1. The CRUSTACEOUS, which adheres firmly to the substratum (bark,
stone) throughout its entire surface, without being raised into any
free patches or lobes. It has, in many instances, no definite outline,
and hyphal-branches from it often penetrate deeply into the substratum.
It grows at the circumference and sometimes dies away in the centre
(Figs. 138, 139, 140).
2. The FOLIACEOUS. This also lies flat upon the substratum, but is not
firmly attached to and has a definite outline. It grows at the margin,
and raises itself a little by free outgrowths and lobes (Fig. 141). The
rhizoid-strands spring out from its whitish under surface (Fig. 131,
_r_).
[Illustration: FIG. 136.--Portion of a hymenium: _d_ a thin
stratum on which the asci (_s_) are situated.]
[Illustration: FIG. 137.--Spores of, _a Cladonia_, _Lecanora_
and _Pertusaria_; _b Bæomyces_; _c Sphinctrina_; _d_, _e_,
_f_ various species of _Parmelia_; _g_, _h Verrucaria_ in its
younger and older condition; _i_, _k_ species of _Leptogium_.]
3. The FRUTICOSE, which is attached to its substratum at a small point
from which it projects freely, either erect or pendulous. It is more
or less tufted, in the form of a bush (Figs. 142, 143). These three
thallus-forms gradually pass over by many intermediate forms into one
another.
[Illustration: FIG. 138.--_Lecanora subfusca_: _a_ the bark on
which it is situated; _l_ the thallus; _s_ the ascocarp; _s’_ an
ascocarp.]
[Illustration: FIG. 139.--_Graphis_ (two species).]
[Illustration: FIG. 140.--_Pertusaria communis._]
The Lichens, like other Ascomycetes, have very variously constructed
ascospores (Fig. 137), which are enclosed in asci (Fig. 136), usually
surrounded by paraphyses attached together. Furthermore they possess
pycnidia (Fig. 141) containing numerous microconidia. These were
formerly considered as organs of fructification, and were termed
“spermatia,” and the pycnidia, “spermogonia.” Alfr. Möller proved,
in 1887, that the microconidia are able to germinate and produce a
mycelium with new conidia, just as in other Ascomycetes.
VEGETATIVE REPRODUCTION takes place by _soredia_, which to the naked
eye appear as whitish powder on the surface of the thallus. They are
small round bodies, formed by one or a group of gonidia, which are
surrounded by a mass of felted hyphæ. After the rupture of the cortex
they are set free, and readily carried by the wind to other places,
where under favourable circumstances they establish a new thallus.
[Illustration: FIG. 141.--_A_ A portion of the thallus of
_Parmelia parietina_ with ascocarps (_a_) and pycnidia (_b_). _B_
A portion of the thallus of _Cetraria islandica_ with pycnidia
at the end of small lobes. _C_ A lobe with pycnidia and ejected
microconidia. (Magnified).]
GEOGRAPHICAL DISTRIBUTION. The Lichens are the most hardy plants, and
are the first to appear on hitherto bare rocks which they gradually
disintegrate, and hence prepare the way for the growth of other plants.
They are to be found from the Polar regions to the Equator; from the
highest snow-free mountain-peaks down to the level of the sea; on the
stems of trees; on rocks, soil, some even on inundated places; on
stones in woodland streams, and on beaches; but they are never found
upon rotten organic remains. Some grow gregariously in enormous masses,
and form wide-stretching carpets, _e.g._ Reindeer Moss (_Cladonia
rangiferina_), species of _Cetraria_ and other fruticose Lichens.
USES. On account of the cell-wall being composed of Lichenstarch
(Lichenin), the Iceland-Lichen and Manna-Lichen (_Lecanora esculenta_)
are used as food; the latter grows on stones, in the deserts of Asia
and North Africa, and is often torn loose in large masses and carried
away by the wind. The Reindeer-Lichen is not only the principal food of
the reindeer, but it is also used in the manufacture of Danish brandy.
_Cetraria islandica_ (Lichen islandicus) is OFFICINAL. Colouring
materials (lacmus, orseille, persio) are made from several species,
especially from _Roccella tinctoria_ (from the rocky coasts of the
Mediterranean). _Parmelia saxatilis_ and particularly _Lecanora
tartarea_ are used for colouring purposes in the Northern countries.
About 2,000 species of Lichens have been described. If we disregard the
Basidiolichenes, which will be considered on page 176, the remaining
Lichens (Ascolichenes) may be divided into the two following orders
according to the structure of the fruit-bodies:--
Order 1. =Pyrenolichenes.= The ascocarps (apothecia) are spherical or
flask-shaped, as in the Pyrenomycetes, more rarely linear (_Graphis_).
According to the nature of the thallus, these Lichens may be
divided into:--
_a._ Thallus homoiomerous, but not gelatinous, branching
according to the mode of growth of the Algæ: _Ephebe_ (Fig.
133), with Algæ of the genus _Stigonema_.
_b._ Thallus homoiomerous, gelatinous: _Lichina_.
_c._ Thallus heteromerous, crustaceous: _Verrucaria_,
_Pyrenula_; _Graphis_ (Fig. 139), which may be considered as
Hysteriaceæ with gonidia; several species of _Graphis_ are
common on bark.
_d._ Thallus heteromerous, foliaceous: _Endocarpon_.
_e._ Thallus heteromerous, fruticose: _Sphærophorus_.
Order 2. =Discolichenes.= These, as in the Discomycetes, have open
apothecia, which, as a rule, are cupular, more rarely hemispherical
(_Cladonia_).
According to the nature of the thallus, these Lichens may be
divided into:--
_a._ Thallus homoiomerous, but not gelatinous, branching
according to the mode of growth of the Algæ: _Cœnogonium_.
_b._ Thallus homoiomerous, gelatinous: _Collema_ (Fig. 132),
with Algæ of the genus _Nostoc_; _Leptogium_.
_c._ Thallus heteromerous, crustaceous: _Pertusaria_ (Fig.
140), _Lecidea_, with apothecia open from the beginning;
_Lecanora_, with apothecia, which in the beginning are closed,
later on open, but with a rim formed by the thallus (Fig. 138);
_Bæomyces_, whose apothecia are borne on a stem formed by the
thallus.
_d._ Thallus heteromerous, foliaceous: _Parmelia_ (_P.
saxatilis_; _P. parietina_, Wall-Lichen, Fig. 141, is yellow,
very frequent on tree-stems, stone-walls, tiles); _Physcia_ (_P.
ciliaris_, frequent on tree-stems); _Sticta_ (_S. pulmonacea_,
Lung-Lichen, on tree-stems); _Peltigera_, especially on the Moss
among trees; _Umbilicaria_, on rocks.
_e._ Thallus heteromerous, fruticose: _Cetraria_ (_C.
islandica_), “Iceland Moss,” with an olive-brown, flat,
furrowed, fringed thallus, on heaths; _C. nivalis_, white, in
the Polar regions; _Evernia_, _Ramalina_, _Usnea_ (_U. barbata_,
Beard-Lichen, Fig. 143); _Roccella_, _Stereocaulon_, _Cladonia_,
of which the genus _C. rangiferina_, Reindeer-Moss (Fig. 142) is
important; _Cladonia_ has two kinds of thallus, one scaly and
leaf-like, the other erect, which bears the apothecia and may be
fruticose (Fig. 142), or cupular (Fig. 144); they grow in soil
in forests and on heaths.
[Illustration: FIG. 142.--_Cladonia rangiferina_: _s_ ascocarp.]
[Illustration: FIG. 143.--_Usnea barbata_: _s_ ascocarp.
(Slightly magnified.)]
[Illustration: FIG. 144.--_Cladonia pyxidata._]