A, Carpogonium, c, with its trichogyne t.
B, Apex of the trichogyne with the spermatium, s, attached.]
_Ejection of Spores._--The spores are ejected from the apothecia and
perithecia as in the fungi by forcible ejaculation from the asci. In
the majority of forms it is clear that the soredia rather than the
ascospore must play the more important part in lichen distribution as
the development of the ordinary spores is dependent on their finding
the proper alga on the substratum on which they happen to fall. In a
number of forms (_Endocarpon pusillum_, _Stigmaatonima cataleptum_,
various species of _Staurothele_), however, there is a special
arrangement by which the spores are, on ejection, associated with
gonidia. In these forms gonidia are found in connexion with the young
fruit; such algal cells undergo numerous divisions becoming very small
in size and penetrating into the hymenium among the asci and
paraphyses. When the spores are thrown out some of these hymenial
gonidia, as they are called, are carried with them. When the spores
germinate the germ-tubes surround the algal cells, which now increase
in size and become the normal gonidia of the thallus.
_Basidiolichens._
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission
of Gustav Fischer.
FIG. 17.--_Cora pavonia._ A, Viewed from above; B, From below; _hym_,
hymenium. (Nat. size.)]
As is clear from the above, nearly all the lichens are produced by the
association of an ascomycetous fungus with algae. For some obscure
reason the Basidiomycetes do not readily form lichens, so that only a
few forms are known in which the fungal element is a member of this
family. The two best-known genera are _Cora_ and _Dictyonema_;
_Corella_, whose hymenium is unknown, is also placed here by Wainio. The
so-called Gasterolichens, _Trichocoma_ and _Emericella_, have been shown
to be merely ascomycetous fungi. _Clavaria mucida_, however, has
apparently some claims to be considered as a Basidiolichen, since the
base of the fruit body and the thallus from which it arises, according
to Coker, always shows a mixture of hyphae and algae.
The best-known species is _Cora pavonia_, which is found in tropical
regions growing on the bare earth and on trees; the gonidia belong to
the genus _Chroococcus_ while the fungus belongs, apparently, to the
Thelephoreae (see FUNGI). This lichen seems unique in the fact that the
fungal element is also found growing and fruiting entirely devoid of
algae, while in the ascolichens the fungus portion seems to have become
so specialized to its symbiotic mode of life that it is never found
growing independently.
The genus _Dictyonema_ has gonidia belonging to the blue-green alga,
_Scytonema_. When the fungus predominates in the thallus it has a
bracket-like mode of growth and is found projecting from the branches of
trees with the hymenium on the under side. When the alga is predominant
it forms felted patches on the bark of trees, the _Laudatea_ form. It is
said that the fungus of _Cora pavonia_ and of _Dictyonema_ is identical,
the difference being in the nature of the alga.
_Mode of Life._
Lichens are found growing in various situations such as bare earth, the
bark of trees, dead wood, the surface of stones and rocks, where they
have little competition to fear from ordinary plants. As is well known,
the lichens are often found in the most exposed and arid situations; in
the extreme polar regions these plants are practically the only
vegetable forms of life. They owe their capacity to live under the most
inhospitable conditions to the dual nature of the organism, and to their
capacity to withstand extremes of heat, cold and drought without
destruction. On a bare rocky surface a fungus would die from want of
organic substance and an alga from drought and want of mineral
substances. The lichen, however, is able to grow as the alga supplies
organic food material and the fungus has developed a battery of acids
(see below) which enable it actually to dissolve the most resistant
rocks. It is owing to the power of disintegrating by both mechanical and
chemical means the rocks on which they are growing that lichens play
such an important part in soil-production. The resistance of lichens is
extraordinary; they may be cooled to very low temperatures and heated to
high temperatures without being killed. They may be dried so thoroughly
that they can easily be reduced to powder yet their vitality is not
destroyed but only suspended; on being supplied with water they absorb
it rapidly by their general surface and renew their activity. The life
of many lichens thus consists of alternating periods of activity when
moisture is plentiful, and completely suspended animation under
conditions of dryness. Though so little sensitive to drought and
extremes of temperature lichens appear to be very easily affected by the
presence in the air of noxious substances such as are found in large
cities or manufacturing towns. In such districts lichen vegetation is
entirely or almost entirely absent. The growth of lichens is extremely
slow and many of them take years before they arrive at a spore-bearing
stage. _Xanthoria parietina_ has been known to grow for forty-five years
before bearing apothecia. This slowness of growth is associated with
great length of life and it is probable that individuals found growing
on hard mountain rocks or on the trunks of aged trees are many hundreds
of years old. It is possible that specimens of such long-lived species
as _Lecidea geographica_ actually outrival in longevity the oldest
trees.
_Relation of Fungus and Alga._
The relation of the two constituents of the lichen have been briefly
stated in the beginning of this article. The relation of the fungus to
the alga, though it may be described in general terms as one of
symbiosis, partakes also somewhat of the nature of parasitism. The algal
cells are usually controlled in their growth by the hyphae and are
prevented from forming zoospores, and in some cases, as already
described, the algal cells are killed sooner or later by the fungus. The
fungus seems, on the other hand, to stimulate the algal cells to special
development, for those in the lichen are larger than those in the free
state, but this is not necessarily adverse to the idea of parasitism,
for it is well known that an increase in the size of the cells of the
host is often the result of the attacks of parasitic fungi. It must be
borne in mind that the exact nutritive relations of the two constituents
of the lichen have not been completely elucidated, and that it is very
difficult to draw the line between symbiosis and parasitism. The lichen
algae are not alone in their specialization to the symbiotic (or
parasitic) mode of life, for, as stated earlier, the fungus appear in
the majority of cases to have completely lost the power of independent
development since with very rare exceptions they are not found alone.
They also differ very markedly from free living fungi in their chemical
reactions.
_Chemistry of Lichens._
The chemistry of lichens is very complex, not yet fully investigated
and can only be very briefly dealt with here. The wall of the hyphae
of the fungus give in the young state the ordinary reactions of
cellulose but older material shows somewhat different reactions,
similar to those of the so-called fungus-cellulose. In many
lichen-fungi the wall shows various chemical modifications. In
numerous lichens, e.g. _Cetraria islandica_, the wall contains
Lichenin (C6H10O5), a gummy substance which swells in cold water and
dissolves in hot. Besides this substance, a very similar one,
Isolichenin, is also found which is distinguished from lichenin by the
fact that it dissolves in cold water and turns blue under the reaction
of Iodine. Calcium oxalate is a very common substance, especially in
crustaceous lichens; fatty oil in the form of drops or as an
infiltration in the membrane is also common; it sometimes occurs in
special cells and in extreme cases may represent 90% of the dry
substance as in _Verrucaria calciseda_, _Biatora immersa_.
_Colouring Matters._--Many lichens, as is well known, exhibit a vivid
colouring which is usually due to the incrustation of the hyphae with
crystalline excretory products. These excretory products have usually
an acid nature and hence are generally known as lichen-acids. A large
number of these acids, which are mostly benzene derivatives, have been
isolated and more or less closely investigated. They are characterized
by their insolubility or very slight solubility in water; as examples
may be mentioned erythrinic acid in _Roccella_ and _Lecanora_; evernic
acid in species of _Evernia_, _Ramalina_ and _Cladonia_; lecanoric
acid in _Lecanora_, _Gyrophora_. The so-called chrysophanic acid found
in _Xanthoria_ (Physcia) _parietina_ is not an acid but a quinone and
is better termed physcion.
_Colour Reactions of Lichens._--The classification of lichens is
unique in the fact that chemical colour reactions are used by many
lichenologists in the discrimination of species, and these reactions
are included in the specific diagnoses. The substances used as tests
in these reactions are caustic potash and calcium hypochlorite; the
former being the substance dissolved in an equal weight of water and
the latter a saturated extract of bleaching powder in water. These
substances are represented by lichenologists by the signs K and CaCl
respectively, and the presence or absence of the colour reactions are
represented thus, K+, CaCl+, or K-, CaCl-. If the cortical layer
should exhibit positive reaction and the medulla of the same species a
negative reaction with both reagents, the result is represented thus,
K±CaCl±. If a reaction is only produced after the consecutive addition
of the two reagents, this is symbolized by K(CaCl)+. A solution of
iodine is also used as a test owing to the blue or wine-red colour
which the thallus, hymenium or spores may give with this reagent. The
objection to the case of these colour reactions is due to the
indefinite nature of the reaction and the doubt as to the constant
presence of a definite chemical compound in a given species. A yellow
colour with caustic potash solution is produced not only by atranoric
acid but also by evernic acid, thamnolic acid, &c. Again in the case
of _Xanthoria parietina_ vulpinic acid is only to be found in young
thalli growing on sandstone; in older forms or in those growing on
another substratum it is not to be detected. A similar relation
between oil formation and the nature of the substratum has been
observed in many lichens. Considerations such as these should make one
very wary in placing reliance on these colour reactions for the
purposes of classification.
_Economic Uses of Lichens._
In the arts, as food and as medicine, many lichens have been highly
esteemed, though others are not now employed for the same purposes as
formerly.
1. _Lichens Used in the Arts._--Of these the most important are such as
yield, by maceration in ammonia, the dyes known in commerce as archil,
cudbear and litmus. These, however, may with propriety be regarded as
but different names for the same pigmentary substance, the variations in
the character of which are attributable to the different modes in which
the pigments are manufactured. Archil proper is derived from several
species of _Roccella_ (e.g. _R. Montaguei_, _R. tinctoria_), which yield
a rich purple dye; it once fetched a high price in the market. Of
considerable value is the "perelle" prepared from _Lecanora parella_,
and used in the preparation of a red or crimson dye. Inferior to this is
"cudbear," derived from _Lecanora tartarea_, which was formerly very
extensively employed by the peasantry of north Europe for giving a
scarlet or purple colour to woollen cloths. By adding certain alkalies
to the other ingredients used in the preparation of these pigments, the
colour becomes indigo-blue, in which case it is the litmus of the Dutch
manufacturers. Amongst other lichens affording red, purple or brown dyes
may be mentioned _Ramalina scopulorum_, _Parmelia_, _saxatilis_ and _P.
amphalodes_, _Umbilicaria pustulata_ and several species of _Gyrophora_,
_Urceolaria scruposa_, all of which are more or less employed as
domestic dyes. Yellow dyes, again, are derived from _Chlorea vulpina_,
_Platysma juniperinum_, _Parmelia caperata_ and _P. conspersa_, _Physcia
flavicans_, _Ph. parietina_ and _Ph. lychnea_, though like the preceding
they do not form articles of commerce, being merely used locally by the
natives of the regions in which they occur most plentifully. In addition
to these, many exotic lichens, belonging especially to _Parmelia_ and
_Sticta_ (e.g. _Parmelia tinctorum_, _Sticta argyracea_), are rich in
colouring matter, and, if obtained in sufficient quantity, would yield a
dye in every way equal to archil. These pigments primarily depend upon
special acids contained in the thalli of lichens, and their presence may
readily be detected by means of the reagents already noticed. In the
process of manufacture, however, they undergo various changes, of which
the chemistry is still but little understood. At one time also some
species were used in the arts for supplying a gum as a substitute for
gum-arabic. These were chiefly _Ramalina fraxinea_, _Evernia prunastri_
and _Parmelia physodes_, all of which contain a considerable proportion
of gummy matter (of a much inferior quality, however, to gum-arabic),
and were employed in the process of calico-printing and in the making of
parchment and cardboard. In the 17th century some filamentose and
fruticulose lichens, viz. species of _Usnea_ and _Ramalina_, also
_Evernia furfuracea_ and _Cladonia rangiferina_, were used in the art of
perfumery. From their supposed aptitude to imbibe and retain odours,
their powder was the basis of various perfumes, such as the celebrated
"Poudre de Cypre" of the hairdressers, but their employment in this
respect has long since been abandoned.
2. _Nutritive Lichens._--Of still greater importance is the capacity of
many species for supplying food for man and beast. This results from
their containing starchy substances, and in some cases a small quantity
of saccharine matter of the nature of mannite. One of the most useful
nutritious species is _Cetraria islandica_, "Iceland moss," which, after
being deprived of its bitterness by boiling in water, is reduced to a
powder and made into cakes, or is boiled and eaten with milk by the poor
Icelander, whose sole food it often constitutes. Similarly _Cladonia
rangiferina_ and _Cl. sylvatica_, the familiar "reindeer moss," are
frequently eaten by man in times of scarcity, after being powdered and
mixed with flour. Their chief importance, however, is that in Lapland
and other northern countries they supply the winter food of the reindeer
and other animals, who scrape away the snow and eagerly feed upon them.
Another nutritious lichen is the "Tripe de Roche" of the arctic regions,
consisting of several species of the _Gyrophorei_, which when boiled is
often eaten by the Canadian hunters and Red Indians when pressed by
hunger. But the most singular esculent lichen of all is the "manna
lichen," which in times of drought and famine has served as food for
large numbers of men and cattle in the arid steppes of various countries
stretching from Algiers to Tartary. This is derived chiefly from
_Lecanora esculenta_, which grows unattached on the ground in layers
from 3 to 6 in. thick over large tracts of country in the form of small
irregular lumps of a greyish or white colour. In connexion with their
use as food we may observe that of recent years in Scandinavia and
Russia an alcoholic spirit has been distilled from _Cladonia
rangiferina_ and extensively consumed, especially in seasons when
potatoes were scarce and dear. Formerly also _Sticta pulmonaria_ was
much employed in brewing instead of hops, and it is said that a Siberian
monastery was much celebrated for its beer which was flavoured with the
bitter principle of this species.
3. _Medicinal Lichens._--During the middle ages, and even in some
quarters to a much later period, lichens were extensively used in
medicine in various European countries. Many species had a great repute
as demulcents, febrifuges, astringents, tonics, purgatives and
anthelmintics. The chief of those employed for one or other, and in
some cases for several, of these purposes were _Cladonia pyxidata_,
_Usnea barbata_, _Ramalina farinacea_, _Evernia prunastri_, _Cetraria
ìslandica_, _Sticla pulmonaria_, _Parmelia saxatilis_, _Xanthoria
parietina_ and _Pertusaria amara_. Others again were believed to be
endowed with specific virtues, e.g. _Peltigera canina_, which formed the
basis of the celebrated "pulvis antilyssus" of Dr Mead, long regarded as
a sovereign cure for hydrophobia; _Platysma juniperinum_, lauded as a
specific in jaundice, no doubt on the _similia similibus_ principle from
a resemblance between its yellow colour and that of the jaundiced skin;
_Peltidea aphthosa_, which on the same principle was regarded by the
Swedes, when boiled in milk, as an effectual remedy for the _aphthae_ or
rash on their children. Almost all of these virtues, general or
specific, were imaginary; and at the present day, except perhaps in some
remoter districts of northern Europe, only one of them is employed as a
remedial agent. This is the "Iceland moss" of the druggists' shops,
which is undoubtedly an excellent demulcent in various dyspeptic and
chest complaints. No lichen is known to be possessed of any poisonous
properties to man, although _Chlorea vulpina_ is believed by the Swedes
to be so. Zukal has considered that the lichen acids protect the lichen
from the attacks of animals; the experiments of Zopf, however, have cast
doubt on this; certainly lichens containing very bitter acids are eaten
by mites though some of the acids appear to be poisonous to frogs.
_Classification._
The dual nature of the lichen thallus introduces at the outset a
classificatory difficulty. Theoretically the lichens may be classified
on the basis of their algal constituent, on the basis of their fungal
constituent, or they may be classified as if they were homogeneous
organisms. The first of these systems is impracticable owing to the
absence of algal reproductive organs and the similarity of the algal
cells (gonidia) in a large number of different forms. The second system
is the most obvious one, since the fungus is the dominant partner and
produces reproductive organs. The third system was that of Nylander and
his followers, who did not accept the Schwenderian doctrine of duality.
In actual practice the difference between the second and third methods
is not very great since the fungus is the producer of the reproductive
organs and generally the main constituent. Most systems agree in
deriving the major divisions from the characters of the reproductive
organs (perithecia, apothecia, or basidiospore bearing fructification),
while the characters of the algal cells and those of the thallus
generally are used for the minor divisions. The difference between the
various systems lies in the relative importance given to the
reproductive characters on the one hand and the vegetative characters on
the other. In the system (1854-1855) of Nylander the greater weight is
given to the latter, while in more modern systems the former characters
receive the more attention.
A brief outline of a system of classification, mainly that of
Zahlbruckner as given in Engler and Prantl's _Pflanzenfamilien_, is
outlined below.
There are two main divisions of lichens, _Ascolichenes_ and
_Basidiolichenes_, according to the nature of the fungal element,
whether an ascomycete or basidiomycete. The Ascolichenes are again
divided into _Pyrenocarpeae_ or _Pyrenolichenes_ and _Gymnocarpeae_ or
_Discolichenes_; the first having an ascocarp of the nature of a
perithecium, the second bearing their ascospores in an open apothecium.