The Lichens were formerly classed among the Thallophyta as a group
quite distinct from the Algæ and Fungi. Investigations during the last
twenty-five years, however, have conclusively proved that the Lichens
are Fungi which reproduce in the same manner as the Ascomycetes, or,
more rarely, the Basidiomycetes, and have entered into a peculiar
_symbiotic relation with Algæ_, especially the Cyanophyceæ and
Protococcoideæ, with which they associate, and without which they
would be unable to exist. The Fungus forms the largest portion of the
Lichen, enclosing the Alga with which it may be said to be commensal.
The Fungus especially produces reproductive bodies and absorbs the
inorganic nourishment through the rhizoids, whilst the Alga supplies
it with the organic materials. In consequence of this the Lichens, in
contradistinction to other Fungi, need light for the development of
their nutritive organs, and are therefore, in any case internally, of
a more or less greenish colour. The form and condition of the thallus
is unusual among the Fungi, and they can grow upon rocks and in other
places where no dead organic matter, such as would be required by other
Fungi, is obtainable.
[Illustration: FIG. 131.--Transverse section through the thallus
of _Sticta fuliginosa_ (× 500): _r-r_ rhizoid-strands, which
arise from the under side; _g-g_ gonidial layer; _m_ medullary
layer; _o_ upper, _u_ lower cortex.]
Two cellular forms are therefore to be found in each Lichen:
1. The cells which belong to the Fungus. These are generally septate,
branched _hyphæ_ without any trace of chlorophyll. In the thallus of
the majority of Lichens there may be found a medullary layer (Fig.
131 _m_) of loosely-woven hyphæ, between which there are large air
chambers; and an _external layer_ (cortex) (Fig. 131 _o_, _u_) formed
of closely-woven hyphæ without any intercellular spaces. In some
Lichens (Collemaceæ) the hyphæ wind about in the thallus, being equally
distributed throughout, without forming any decided strata. These
Lichens moreover become gelatinous when exposed to moisture (Fig. 132),
on account of the swelling of the walls of the Algæ. The hyphæ contain
protoplasm with drops of oil, but never starch; their walls easily
swell when exposed to damp after having been dried, and in some (_e.g._
_Cetraria islandica_) they become gelatinous when cooked. Certain
strata of hyphæ become blue on treatment with iodine alone, from which
it is inferred that the wall is allied, in its chemical nature, to
starch.
2. The enclosed Algæ, termed “gonidia.” Some belonging to the
Cyanophyceæ, Protococcoideæ, (especially _Pleurococcus_) and
Chroococcaceæ, are spherical and are found isolated, or in irregular
_groups_ of cells (Fig. 131 _g_); some belonging to _Nostoc_ (Fig. 132
_g_), Lyngbyaceæ, etc., are placed in cell-rows. Each Lichen, as a
rule, has only one definite Algal-form for its gonidium.
The gonidia either lie together in a certain stratum between the
cortex and the medullary layer (Fig. 131 _g_), or are scattered
irregularly throughout the entire thallus (Fig. 132). The thallus is
in the first instance termed “heteromerous,” in the second instance,
“homoiomerous.” The Fungal-hyphæ embrace the gonidia and apply
themselves closely to, or even penetrate them, and hence it has been
difficult to decide whether the one cellular form does or does not
develop from the other (Figs. 134, 135).
This theory regarding the symbiosis of Fungi and Algæ to form
a Lichen is termed the Schwendenerian theory, after the first
scientist who advanced it with any weight. It had been already
indicated by De Bary, and further arguments in its support have
at a later time been adduced by Bornet, Stahl, Treub, Frank,
Bonnier, Alfr. Möller and others.
[Illustration: FIG. 132.--_Collema microphyllum._ Transverse
section through the thallus; _g Nostoc_-chains; _h_ hyphæ.]
[Illustration: FIG. 133.--_Ephebe pubescens._ The apex of a
branch of the thallus with two lateral branches (_s_): _h_ its
hyphæ; _g_ the apical gonidium of the main branch.]
[Illustration: FIG. 134.--_Nostoc lichenoides_, which is attached
by a germinating thread (_h_) of _Collema glaucescens_.]
[Illustration: FIG. 135.--_A_ Germinating spore of _Physcia
parietina_ with _Protococcus viridis_. _B Synalissa symphorea_
with _Glæocapsa_. _C Cladonia furcata_ with _Protococcus_.]