hyphae of the thallus.
C, _Cladonia novae Angliae_, Delise; sterigmata with spermatia from
the spermogonium.]
The two principal parts of which an apothecium consists are the
_hypothecium_ and the hymenium, or thecium. The _hypothecium_ is the
basal part of the apothecium on which the _hymenium_ is borne; the
latter consists of asci (thecae) with ascospores, and paraphyses. The
paraphyses (which may be absent entirely in the Pyrenolichens) are
erect, colourless filaments which are usually dilated and coloured at
the apex; the apices are usually cemented together into a definite
layer, the _epithecium_ (fig. 14). The spores themselves may be
unicellular without a septum or multicellular with one or more septa.
Sometimes the two cavities are restricted to the two ends of the
spore, the _polari-bilocular_ type and the two loculi may be united
by a narrow channel (fig. 15). At other times the spores are divided
by both transverse and longitudinal septa producing the muriform
(murali-divided) spore so called from the resemblance of the
individual chambers to the stones in a wall. The very large single
spores of _Pertusaria_ have been shown to contain numerous nuclei and
when they germinate develop a large number of germ tubes.
[Illustration: After Darbishire, from _Berichte der deutschen
botanischen Gesellschaft_, by permission of Borntraeger & Co.
FIG. 14.--Diagram showing Apothecium in Section and surrounding
Portion of Thallus, and special terms used to designate these
parts.]
_Development of the Ascocarps._--As the remarks on the nature of the
spermatia show, the question of the sexuality of the lichens has been
hotly disputed in common with that of the rest of the Ascomycetes. As
indicated above, the weight of evidence seems to favour what has been
put forward in the case of the non-lichen-forming fungi (see FUNGI),
that in some cases the ascogonia develop as a result of a previous
fertilization by spermatia, in other cases the ascogonia develop
without such a union, while in still other cases the reduction goes
still farther and the ascogenous hyphae instead of developing from the
ascogonia are derived directly from the vegetative hyphae.
[Illustration: FIG. 15.--Vertical Section of Apothecium of _Xanthoria
parietina_.
a, Paraphyses.
b, Asci (thecae) with bilocular spores.
c, Hypothecium.]
The first exact knowledge as to the origin of the ascocarp was the
work of Stahl on _Collema_ in 1877. He showed that the archicarp
consisted of two parts, a lower coiled portion, the ascogonium, and an
upper portion, the trichogyne, which projected from the thallus. Only
when a spermatium was found attached to the trichogyne did the further
development of the ascogonium take place. From these observations he
drew the natural conclusion that the spermatium was a male, sexual
cell. This view was hotly contested by many workers and it was sought
to explain the trichogyne--without much success--as a respiratory
organ, or as a boring organ which made a way for the developing
apothecium. It was not till 1898, however, that Stahl's work received
confirmation and addition at the hands of Baur (fig. 16). The latter
showed that in _Collema crispum_ there are two kinds of thalli, one
with numerous apothecia, the other quite sterile or bearing only a
few. The sterile thalli possessed no spermogonia, but were found to
show sometimes as many as 1000 archicarps with trichogynes; yet none
or very few came to maturity. The fertile thalli were shown to bear
either spermogonia or to be in immediate connexion with
spermogonia-bearing thalli. Furthermore Baur showed that after the
fusion of the spermatium with the trichogyne the transverse walls of
that organ became perforated. There was thus very strong
circumstantial evidence in favour of fertilization, although the male
nucleus was not traced. The further work of Baur, and that of
Darbishire, Funfstuck and Lindau, have shown that in a number of other
cases trichogynes are present. Thus ascogonia with trichogynes have
been observed in _Endocarpon_, _Collema_, _Pertusaria_, _Lecanora_,
_Gyrophora_, _Parmelia_, _Ramalina_, _Physcia_, _Anaptychia_ and
_Cladonia_. In _Nephroma_, _Peltigera_, _Peltidea_ and _Solorina_ a
cogonia without trichogynes have been observed. In _Collema_ and a
form like _Xanthoria parietina_ it is probable that actual
fertilization takes place, and possibly also in some of the other
forms. It is probable, however, that in the majority of cases the
ascogonia develop without normal fertilization, as is necessarily the
case where the ascogonia have no trichogynes or the spermatia are
absent. In these cases we should expect to find some reduced process
of fertilization similar to that of _Humaria granulata_ among the
ordinary Ascomycetes, where in the absence of the antheridia the
female nuclei fuse in pairs. In other lichens we should expect to find
the ascogenous hyphae arising directly from the vegetative hyphae as
in _Humaria rutilans_ among the ordinary fungi, where the process is
associated with the fusion of vegetative nuclei. It is possible that
_Solorina saccata_ belongs to this class. Cytological details of
nuclear behaviour among the lichens are, however, difficult to obtain
owing to the slow growth of these forms and the often refractory
nature of the material in the matter of preparation for microscopical
examination.
[Illustration: After E. Baur, from Strasburger's _Lehrbuch der
Botanik_, by permission of Gustav Fischer.