A, Optical longitudinal section of the extremity of a thin branch of
the thallus which has become transparent in solution of potash.
B, Transverse section through a stronger branch with the point of
origin of an adventitious branch (sa).
r, Cortical layer.
m, Medullary layer.
x, Stout axile strand.
g, The algal zone (_Cystococcus_).
s, Apex of the branch.]
_Spermatia._--In the majority of genera of lichens small flask-shaped
structures are found embedded in the thallus (fig. 13). These were
investigated by Tulasne in 1853, who gave them the name _spermogonia_
The lower, ventral portion of the spermogonium is lined by delicate
hyphae, the _sterigmata_, which give origin to minute colourless
cells, the _spermatia_. The sterigmata are either simple (fig. 13, C)
or septate--the so-called arthrosterigmata (fig. 13, B). The
spermogonia open by a small pore at the apex, towards which the
sterigmata converge and through which the spermatia escape (fig. 13).
There are two views as to the nature of the spermatia. In one view
they are mere asexual conidia, and the term _pycnoconidia_ is
accordingly applied since they are borne in structures like the
non-sexual _pycnidia_ of other fungi. In the other view the spermatia
are the male sexual cells and thus are rightly named; it should,
however, be pointed out that this was not the view of Tulasne, though
we owe to him the designation which carries with it the sexual
significance. The question is one very difficult to settle owing to
the fact that the majority of spermatia appear to be functionless. In
favour of the conidial view is the fact that in the case of _Collema_
and a few other forms the spermatia have been made to germinate in
artificial cultures, and in the case of _Calicium parietinum_ Möfler
succeeded in producing a spermogonia bearing thallus from a
spermatium. For the germination of the spermatia in nature there is
only the observation of Hedlund, that in _Catillaria denigrata_ and
_C. prasena_ a thallus may be derived from the spermatia under natural
conditions. In relation to the view that the spermatia are sexual
cells, or at least were primitively so, it must be pointed out that
although the actual fusion of the spermatial nucleus with a female
nucleus has not been observed, yet in a few cases the spermatia have
been seen to fuse with a projecting portion (trichogyne) of the
ascogonium, as in _Collema_ and _Physcia_, and there is very strong
circumstantial evidence that fertilization takes place (see later in
section on development of ascocarp). The resemblance of the spermatia
and spermogonia to those of Uredineae should be pointed out, where
also there is considerable evidence for their original sexual nature,
though they appear in that group to be functionless in all cases. The
observations of Möller, &c., on the germination cannot be assumed to
negative the sexual hypothesis for the sexual cells of _Ulothrix_ and
_Ectocarpus_, for example are able to develop with or without fusion.
The most satisfactory view in the present state of our knowledge seems
to be that the spermatia are male cells which, while retaining their
fertilizing action in a few cases are now mainly functionless. The
female sexual organs, the ascogonia, would thus in the majority of
cases develop by the aid of some reduced sexual process or the
ascocarps be developed without relation to sexual organs. A further
argument in support of this view is that it is in complete agreement
with what we know of the sexuality of the ordinary, free-living
ascomycetes, where we find both normal and reduced forms (see FUNGI).
[Illustration: From _Beiträge zur Wissenschaftlichen Botanik_.
FIG. 9.--Section of Heteromerous Lichen Thallus.
a, Upper cortical layer.
d, Lower cortical layer.
c, Medullary layer.
b, Gonidial layer.]
[Illustration: After Bornet, from De Bary's _Vergleichende Morphologie
und Biologie der Pilze, Myceiozoen und Bacterien_, by permission of
Wilhelm Engelmann.
FIG. 11.--Lichen-forming Algae. (A, C, D, E mag. 950, B 650 times.)
The alga is in all cases indicated by the letter _g_, the assailing
hyphae by _h_.
A, _Pleurococcus_, Ag. (_Cystococcus_, Näg.) attacked by the germ-tube
from a spore of _Physica parietina_.
B, _Scytonema_ from the thallus of _Stereocaulon famulosum_.
C, _Nostoc_ from the thallus of _Physma chalazanum_.
D, _Gloeocapsa_ from the thallus of _Synalissa Symphorea_.
E, _Pleurococcus_ Sp. (_Cystococcus_) from the thallus of _Cladonia
furcata_.]
_Fruit Bodies._--We find two chief types of fruit bodies in the lichens,
the _perithecium_ and _apothecium_; the first when the fungal element is
a member of the Pyrenomycetes division of the Ascomycetes, the second
when the fungus belongs to the Discomycetes division. In the two genera
of lichens--the _Basidiolichens_--in which the fungus is a member of the
Basidiomycetes, we have the fructification characteristic of that class
of fungi: these are dealt with separately. The perithecium is very
constant in form and since the gonidia take no part in the formation of
this organ or that of the apothecium it has the general structure
characteristic of that division of fungi. The apothecia, though of the
normal fungal type and usually disk-shaped, are somewhat more variable,
and since the variations are of value in classification some more
details may be added.
[Illustration: After Schwendener, from De Bary's _Vergleichende
Morphologie und Biologie der Pilze Mycetozoen und Bacterien,_ by
permission of Wilhelm Engelmann.