PHYLOGENY
I. GENERAL STATEMENT
A. ORIGIN OF LICHENS
Though lichens are very old members of the vegetable kingdom, as
symbiotic plants they yet date necessarily from a time subsequent to the
evolution of their component symbionts. Phylogeny of lichens begins with
symbiosis.
The algae, which belong to those families of Chlorophyceae and
Myxophyceae that live on dry land, had become aerial before their
association with fungi to form lichens. They must have been as fully
developed then as now, since it is possible to refer them to the genus
or sometimes even to the species of free-living forms. The fungus hyphae
have combined with a considerable number of different algae, so that,
even as regards the algal symbiont, lichens are truly polyphyletic in
origin.
The fungus is, however, the dominant partner, and the principal line of
development must be traced through it, as it provides the reproductive
organs of the plant. Representatives of two great groups of fungi are
associated with lichens: Basidiomycetes, found in only a few genera,
and Ascomycetes which form with the various algae the great bulk of
lichen families. In respect of their fungal constituents lichens are
also polyphyletic, and more especially in the Ascolichens which can be
traced back to several starting points. But though lichens have no common
origin, the manner of life is common to them all and has influenced them
all in certain directions: they are fitted for a much longer existence
than that of the fungi from which they started; and both the thallus and
the fruiting bodies—at least in the sub-class Ascolichens—can persist
through great climatic changes, and can pass unharmed through prolonged
periods of latent or suspended vitality.
Another striking note of similarity that runs through the members of
this sub-class, with perhaps the exception of the gelatinous lichens, is
the formation of lichen-acids which are excreted by the fungus. These
substances are peculiar to lichens and go far to mark their autonomy. The
production of the acids and the many changes evolved in the vegetative
thallus suggest the great antiquity of lichens.
B. ALGAL ANCESTORS
It is unnecessary to look far for the algae as they have persisted
through the ages in the same form both without and within the lichen
thallus. By many early lichenologists the free-living algae, similar
in type to lichen algae, were even supposed to be lichen gonidia in a
depauperate condition and were, for that reason, termed by Wallroth
“unfortunate brood-cells.” In the condition of symbiosis they may be
considerably modified, but they revert to their normal form, and resume
their normal life-history of spore production, etc., under suitable and
free culture. The different algae taking part in lichen-formation have
been treated in an earlier chapter[980].
C. FUNGAL ANCESTORS
_a._ HYMENOLICHENS. The problem of the fungal origin in this sub-class
is comparatively simple. It contains but three genera of tropical
lichens which are all associated with Myxophyceae, and the fungus in
them, to judge from the form and habit of the plants, is a member of the
Thelephoraceae. It may be that Hymenolichens are of comparatively recent
origin and that the fungi belonging to the Basidiomycetes had, in the
course of time, become less labile and less capable of originating a new
method of existence. Whatever the reason, they lag immeasurably behind
Ascomycetes in the formation of lichens.
_b._ ASCOLICHENS. Lichens are again polyphyletic within this sub-class.
The main groups from which they are derived are evident. Whether there
has been a series of origins within the different groups or a development
from one starting point in each it would be difficult to determine.
In any case great changes have taken place after symbiosis became
established.
The main divisions within the Ascolichens are related to fungi thus:
Series 1. Pyrenocarpineae } to Pyrenomycetes.
2. Coniocarpineae }
3. Graphidineae to Hysteriaceae.
4. Cyclocarpineae to Discomycetes.
II. THE REPRODUCTIVE ORGANS
A. THEORIES OF DESCENT IN ASCOLICHENS
It has been suggested that ascomycetous fungi, from which Ascolichens are
directly derived, are allied to the Florideae, owing to the appearance
of a trichogyne in the carpogonium of both groups. That organ in the
red seaweeds is a long delicate cell in direct communication with the
egg-cell of the carpogonium. It is a structure adapted to totally
submerged conditions, and fitted to attach the floating spermatia.
In fungi there is also a structure considered as a trichogyne[981],
which, in the Laboulbeniales, is a free, simple or branching organ. There
is no other instance of any similar emergent cell or cells connected with
the ascogonium of the Ascomycetes, though the term has been applied in
these fungi to certain short hyphal branches from the ascogonium which
remain embedded in the tissue. In the Ascomycetes examined all traces of
emergent receptive organs, if they ever existed, have now disappeared;
in some few there are possible internal survivals which never reach the
surface.
In Ascolichens, on the contrary, the “trichogyne,” a septate hyphal
branch extending upwards from the ascogonium, and generally reaching the
open, has been demonstrated in all the different groups except, as yet,
in the Coniocarpineae which have not been investigated. Its presence is
a strong point in the argument of those who believe in the Floridean
ancestry of the Ascomycetes. It should be clearly borne in mind that
Ascolichens are evolved from the Ascomycetes: these latter stand between
them and any more remote ancestry.
In the Ascomycetes, there is a recognized progression of development
in the form of the sporophore from the closed perithecium of the
Pyrenomycetes and possibly through the Hysteriaceae, which are partially
closed, to the open ascocarp of the Discomycetes. If the fungal and
lichenoid “trichogyne” is homologous with the carpogonial organ in
the Florideae, then it must have been retained in all the groups of
Ascomycetes as an emergent structure, and as such passed on from them
to their lichen derivatives. Has that organ then disappeared from fungi
since symbiosis began? There is no trace of it now, except as already
stated in Laboulbeniales with which lichens are unconnected.
Were Ascolichens monophyletic in origin, one could more easily suppose
that both the fungal and lichen series might have started at some
early stage from a common fungal ancestor possessing a well-developed
trichogyne which has persisted in lichens, but has been reduced to
insignificance in fungi, while fruit development proceeded on parallel
lines in both. There is no evidence that such progression has taken place
among lichens; the theory of a polyphyletic origin for the different
series seems to be unassailable. At the same time, there is no evidence
to show in which series symbiosis started first.
It is more reasonable to accept the polyphyletic origin, as outlined
above, from forms that had already lost the trichogyne, if they ever
really possessed it, and to regard the lichen trichogyne as a new organ
developing in lichens in response to some requirement of the deep-seated
ascogonium. Its sexual function still awaits satisfactory proof, and
it is wiser to withhold judgment as to the service it renders to the
developing fruit.
B. RELATION OF LICHENS TO FUNGI
_a._ PYRENOCARPINEAE. In Phycolichens (containing blue-green gonidia)
and especially in the gelatinous forms, fructification is nearly always
a more or less open apothecium. The general absence of the perithecial
type is doubtless due to the gelatinous consistency of the vegetative
structure; it is by the aid of moisture that the hymenial elements become
turgid enough to secure the ejection of the spores through the narrow
ostiole of the perithecium, and this process would be frustrated were
the surrounding and enveloping thallus also gelatinous. There is only
one minutely foliose or fruticose gelatinous family, the Pyrenidiaceae,
in which Pyrenomycetes are established, and the gonidia, even though
blue-green, have lost the gelatinous sheath and do not swell up.
In Archilichens (with bright-green gonidia), perithecial fruits occur
frequently; they are nearly always simple and solitary; in only a few
families with a few representatives, is there any approach to the stroma
formation so marked among fungi. The single perithecium is generally
semi-immersed in the thallus. It may be completely surrounded by a
hyphal “entire” wall, either soft and waxy or dark coloured and somewhat
carbonaceous. In numerous species the outer protective wall covers
only the upper portion that projects beyond the thallus, and such a
perithecium is described as “dimidiate,” a type of fruit occurring in
several genera, though rare among fungi.
As to internal structure, there is a dissolution and disappearance of
the paraphyses in some genera, their protective function not being so
necessary in closed fruits, a character paralleled in fungi. There is a
great variety of spore changes, from being minute, simple and colourless,
to varied septation, general increase in size, and brown colouration. The
different types may be traced to fungal ancestors with somewhat similar
spores, but more generally they have developed within the lichen series.
From the life of the individual it is possible to follow the course of
evolution, and the spores of all species begin as simple, colourless
bodies; in some genera they remain so, in others they undergo more or
less change before reaching the final stage of colour or septation that
marks the mature condition.
As regards direct fungal ancestors, the Pyrenocarpineae, with solitary
perithecia, are nearest in fruit structure to the Mycosphaerellaceae, in
which family are included several fungus genera that are parasitic on
lichens such as _Ticothecium_, _Müllerella_, etc. In that family occurs
also the genus _Stigmatea_, in which the perithecia in form and structure
are very similar to dimidiate _Verrucariae_.
Zahlbruckner[982] has suggested as the starting point for the
Verrucariaceae the fungus genus _Verrucula_. It was established by
Steiner[983] to include two species, _V. cahirensis_ and _V. aegyptica_,
their perithecia being exactly similar to those of _Verrucaria_[984] in
which genus they were originally placed. Both are parasitic on species
of _Caloplaca_ (_Placodium_). The former, on _C. gilvella_, transforms
the host thallus to the appearance of a minutely lobed _Placodium_; the
latter occupies an island-like area in the centre of the thallus of
_Caloplaca interveniens_, and gives it, with its accompanying parasite,
the character of an _Endopyrenium_ (_Dermatocarpon_), while the rest of
the thallus is normal and fertile.
Zahlbruckner may have argued rightly, but it is also possible to regard
these rare desert species as reversions from an originally symbiotic to
a purely parasitic condition. Reinke came to the conclusion that if a
parasitic species were derived directly from a lichen type, then it must
still rank as a lichen, a view that has a direct bearing on the question.
The parallel family of Pyrenulaceae which have _Trentepohlia_ gonidia
is considered by Zahlbruckner to have originated from the fungus genus
_Didymella_.
Compound or stromatoid fructifications occur once and again in
lichen families; but, according to Wainio[985], there is no true
stroma formation, only a pseudostroma resulting from adhesions and
agglomerations of the thalline envelopes or from cohesions of the margins
of developing fruit bodies. These pseudostromata are present in the
genera _Chiodecton_ and _Glyphis_ (Graphidineae) and in _Trypethelium_,
_Mycoporium_, etc. (Pyrenocarpineae). This view of the nature of the
compound fruits is strengthened, as Wainio points out, by the presence in
certain species of single apothecia or perithecia on the same specimen as
the stromatoid fruits.
_b._ CONIOCARPINEAE. This subseries is entirely isolated. Its peculiarity
lies in the character of the mature fruit in which the spores, owing
to the early breaking down of the asci, lie as a loose mass in the
hymenium, while dispersal is delayed for an indefinite time. This type of
fruit, termed a _mazaedium_ by Acharius, is in the form of a stalked or
sessile roundish head—the capitulum—closed at first and only half-open
at maturity rarely, as in _Cyphelium_, an exposed disc. There is a
suggestion, but only a suggestion, of a similar fructification in the
tropical fungus _Camillea_ in which there is sometimes a stalk with one
or more perithecia at the tip, and in some species early disintegration
of the asci, leaving spore masses[986]. But neither in fungi nor in
other lichens is there any obvious connection with Coniocarpineae. In
some of the genera the fungus alone forms the stalk and the wall of the
capitulum; in others the thallus shares in the fruit-formation growing
around it as an amphithecium.
The semi-closed fruits point to their affinity with Pyrenolichens,
though they are more advanced than these judging from the thalline
wall that is present in some genera and also from the half-open disc
at maturity. The latter feature has influenced some systematists to
classify the whole subseries among Cyclocarpineae. The thallus, as in
_Sphaerophorus_, reaches a high degree of fruticose development; in
other genera it is crustaceous without any formation of cortex, while in
several genera or species it is non-existent, the fruits being parasites
on the thalli of other lichens or saprophytes on dead wood, humus, etc.
These latter—both parasites and saprophytes—are included by Rehm[987]
and others among fungi, which has involved the breaking up of this very
distinctive series. Rehm has thus published as Discomycetes the lichen
genera _Sphinctrina_, _Cyphelium_, _Coniocybe_, _Acolium_, _Calicium_ and
_Stenocybe_, since some or all of their species are regarded by him as
fungi.
Reinke[988] in his lichen studies states that it might not be impossible
for a saprophytic fungus to be derived from a crustaceous lichen—a
case of reversion—but that no such instance was then known. More exact
studies[989] of parasymbiosis and antagonistic symbiosis have shown the
wide range of possible life-conditions, and such a reversion does not
seem improbable. We must also bear in mind that in suitable cultures,
lichen hyphae can be grown without gonidia: they develop in that case as
saprophytes.
On Reinke’s[988] view, however, that these saprophytic species, belonging
to different genera in the Coniocarpineae, are true fungi, they would
represent the direct and closely related ancestors of the corresponding
lichen genera, giving a polyphyletic origin within this group. As fungus
genera he has united them in Protocaliciaceae, and the representatives
among fungi he distinguishes, as does Wainio[990], under such names as
_Mycocalicium_ and _Mycoconiocybe_.
If we might consider the saprophytic forms as also retrogressive
lichens, a monophyletic origin from some remote fungal ancestor would
prove a more satisfactory solution of the inheritance problem. This
view is even supported by a comparison Reinke himself has drawn between
the development of the fructification in _Mycocalicium parietinum_, a
saprophyte, and in his view a fungus, and _Chaenotheca chrysocephala_,
a closely allied lichen. Both grow on old timber. In the former (the
fungus), the mycelium pervades the outer weathered wood-cells, and the
fruit stalk rises from a clump of brownish hyphae; there is no trace of
gonidia. _Chaenotheca chrysocephala_ differs in the presence of gonidia
which are associated with the mycelium in scattered granular warts; but
the fruit stalk here also rises directly from the mycelium between the
granules. The presence of a lichen thallus chiefly differentiates between
the two plants, and this thallus is not a casual or recent association;
it is constant and of great antiquity as it is richly provided with
lichen-acids.
Reinke has indicated the course of evolution within the series but that
is on the lines of thalline development and will be considered later.
_c._ GRAPHIDINEAE. This series contains a considerable variety of lichen
forms, but all possess to a more or less marked degree the linear form
of fructification termed a “lirella” which has only a slit-like opening.
There is a tendency to round discoid fruits in the _Roccellae_ and
also in the _Arthoniae_; the apothecia of the latter, called by early
lichenologists “ardellae,” are without margins. In nearly all there is
a formation of carbonaceous black tissue either in the hypothecium or
in the proper margins. In some of them the paraphyses are branched and
dark at the tips, the branches interlocking to form a strong protective
epithecium. There are, however, constant exceptions, in some particular,
to any generalization in genera and in species. Müller-Argau’s[991]
pronouncement might be held to have special reference to Graphidineae:
“that in any genus, species or groups of species are to be found which
outwardly shew something that is peculiar, though of slight importance.”
The most constant type of gonidium is _Trentepohlia_, but _Palmella_ and
_Phycopeltis_ occasionally occur. The spores are various in colour and
form; they are rarely simple.
The genus _Arthonia_ is derived from a member of the Patellariaceae,
from which family many of the Discomycetes have arisen. The course
of development does not follow from a closed to an open fruit; the
apothecium is open from the first, and growth proceeds from the centre
outwards, the fertile cells gradually pushing aside the sterile tissue
of the exterior. The affinity of _Xylographa_ (with _Palmella_ gonidia)
is to be found in _Stictis_ in the fungal family Stictidaceae, the
apothecia of _Stictis_ being at first closed, then open, and with a thick
margin; _Xylographa_ has a more elongate lirella fruit, though otherwise
very similar, and has a very reduced thallus. Rehm[992] has classified
_Xylographa_ as a fungus.
The genera with linear apothecia are closely connected with Hysteriaceae,
and evidently inherit their fruit form severally from that family. There
is thus ample evidence of polyphyletic descent in the series. Stromatoid
fruits occur in Chiodectonaceae, with deeply sunk, almost closed disc,
but they have evidently evolved within the series, possibly from a
dividing up of the lirellae.
In Graphidineae there are also forms, more especially in Arthoniaceae,
on the border line between lichens and fungi: those with gonidia being
classified as lichens, those without gonidia having been placed in
corresponding genera of fungi. These latter athalline species live as
parasites or saprophytes.
The larger number of genera have a poorly developed thallus; in many of
them it is embedded within the outer periderm-cells of trees, and is
known as “hypophloeodal.” But in some families, such as Roccellaceae,
the thallus attains a very advanced form and a very high production of
acids.
The conception of Graphidineae as a whole is puzzling, but one or other
characteristic has brought the various members within the series. It is
in this respect an epitome of the lichen class of which the different
groups, with all their various origins and affinities, yet form a
distinct and well-defined section of the vegetable kingdom.
_d._ CYCLOCARPINEAE. This is by far the largest series of lichens. The
genera are associated with algae belonging both to the Myxophyceae and
the Chlorophyceae, and from the many different combinations are produced
great variations in the form of the vegetative body. The fruit is an
emergent, round or roundish disc or open apothecium in all the members
of the series except Pertusariaceae, where it is partially immersed in
thalline “warts.” In its most primitive form, described as “biatorine” or
“lecideine,” it may be soft and waxy (_Biatora_) or hard and carbonaceous
(_Lecidea_), in the latter the paraphyses being mostly coloured at the
tips; these are either simple or but sparingly branched, so that the
epithecium is a comparatively slight structure. The outer sterile tissue
forms a protective wall or “proper margin” which may be entirely pushed
aside, but generally persists as a distinct rim round the disc.
A great advance within the series arose when the gonidial elements of
the thallus took part in fruit-formation. In that case not only is the
hymenium generally subtended by a layer of algae, but thalline tissue
containing algae grows up around the fruit, and forms a second wall or
thalline margin. This type of apothecium, termed “lecanorine,” is thus
intimately associated with the assimilating tissue and food supply,
and it gains in capacity of ascus renewal and of long duration. This
development from non-marginate to marginate ascomata is necessarily an
accompaniment of symbiosis.
There is no doubt that the Cyclocarpineae derive from some simple form or
forms of Discomycete in the Patellariaceae. The relationship between that
family and the lower _Lecideae_ is very close. Rehm[993] finds the direct
ancestors of _Lecidea_ itself in the fungus genus, _Patinella_, in which
the apothecia are truly lecideine in character—open, flat and slightly
margined, the hypothecium nearly always dark-coloured and the paraphyses
branched, septate, clavate and coloured at the tips, forming a dark
epithecium. More definitely still he describes _Patinella atroviridis_,
a new species he discovered, as in all respects a _Lecidea_, but without
gonidia.
In the crustaceous Lecideaceae, a number of genera have been delimited
on spore characters—colourless or brown, and simple or variously
septate. In Patellariaceae as described by Rehm are included a number
of fungus genera which correspond to these lichen genera. Only two of
them—_Patinella_ and _Patellaria_—are saprophytic; in all the other
genera of the family, the species with very few exceptions are parasitic
on lichens: they are parasymbionts sharing the algal food supply; in any
case, they thrive on a symbiotic thallus.
Rehm unhesitatingly derives the corresponding lichen genera from these
fungi. He takes no account of the difficulty that if these parasitic (or
saprophytic) fungi are primitive, they have yet appeared either later in
time than the lichens on which they exist, or else in the course of ages
they have entirely changed their substratum.
He has traced, for instance, the lichen, _Buellia_, to a saprophytic
fungus species, _Karschia lignyota_, to a genus therefore in which
most of the species are parasitic on lichens and have generally been
classified as parasitic lichens. There is no advance in apothecial
characters from the fungus, _Karschia_, to _Buellia_, merely the change
to symbiosis. It therefore seems more in accordance with facts to regard
_Buellia_ as a genus evolved within the lichen series from _Patinella_
through _Lecidea_, and to accept these species of _Karschia_ on the
border line as parasitic, or even as saprophytic, reversions from
the lichen status. We may add that while these brown-spored lichens
are fairly abundant, the corresponding athalline or fungus forms are
comparatively few in number, which is exactly what might be expected from
plants with a reversionary history.
Occasionally in biatorine or lecideine species with a slight thalline
development all traces of the thallus disappear after the fructification
has reached maturity. The apothecia, if on wood or humus, appear to
be saprophytic and would at first sight be classified as fungi. They
have undoubtedly retained the capacity to live at certain stages, or in
certain conditions, as saprophytes.
The thallus disappears also in some species of the crustaceous genera
that possess apothecia with a thalline margin, and the fruits may be left
stranded and solitary on the normal substratum, or on some neighbouring
lichen thallus where they are more or less parasitic; but as the thalline
margin persists, there has been no question as to their nature and
affinity.
Rehm suggests that many species now included among lichens may be
ultimately proved to be fungi; but it is equally possible that the
reverse may be the case, as for instance _Bacidia flavovirescens_, held
by Rehm and others to be a parasitic fungus species, but since proved by
Tobler[994] to be a true lichen.
A note by Lightfoot[995], one of our old-time botanists who gave lichens
a considerable place in his Flora, foreshadows the theory of evolution
by gradual advance, and his views offer a suggestive commentary on the
subject under discussion. He was debating the systematic position of the
maritime lichen genus _Lichina_, considered then a kind of _Fucus_, and
had observed its similarity with true lichens. “The cavity,” he writes,
“at the top of the fructification (in _Lichina_) is a proof how nearly
this species of _Fucus_ is related to the scutellated lichens. Nature
disdains to be limited to the systematic rules of human invention. She
never makes any sudden starts from one class or genus to another, but is
regularly progressive in all her works, uniting the various links in the
chain of beings by insensible connexions.”
III. THE THALLUS
A. GENERAL OUTLINE OF DEVELOPMENT
_a._ PRELIMINARY CONSIDERATIONS. The evolution of lichens, as such,
has reference mainly to the thallus. Certain developments of the
fructification are evident, but the changes in the reproductive organs
have not kept pace with those of the vegetative structures: the highest
type of fruit, for instance, the apothecium with a thalline margin,
occurs in genera and species with a very primitive vegetative structure
as well as in those that have attained higher development.
Lichens are polyphyletic as regards their algal, as well as their fungal,
ancestors, so that it is impossible to indicate a straight line of
progression, but there is a general process of thalline development which
appears once and again in the different phyla. That process, from simpler
to more complicated forms, follows on two lines: on the one there is the
endeavour to increase the assimilating surface, on the other the tendency
to free the plant from the substratum. In both, the aim has been the
same, to secure more favourable conditions for assimilation and aeration.
Changes in structure have been already described[996], and it is only
needful to indicate here the main lines of evolution.
_b._ COURSE OF EVOLUTION IN HYMENOLICHENS. There is but little trace of
development in these lichens. The fungus has retained more or less the
form of the ancestral _Thelephora_ which has a wide-spreading superficial
basidiosporous hymenium. Three genera have been recognized, the
differences between them being due to the position within the thallus,
and the form of the _Scytonema_ that constitutes the gonidium. The
highest stage of development and of outward form is reached in _Cora_, in
which the gonidial zone is central in the tissue and is bounded above and
below by strata of hyphae.
_c._ COURSE OF EVOLUTION IN ASCOLICHENS. It is in the association with
Ascomycetes that evolution and adaptation have had full scope. In that
sub-class there are four constantly recurring and well-marked stages
of thalline development. (1) The earliest, most primitive stage, is
the crustaceous: at first an accretion of separate granules which may
finally be united into a continuous crust with a protective covering
of thick-walled amorphous hyphae forming a “decomposed” cortex. The
extension of a granule by growth in one direction upwards and outwards
gives detachment from the substratum, and originates (2) the squamule
which is, however, often of primitive structure and attached to the
support, like the granule, by the medullary hyphae. Further growth of
the squamule results in (3) the foliose thallus with all the adaptations
of structure peculiar to that form. In all of these, the principal area
of growth is round the free edges of the thallus. A greater change
takes place in the advance to (4) the fruticose type in which the more
active growing tissue is restricted to the apex, and in which the frond
or filament adheres at one point only to the support, a new series of
strengthening and other structures being evolved at the same time.
The lichen fungi associate, as has been already stated, with two
different types of algae: those combined with the Myxophyceae have been
designated _Phycolichenes_, those with Chlorophyceae as _Archilichenes_.
The latter predominate, not only in the number of lichens, but also in
the more varied advance of the thallus, although, in many instances,
genera and species of both series may be closely related.
B. COMPARATIVE ANTIQUITY OF ALGAL SYMBIONTS
One of the first questions of inheritance concerns the comparative
antiquity of the two gonidial series: with which kind of alga did the
fungus first form the symbiotic relationship? No assistance in solving
the problem is afforded by the type of fructification. The fungus in
Archilichens is frequently one of the more primitive Pyrenomycetes,
though more often a Discomycete, while in Phycolichens Pyrenomycetes are
very rare. There is, as already stated, no correlation of advance between
the fruit and the thallus, as the most highly evolved apothecia with
well-formed thalline margins are constantly combined with thalli of low
type.
Forssell[997] gave considerable attention to the question of
antiquity in his study of gelatinous crustaceous lichens in the
family Pyrenopsidaceae, termed by him Gloeolichens, and he came to
the conclusion that Archilichens represented the older combination,
Phycolichens being comparatively young.
His view is based on a study of the development of certain lichen
fungi that seem able to adapt themselves to either kind of algal
symbiont. He found[997] in _Euopsis_ (_Pyrenopsis_) _granatina_, one
of the Pyrenopsidaceae, that certain portions of the thallus contained
blue-green algae, while others contained _Palmella_, and that these
latter, though retrograde in development, might become fertile. The
granules with blue-green gonidia were stronger, more healthy and capable
of displacing those with _Palmella_, but not of bearing apothecia, though
spermogonia were embedded in them—a first step, according to Forssell,
towards the formation of apothecia. These granules, not having reached
a fruiting stage, were reckoned to be of a more recent type than those
associated with _Palmella_. In other instances, however, the line of
evolution has been undoubtedly from blue-green to more highly evolved
bright-green thalli.
The striking case of similarity between _Psoroma hypnorum_ (bright-green)
and _Pannaria rubiginosa_ (blue-green) may also be adduced. Forssell
considers that _Psoroma_ is the more ancient form, but as the fungus is
adapted to associate with either kind of alga, the type of squamules
forming the thallus may be gradually transformed by the substitution of
blue-green for the earlier bright-green—the _Pannaria_ superseding the
_Psoroma_. There is a close resemblance in the fructification—that is of
the fungus—in these two different lichens.
Hue[998] shares Forssell’s opinion as to the greater antiquity of
the bright-green gonidia and cites the case of _Solorina crocea_. In
that lichen there is a layer of bright-green gonidia in the usual
dorsiventral position, below the upper cortex. Below this zone there is
a second formed entirely of blue-green cells. Hue proved by his study of
development in _Solorina_ that the bright-green were the normal gonidia
of the thallus, and were the only ones present in the growing peripheral
areas; the blue-green were a later addition, and appeared first in small
groups at some distance from the edge of the lobes.
The whole subject of cephalodia-development[999] has a bearing on this
question. These bodies always contain blue-green algae, and are always
associated with Archilichens. Mostly they occur as excrescences, as in
_Stereocaulon_ and in _Peltigera_. The fungus of the host-lichen though
normally adapted to bright-green algae has the added capacity of forming
later a symbiosis with the blue-green. This tendency generally pervades a
whole genus or family, the members of which, as in Peltigeraceae, are too
closely related to allow as a rule of separate classification even when
the algae are totally distinct.
C. EVOLUTION OF PHYCOLICHENS
The association of lichen-forming fungi with blue-green algae may have
taken place later in time, or may have been less successful than with
the bright-green: they are fewer in number, and the blue-green type of
thallus is less highly evolved, though examples of very considerable
development are to be found in such genera as _Peltigera_, _Sticta_ or
_Nephromium_.
_a._ GLOEOLICHENS. Among crustaceous forms the thallus is generally
elementary, more especially in the Gloeolichens (Pyrenopsidaceae). The
algae of that family, _Gloeocapsa_, _Xanthocapsa_ or _Chroococcus_, are
furnished with broad gelatinous sheaths which, in the lichenoid state,
are penetrated and traversed by the fungal filaments, a branch hypha
generally touching with its tip the algal cell-wall. Under the influence
of symbiosis, the algal masses become firmer and more compact, without
much alteration in form; algae entirely free from hyphae are often
intermingled with the others. Even among Gloeolichens there are signs
of advancing development both in the internal structure and in outward
form. Lobes free from the substratum, though very minute, appear in
the genus _Paulia_, the single species of which comes from Polynesia.
Much larger lobes are characteristic of _Thyrea_, a Mediterranean and
American genus. The fruticose type, with upright fronds of minute size,
also appears in our native genus _Synalissa_. It is still more marked in
the coralloid thalli of _Peccania_ and _Phleopeccania_. In most of these
genera there is also a distinct tendency to differentiation of tissues,
with the gonidia congregating towards the better lighted surfaces. The
only cortex formation occurs in the crustaceous genus _Forssellia_ in
which, according to Zahlbruckner[1000], it is plectenchymatous above, the
thallus being attached below by hyphae penetrating the substratum. In
another genus, _Anema_[1001], which is minutely lobate-crustaceous, the
internal hyphae form a cellular network in which the algae are immeshed.
As regards algal symbionts, the members of this family are polyphyletic
in origin.
_b._ EPHEBACEAE AND COLLEMACEAE. In Ephebaceae the algae are tufted and
filamentous, _Scytonema_, _Stigonema_ or _Rivularia_, the trichomes of
which are surrounded by a common gelatinous sheath. The hyphae travel in
the sheath alongside the cell-rows, and the symbiotic plant retains the
tufted form of the alga as in _Lichina_ with _Rivularia_, _Leptogidium_
with _Scytonema_, and _Ephebe_ with _Stigonema_. The last named lichen
forms a tangle of intricate branching filaments about an inch or more in
length. The fruticose habit in these plants is an algal characteristic;
it has not been acquired as a result of symbiosis, and does not signify
any advance in evolution.
A plectenchymatous cortex marks some progress here also in
_Leptodendriscum_, _Leptogidium_ and _Polychidium_, all of which are
associated with _Scytonema_. These genera may well be derived from an
elementary form such as _Thermutis_. They differ from each other in spore
characters, etc., _Polychidium_ being the most highly developed with its
cortex of two cell-rows and with two-celled spores.
_Nostoc_ forms the gonidium of Collemaceae. In its free state it is
extremely gelatinous and transmits that character more or less to the
lichen. In the crustaceous genus _Physma_, which forms the base of the
_Collema_ group or phylum, there is but little difference in form
between the thalline warts of the lichen crust and the original small
_Nostoc_ colonies such as are to be found on damp mosses, etc.
In _Collema_ itself, the less advanced species are scarcely more than
crusts, though the more developed show considerable diversity of lobes,
either short and pulpy, or spreading out in a thin membrane. The _Nostoc_
chains pervade the homoiomerous thallus, but in some species they lie
more towards the upper surface. There is no cortex, though once and again
plectenchyma appears in the apothecial margin, both in this genus and in
_Leprocollema_ which is purely crustaceous.
_Leptogium_ is a higher type than _Collema_, the thallus being
distinguished by its cellular cortex. The tips of the hyphae, lying
close together at the surface, are cut off by one or more septa, giving
a one- or several-celled cortical layer. The species though generally
homoiomerous are of thinner texture and are less gelatinous than those of
_Collema_.
_c._ PYRENIDIACEAE. This small family of pyrenocarpous Phycolichens
may be considered here though its affinity, through the form of the
fruiting body, is with Archilichens. The gonidia are species of _Nostoc_,
_Scytonema_ and _Stigonema_. There are only five genera; one of these,
_Eolichen_, contains three species, the others are monotypic.
The crustaceous genera have a non-corticate thallus, but an advance
to lobate form takes place in _Placothelium_, an African genus. The
two genera that show most development are both British: _Coriscium_
(_Normandina_), which is lobate, heteromerous and corticate—though
always sterile—and _Pyrenidium_ which is fruticose in habit; the latter
is associated with _Nostoc_ and forms a minute sward of upright fronds,
corticate all round; the perithecium is provided with an entire wall and
is immersed in the thallus.
If the thallus alone were under consideration these lichens would rank
with Pannariaceae.
_d._ HEPPIACEAE AND PANNARIACEAE. The next stage in the development of
Phycolichens takes place through the algae, _Scytonema_ and _Nostoc_,
losing not only their gelatinous sheaths, but also, to a large extent,
their characteristic forms. Chains of cells can frequently be observed,
but accurate and certain identification of the algal genus is only
possible by making separate cultures of the gonidia.
_Scytonema_ forms the gonidium of the squamulose Heppiaceae consisting
of the single genus _Heppia_. The ground tissue of the species is either
wholly of plectenchyma with algae in the interstices, or the centre is
occupied by a narrow medulla of loose filaments.
In the allied family Pannariaceae, a number of genera contain _Scytonema_
or _Nostoc_, while two, _Psoroma_ and _Psoromaria_, have bright-green
gonidia. The thallus varies from crustaceous or minutely squamulose, to
lobes of fair dimension in _Parmeliella_ and in _Hydrothyria venosa_,
an aquatic lichen. Plectenchyma appears in the upper cortex of both of
these, and in the proper margin of the apothecia, while the under surface
is frequently provided with rhizoidal filaments.
These two families form a transition between the gelatinous, and mostly
homoiomerous thallus, and the more developed entirely heteromerous
thallus of much more advanced structure. The fructification in all of
them, gelatinous and non-gelatinous, is a more or less open apothecium,
sometimes immarginate, and biatorine or lecideine, but often, even
in species nearly related to these, it is lecanorine with a thalline
amphithecium. Rarely are the sporiferous bodies sunk in the tissue, with
a pseudo-perithecium, as in _Phylliscum_. It would be difficult to trace
advance in all this group on the lines of fruit development. The two
genera with bright-green gonidia, _Psoroma_ and _Psoromaria_, have been
included in Pannariaceae owing to the very close affinity of _Psoroma
hypnorum_ with _Pannaria rubiginosa_; they are alike in every respect
except in their gonidia. _Psoromaria_ is exactly like _Psoroma_, but
with immarginate biatorine apothecia, representing therefore a lower
development in that respect.
These lichens not only mark the transition from gelatinous to
non-gelatinous forms, but in some of them there is an interchange of
gonidia. The progression in the phylum or phyla has evidently been
from blue-green up to some highly evolved forms with bright-green
algae, though there may have been, at the beginning, a substitution of
blue-green in place of earlier bright-green algae, Phycolichens usurping
as it were the Archilichen condition.
_e._ PELTIGERACEAE AND STICTACEAE. The two families just examined
marked a great advance which culminated in the lobate aquatic lichen
_Hydrothyria_. This lichen, as Sturgis pointed out, shows affinity with
other Pannariaceae in the structure of the single large-celled cortical
layer as well as with species of _Nephroma_ (Peltigeraceae). A still
closer affinity may be traced with _Peltigera_ in the presence in both
plants of veins on the under surface. The capacity of _Peltigera_ species
to grow in damp situations may also be inherited from a form like the
submerged _Hydrothyria_. In both families there are transitions from
blue-green to bright-green gonidia, or _vice versa_, in related species.
Thus in Peltigeraceae we find _Peltigera_ containing _Nostoc_ in the
gonidial zone, with _Peltidea_ which may be regarded as a separate genus,
or more naturally as a section of _Peltigera_; it contains bright-green
gonidia, but has cephalodia containing _Nostoc_ associated with its
thallus.
The genus _Nephroma_ is similarly divided into species with a
bright-green gonidial zone, chiefly Arctic or Antarctic in distribution,
and species with _Nostoc_ (subgenus _Nephromium_) more numerous and more
widely distributed.
_Peltigera_ and _Nephroma_ are also closely related in the character of
the fructification. It is a flat non-marginate disc borne on the edge of
the thallus: in _Peltigera_ on the upper surface, in _Nephroma_ on the
under surface. The remaining genus _Solorina_ contains normally a layer
of bright-green algae, but, along with these, there are always present
more or fewer _Nostoc_ cells, either in a thin layer as in _S. crocea_ or
as cephalodia in others, while, in three species the algae are altogether
blue-green.
The members of the Peltigeraceae have a thick upper cortex of
plectenchyma and in some cases strengthening veins, and long rhizinae on
the lower side. Some of the species attain a large size, and, in some,
soredia are formed, an evidence of advance, this being a peculiarly
lichenoid form of reproduction.
The Stictaceae form a parallel but more highly organized family, which
also includes closely related bright-green and blue-green series. They
are all dorsiventral, but they are mostly attached by a single hold-fast
and the lobes in some species suggest the fruticose type in their long
narrow form. A wide cortex of plectenchyma protects both the upper and
the lower surface and a felt of hairs replaces the rhizinae of other
foliose lichens. In the genus _Sticta_ (including the section _Stictina_)
special aeration organs, cyphellae or pseudocyphellae, are provided; in
_Lobaria_ these are replaced by naked areas which serve the same purpose.
Nylander[1002] regarded the Stictaceae as the most highly developed of
all lichens, and they easily take a high place among dorsiventral forms,
but it is generally conceded that the fruticose type is the more highly
organized. In any case they are the highest reach of the phylum or phyla
that started with Pyrenopsidaceae and Collemaceae; the lowly gelatinous
thalli changing to more elaborate structures with the abandonment of the
gelatinous algal sheath, as in the Pannariaceae, and with the replacement
of blue-green by bright-green gonidia. Reinke[1003], considers the
Stictaceae as evolved from the Pannariaceae more directly from the genus
_Massalongia_. Their relationship is certainly with Pannariaceae and
Peltigeraceae rather than with Parmeliaceae; these latter, as we shall
see, belong to a wholly different series.
D. EVOLUTION OF ARCHILICHENS
The study of Archilichens as of Phycolichens is complicated by the many
different kinds of fungi and algae that have entered into combination;
but the two principal types of algae are the single-celled _Protococcus_
group and the filamentous _Trentepohlia_: as before only the broad lines
of thalline development will be traced.
The elementary forms in the different series are of the simplest type—a
somewhat fortuitous association of alga and fungus, which in time bears
the lichen fructification. It has been stated that the greatest advance
of all took place with the formation of a cortex over the primitive
granule, followed by a restricted area of growth outward or upward
which resulted finally in the foliose and fruticose thalli. Guidance
in following the course of evolution is afforded by the character of
the fructification, which generally shows some great similarity of type
throughout the different phyla, and remains fairly constant during the
many changes of thalline evolution. Development starting from one or many
origins advances point by point in a series of parallel lines.
_a._ THALLUS OF PYRENOCARPINEAE. In this series there are two families
of algae that function as gonidia: Protococcaceae, consisting of single
cells, and Trentepohliaceae, filamentous. _Phyllactidium_ (_Cephaleuros_)
appears in a single genus, _Strigula_, a tropical epiphytic lichen.
Associated with these types of algae are a large number of genera and
species of an elementary character, without any differentiation of
tissue. In many instances the thallus is partly or wholly embedded in the
substratum.
Squamulose or foliose forms make their appearance in Dermatocarpaceae:
in _Normandina_ the delicate shell-like squamules are non-corticate, but
in other genera, _Endocarpon_, _Placidiopsis_, etc., the squamules are
corticate and of firmer texture, while in _Dermatocarpon_, foliose fronds
of considerable size are formed. The perithecial fruits are embedded in
the upper surface.
In only one extremely rare lichen, _Pyrenothamnia Spraguei_ (N. America),
is there fruticose development: the thallus, round and stalk-like at the
base, branches above into broader more leaf-like expansions.
_b._ THALLUS OF CONIOCARPINEAE. At the base of this series are
genera and species that are extremely elementary as regards thalline
formation, with others that are saprophytic and parasitic. The
simplest type of thallus occurs in Caliciaceae, a spreading mycelium
with associated algae (Protococcaceae) collected in small scattered
granules, resembling somewhat a collection of loose soredia. The
species grow mostly on old wood, trunks of trees, etc. In _Calicium_
(_Chaenotheca_) _chrysocephalum_ as described by Neubner[1004] the
first thallus formation begins with these scattered minute granules;
gradually they increase in size and number till a thick granular coating
of the substratum arises, but no cortex is formed and there is no
differentiation of tissue.
The genus _Cyphelium_ (Cypheliaceae) is considered by Reinke to be
more highly developed, inasmuch as the thalline granules, though
non-corticate, are more extended horizontally, and, in vertical
section, show a distinct differentiation into gonidial zone and
medulla. The sessile fruit also takes origin from the thallus, and is
surrounded by a thalline amphithecium, or rather it remains embedded
in the thalline granule. A closely allied tropical genus _Pyrgillus_
has reached a somewhat similar stage of development, but with a more
coherent homogeneous thallus, while in _Tylophoron_, also tropical or
subtropical, the fruit is raised above the crustaceous thallus but is
thickly surrounded by a thalline margin. The alga of that genus is
_Trentepohlia_, a rare constituent of Coniocarpineae.
A much more advanced formation appears in the remaining family
Sphaerophoraceae. In _Calycidium_, a monotypic New Zealand genus, the
thallus consists of minute squamules, dorsiventral in structure but with
a tendency to vertical growth, the upper surface is corticate and the
mazaedial apothecia—always open—are situated on the margins. _Tholurna
dissimilis_, (Scandinavian) still more highly developed, has two kinds
of rather small fronds corticate on both surfaces, the one horizontal in
growth, crenulate in outline, and sterile, the other vertical, about 2
mm. in height, hollow and terminating in a papilla in which is seated the
apothecium.
Two other monotypic subtropical genera form a connecting link with the
more highly evolved forms. In the first, _Acroscyphus sphaerophoroides_,
the fronds are somewhat similar to the fertile ones of _Tholurna_, but
they possess a solid central strand and the apical mazaedium is less
enveloped by the thallus. The other, _Pleurocybe madagascarea_, has
narrow flattish branching fronds about 3 cm. in height, hollow in the
centre and corticate with marginal or surface fruits.
The third genus, _Sphaerophorus_, is cosmopolitan; three of the species
are British and are fairly common on moorlands, etc. They are fruticose
in habit, being composed of congregate upright branching stalks, either
round or slightly compressed and varying in height from about 1 to 8
cm. The structure is radiate with a well-developed outer cortex, and a
central strand which gives strength to the somewhat slender stalks. The
fruits are lodged in the swollen tips and are at first enclosed; later,
the covering thallus splits irregularly and exposes the hymenium.
Coniocarpineae comprise only a comparatively small number of genera
and species, but the series is of unusual interest as being extremely
well defined by the fruit-formation and as representing all the various
stages of thalline development from the primitive crustaceous to the
highly evolved fruticose type. With the primitive thallus is associated
a wholly fungal fruit, both stalk and capitulum, which in the higher
forms is surrounded and protected by the thallus. Lichen-acids are freely
produced even in crustaceous forms, and they, along with the high stage
of development reached, testify to the great antiquity of the series.
_c._ THALLUS OF GRAPHIDINEAE. As formerly understood, this series
included only crustaceous forms with an extremely simple development
of thallus, fungi and algae—whether Palmellaceae, etc., or more
frequently Trentepohliaceae—growing side by side either superficially
or embedded in tree or rock, the presence of the vegetative body being
often signalled only by a deeper colouration of the substratum. The
researches of Almquist, and more recently of Reinke and Darbishire, have
enlarged our conception of the series, and the families Dirinaceae and
Roccellaceae are now classified in Graphidineae.
Arthoniaceae, Graphidaceae and Chiodectonaceae are all wholly
crustaceous. The first thalline advance takes place in Dirinaceae with
two allied genera, _Dirina_ and _Dirinastrum_. Though the thallus is
still crustaceous, it is of considerable thickness, with differentiation
of tissues: on the lower side there is a loosely filamentous medulla from
which hyphae pierce the substratum and secure attachment. _Trentepohlia_
gonidia lie in a zone above the medulla, and the upper cortex is formed
of regular palisade hyphae forming a “fastigiate cortex.” It is the
constant presence of _Trentepohlia_ algae as well as the tendency to
ellipsoid or lirellate fruits that have influenced the inclusion of
Dirinaceae and Roccellaceae in the series.
The thallus of Dirinaceae is crustaceous, while the genera of
Roccellaceae are mostly of an advanced fruticose type, though in one,
_Roccellina_, there is a crustaceous thallus with an upright portion
consisting of short swollen podetia-like structures with apothecia
at the tips; and in another, _Roccellographa_, the fronds broaden to
leafy expansions. They are nearly all rock-dwellers, often inhabiting
wind-swept maritime coasts, and a strong basal sheath has been evolved to
strengthen their foothold. In some genera the sheath contains gonidia;
in others the tissue is wholly of hyphae—in nearly every case it is
protected by a cortex.
In the upright fronds the structure is radiate: generally a rather loose
strand of hyphae more or less parallel with the long axis of the plant
forms a central medulla. The gonidia lie outside the medulla and just
within the outer cortex. The latter, in a few genera, is fibrous, the
parallel hyphae being very closely compacted; but in most members of the
family the fastigiate type prevails, as in the allied family Dirinaceae.
_d._ THALLUS OF CYCLOCARPINEAE. This is by far the largest and most
varied series of Archilichens. It is derived, as regards the fungal
constituent, from the Discomycetes, but in these fungi, the vegetative or
mycelial body gives no aid to the classification which depends wholly on
apothecial characters. In the symbiotic condition, on the contrary, the
thallus becomes of extreme importance in the determination of families,
genera and species. There has been within the series a great development
both of apothecial and of thalline characters in parallel lines or phyla.
_AA._ _LECIDEALES._ The type of fruit nearest to fungi in form and origin
occurs in the Lecideales. It is an open disc developed from the fungal
symbiont alone, the alga taking no part. There are several phyla to be
considered.
_aa._ _COENOGONIACEAE._ There are two types of gonidial algae in this
family, and both are filamentous forms, _Trentepohlia_ in _Coenogonium_
and _Cladophora_ in _Racodium_. The resulting lichens retain the slender
thread-like form of the algae, their cells being thinly invested by the
hyphae and both symbionts growing apically. The thalline filaments are
generally very sparingly branched and grow radially side by side in a
loose flat expansion attached at one side by a sheath, or the strands
spread irregularly over the substratum. Plectenchyma appears in the
apothecial margin in _Coenogonium_. Fruiting bodies are unknown in
_Racodium_.
Coenogoniaceae are a group apart and of slight development, only the one
kind of thallus appearing; the form is moulded on that of the gonidium,
and is, as Reinke[1005] remarks, perfectly adapted to receive the maximum
of illumination and aeration.
_bb._ _LECIDEACEAE AND GYROPHORACEAE._ The origin of this thalline phylum
is distinct from that of the previous family, being associated with a
different type of gonidium, the single-celled alga of the Protococcaceae.
The more elementary species are of extremely simple structure as
exemplified in such species as _Lecidea_ (_Biatora_) _uliginosa_
or _Lecidea granulosa_. These lichens grow on humus-soil and the
thallus consists of a spreading mycelium or hypothallus with more or
less scattered thalline granules containing gonidia, but without any
defined structure. The first advance takes place in the aggregation
and consolidation of such thalline granules and the massing of the
gonidia towards the light, thus substituting the heteromerous for the
homoiomerous arrangement of the tissues. The various characters of
thickness, areolation, colour, etc. of the thallus are constant and
are expressed in specific diagnoses. Frequently an amorphous cortex of
swollen hyphae provides a smooth upper surface and forms a protective
covering for such long-lived species as _Rhizocarpon geographicum_, etc.
The squamulose thallus is well represented in this phylum. The squamules
vary in size and texture but are mostly rather thick and stiff. In
_Lecidea ostreata_ they rise from the substratum in serried rows forming
a dense sward; in _L. decipiens_, also a British species, the squamules
are still larger, and more horizontal in direction; they are thick and
firm and the upper cortex is a plectenchyma of cells with swollen walls.
Solitary hyphae from the medulla pass downwards into the support.
Changes in spore characters also arise in these different thalline
series, as for instance in genera such as _Biatorina_ and _Buellia_,
the one with colourless, the other with brown, two-celled spores. These
variations, along with changes in the thallus, are of specific or generic
importance following the significance accorded to the various characters.
In one lichen of the series, the monotypic Brazilian genus
_Sphaerophoropsis stereocauloides_, the thallus is described by
Wainio[1006] as consisting of minute clavate stalks of interwoven
thick-walled hyphae, with gelatinous algae, like _Gloeocapsa_,
interspersed in groups, though with a tendency to congregate towards the
outer surface.
The highest development along this line of advance is to be found in the
Gyrophoraceae, a family of lichens with a varied foliose character and
dark lecideine apothecia. The thallus may be monophyllous and of fairly
large dimensions or polyphyllous; it is mostly anchored by a central
stout hold-fast and both surfaces are thickly corticate with a layer
of plectenchyma; the under surface is mostly bare, but may be densely
covered with rhizina-like strands of dark hyphae. They are all northern
species and rock-dwellers exposed to severe extremes of illumination
and temperature, but well protected by the thick cortex and the dark
colouration common to them all.
_cc._ _CLADONIACEAE._ This last phylum of Lecideales is the most
interesting as it is the most complicated. It possesses a primary,
generally sterile, thallus which is dorsiventral and crustaceous,
squamulose or in some instances almost foliaceous, along with a secondary
thallus of upright radiate structure and of very varied form, known as
the podetium which bears at the summit the fertile organs.
A double thallus has been suggested in the spreading base, containing
gonidia, of some radiate lichens such as _Roccella_, but the upright
portion of such lichens, though analogous, is not homologous with that of
Cladoniaceae.
The algal cells of the family belong to the Protococcaceae.
Blue-green algae are associated in the cephalodia of _Pilophorus_ and
_Stereocaulon_. The primary thallus is a feature of all the members,
though sometimes very slight and very short-lived, as in _Stereocaulon_
or in the section _Cladina_ of the genus _Cladonia_. Where the primary
thallus is most largely developed, the secondary (the podetium) is less
prominent.
This secondary thallus originates in two different ways: (1) the primary
granule may grow upward, the whole of the tissues taking part in the
new development; or (2) the origin may be endogenous and proceed from
the hyphae only of the gonidial zone: these push upwards in a compact
fascicle, as in the apothecial development of _Lecidea_, but instead of
spreading outward on reaching the surface, they continue to grow in a
vertical direction and form the podetium. In origin this is an apothecial
stalk, but generally it is clothed with gonidial tissue. The gonidia may
travel upwards from the base or they may possibly be wind borne from
the open. The podetium thus takes on an assimilative function and is a
secondary thallus.
The same type of apothecium is common to all the genera; the spores are
colourless and mostly simple, but there are also changes in form and
septation not commensurate with thalline advance, as has been already
noted. Thus in _Gomphillus_, with primitive thallus and podetium, the
spores are long and narrow with about 100 divisions.
1. ORIGIN OF CLADONIA. There is no difficulty in deriving Cladoniaceae
from _Lecidea_, or, more exactly, from some crustaceous species of the
section _Biatora_ in which the apothecia—as in Cladoniaceae—are waxy and
more or less light-coloured and without a thalline margin. In only a very
few isolated instances has a thalline margin grown round the _Cladonia_
fruit.
There are ten genera included in the Cladoniaceae, of which five are
British. Considerable study has been devoted to the elucidation of
developmental problems within the family by various workers, more
especially in the large and varied genus _Cladonia_ which is complicated
by the presence of the two thalli. The family is monophyletic in origin,
though many subordinate phyla appear later.
2. EVOLUTION OF THE PRIMARY THALLUS. At the base of the series we find
here also an elementary granular thallus which appears in some species
of most of the genera. In _Gomphillus_, a monospecific British genus,
the granules have coalesced into a continuous mucilaginous membrane.
In _Baeomyces_, though mostly crustaceous, there is an advance to the
squamulose type in _B. placophyllus_, and in two Brazilian species
described by Wainio, one of which, owing to the form of the fronds,
has been placed in a separate genus _Heteromyces_. The primary thallus
becomes almost foliose also in _Gymnoderma coccocarpum_ from the
Himalayas, with dorsiventral stratose arrangement of the tissues, but
without rhizinae. The greatest diversity is however to be found in
_Cladonia_ where granular, squamulose and almost foliose thalli occur.
The various tissue formations have already been described[1007].
3. EVOLUTION OF THE SECONDARY THALLUS. Most of the interest centres round
the development and function of the podetium. In several genera the
primordium is homologous with that of an apothecium; its elongation to an
apothecial stalk is associated with delayed fructification, and though
it has taken on the function of the vegetative thallus, the purpose of
elongation has doubtless been to secure good light conditions for the
fruit, and to facilitate a wide distribution of spores: therefore, not
only in development but in function, its chief importance though now
assimilative was originally reproductive. The vegetative development of
the podetium is correlated with the reduction of the primary thallus
which in many species bears little relation in size or persistence to the
structure produced from it, as, for instance, in _Cladonia rangiferina_
where the ground thallus is of the scantiest and very soon disappears,
while the podetial thallus continues to grow indefinitely and to
considerable size.
4. COURSE OF PODETIAL DEVELOPMENT. In _Baeomyces_ the podetial primordium
is wholly endogenous in some species, but in others the outer cortical
layer of the primary thallus as well as the gonidial hyphae take part
in the formation of the new structure which, in that case, is simply a
vertical extension of the primary granule. This type of podetium—called
by Wainio[1008] a pseudopodetium—also recurs in _Pilophorus_ and in
_Stereocaulon_. To emphasize the distinction of origin it has been
proposed to classify these two latter genera in a separate family, but in
that case it would be necessary to break up the genus _Baeomyces_. We may
assume that the endogenous origin of the “apothecial stalk” is the more
primitive, as it occurs in the most primitive lecideine lichens, whereas
a vertical thallus is always an advanced stage of vegetative development.
Podetia are essentially secondary structures, and they are associated
both with crustaceous and squamulose primary thalli. If monophyletic in
origin their development must have taken place while the primary thallus
was still in the crustaceous stage, and the inherited tendency to form
podetia must then have persisted through the change to the squamulose
type. In species such as _Cl. caespiticia_ the presence of rudimentary
podetia along with large squamules suggests a polyphyletic origin, but
Wainio’s[1008] opinion is that such instances may show retrogression
from an advanced podetial form, and that the evidence inclines to the
monophyletic view of their origin.
The hollow centre of the podetium arises in the course of development
and is common to nearly all advanced stages of growth. There are however
some exceptions: in _Glossodium aversum_, a soil lichen from New Granada,
and the only representative of the genus, a simple or rarely forked
stalk about 2 cm. in height rises from a granular or minutely squamulose
thallus. The apothecium occupies one side of the flattened and somewhat
wider apex. There is no external cortex and the central tissue is of
loose hyphae. In _Thysanothecium Hookeri_, also a monotypic genus from
Australia, the podetia are about the same height, but, though round at
the base, they broaden upwards into a leaf-like expansion. The central
tissue below is of loose hyphae, but compact strands occur above, where
the apothecium spreads over the upper side. The under surface is sterile
and is traversed by nerve-like strands of hyphae.
5. VARIATION IN CLADONIA. It is in this genus that most variation is
to be found. Characters of importance and persistence have arisen by
which secondary phyla may be traced within the genus: these are mainly
(1) the relative development of the horizontal and vertical structures,
(2) formation of the scyphus and branching of the podetium, with (3)
differences in colour both in the vegetative thallus and in the apothecia.
Wainio has indicated the course of evolution on the following lines: (1)
the crustaceous thallus is monophyletic in origin and here as elsewhere
precedes the squamulose. The latter he considers to be also monophyletic,
though at more than one point the more advanced and larger foliose forms
have appeared: (2) the primitive podetium was subulate and unbranched,
and the apex was occupied by the apothecium. Both scyphus and branching
are later developments indicating progress. They are in both cases
associated with fruit-formation—scyphi generally arising from abortive
apothecia[1009], branching from aggregate apothecia. In forms such as
_Cl. fimbriata_, where both scyphiferous and subulate sterile podetia
are frequent, the latter (subspecies _fibula_) are retrogressive, and
reproduce the ancestral pointed podetium. (3) In subgen. _Cenomyce_,
with a squamulose primary thallus, there is a sharp division into two
main phyla characterized by the colour of the apothecia, brown in
_Ochrophaeae_—the colour being due to a pigment—and red in _Cocciferae_
where the colouring substance is a lichen-acid, rhodocladonic acid.
In the brown-fruited _Ochrophaeae_ there are again several secondary
phyla. Two of these are distinguished primarily by the character of the
branching: (_a_) the _Chasmariae_ in which two or several branches arise
from the same level, entailing perforation of the axils (_Cl. furcata_,
_Cl. rangiformis_, _Cl. squamosa_, etc.), the scyphi also are perforated.
They are further characterized by peltate aggregate apothecia, this
grouping of the apothecia according to Wainio being the primary cause
of the complex branching, the several fruit stalks growing out as
branches. The second group (_b_), the _Clausae_, are not perforated and
the apothecia are simple and broad-based on the edge of the scyphus
(_Cl. pyxidata_, _Cl. fimbriata_, etc.), or on the tips of the podetia
(_Cl. cariosa_, _Cl. leptophylla_, etc.). A third very small group also
of _Clausae_ called (_c_) _Foliosae_ has very large primary squamules
and reduced podetia (_Cl. foliacea_, etc.), while finally (_d_) the
_Ochroleucae_, none of which is British, have poorly developed squamules
and variously formed yellowish podetia with pale-coloured apothecia.
The _Cocciferae_ represent a phylum parallel in development with the
_Ochrophaeae_. The species have perhaps most affinity with the _Clausae_,
the vegetative thallus—both the squamules and the podetia—being very
much alike in several species. Wainio distinguishes two groups based on
a difference of colour in the squamules, glaucous green in one case,
yellowish in the other.
6. CAUSES OF VARIATION. External causes of variation in _Cladonia_ are
chiefly humidity and light, excess or lack of either effecting changes
which may have become fixed and hereditary. Minor changes directly
traceable to these influences are also frequent, viz. size of podetia,
proliferation and the production more or less of soredia or of squamules
on the podetia, though only in connection with species in which these
variations are already an acquired character. The squamules on the
podetium more or less repeat the form of the basal squamules.
7. PODETIAL DEVELOPMENT AND SPORE-DISSEMINATION. In a recent paper by
Hans Sättler[1010] the problem of podetial development in _Cladonia_ is
viewed from a different standpoint. He holds that as the podetia are
apothecial stalks, their service to the plant consists in the raising of
the mature fruit in order to secure a wide distribution of the spores,
and that changes in the form of the podetium are therefore but new
adaptations for the more efficient discharge of this function.
Following out this idea he regards as the more primitive forms those
in which both the spermogonia, as male reproductive bodies, and the
carpogonia occur on the primary thallus, ascogonia and trichogynes being
formed before the podetium emerges from the thallus. Fertilization thus
must take place at a very early period, though the ultimate fruiting
stage may be long delayed. Sättler considers that any doubt as to actual
fertilization is without bearing on the question, as sexuality he holds
must have originally existed and must have directed the course of
evolution in the reproductive bodies. In this primitive group, called
by him the “Floerkeana” group, the podetia are always short and simple,
they are terminated by the apothecium and no scyphi are formed (_Cl.
Floerkeana_, _Cl. leptophylla_, _Cl. cariosa_, _Cl. caespiticia_, _Cl.
papillaria_, etc.).
In his second or “pyxidata” group, he places those species in which the
apothecia are borne at the edge of a scyphus. That structure he follows
Wainio in regarding as a morphological reaction on the failure of the
first formed apical apothecium: it is, he adds, a new thallus in the form
of a spreading cup and bears, as did the primary thallus, both the female
primordia and the spermogonia. In some species, such as _Cl. foliacea_,
there may be either scyphous or ascyphous podetia, and spermogonia
normally accompany the carpogonium appearing accordingly along with it
either on the squamule or on the scyphus.
As the pointed podetia are the more primitive, Sättler points out that
they may reappear as retrogressive structures, and have so appeared
in the “pyxidata” group in such species as _Cl. fimbriata_. He refers
to Wainio’s statement that the abortion of the apothecium being a
retrogressive anomaly, while scyphus formation is an evolutionary
advance, the scyphiferous species present the singular case, “that a
progressive transmutation induced by a retrogressive anomaly has become
constant.”
His third group includes those forms that grow in crowded tufts or
swards such as _Cl. rangiferina_, _Cl. furcata_, _Cl. gracilis_, etc.
They originate, as did the pyxidata group, in some _Floerkeana_-like
form, but in the “rangiferina” group instead of cup-formation there is
extensive branching. In the closely packed phalanx of branches water is
retained as in similar growths of mosses, and moist conditions necessary
for fertilization are thus secured as efficiently as by the water-holding
scyphus.
Sättler in his argument has passed over many important points. Above
all he ignores the fact that whatever may have been the original nature
and function of the podetium, it has now become a thalline structure
and provides for the vegetative life of the plant, and that it is in
its thalline condition that the many variations have been formed; the
scyphus is not, as he contends, a new thallus, it is only an extension of
thalline characters already acquired.
8. PILOPHORUS, STEREOCAULON AND ARGOPSIS. These closely related genera
are classified with _Cladonia_ as they share with it the twofold thallus
and the lecideine apothecia. The origin of the podetium being different
they may be held to constitute a phylum apart, which has however taken
origin also from some _Biatora_ form.
The primary thallus is crustaceous or minutely squamulose and the
podetia of _Pilophorus_, which are short and unbranched (or very
sparingly branched), are beset with thalline granules. The podetia of
_Stereocaulon_ and _Argopsis_ are copiously branched and are more or
less thickly covered with minute variously divided leaflets. Cephalodia
containing blue-green algae occur on the podetia of these latter genera;
in _Pilophorus_ they are intermixed with the primary thallus.
The tissue systems are less advanced in these genera than in _Cladonia_:
there is no cortex present either in _Pilophorus_ or in _Argopsis_ or in
some species of _Stereocaulon_, though in others a gelatinous amorphous
layer covers the podetia and also the stalk leaflets. The stalks are
filled with loose hyphae in the centre.
_BB._ _LECANORALES._ This second group of Cyclocarpineae is distinguished
by the marginate apothecium, a thalline layer providing a protecting
amphithecium. The lecanorine apothecium is of a more or less soft
and waxy consistency, and though the disc is sometimes almost black,
neither hypothecium nor parathecium is carbonaceous as in _Lecidea_. The
affinity of _Lecanora_ is with sect. _Biatora_, and development must
have been from a biatorine form with a persistent thallus. The margin or
amphithecium varies in thickness: in some species it is but scanty and
soon excluded by the over-topping growth of the disc, so that a zone of
gonidia underlying the hypothecium is often the only evidence of gonidial
intrusion left in fully formed fruits.
The marginate apothecium has appeared once and again as we have seen.
It is probable however that its first development was in this group of
lichens, and even here there may have been more than one origin as there
is certainly more than one phylum.
_aa._ _COURSE OF DEVELOPMENT._ At the base of the series, the thallus
is of the crustaceous type somewhat similar to that of _Lecidea_, but
there are none of the very simple primitive forms. _Lecanora_ must
have originated when the crustaceous lecideine thallus was already
well established. Its affinity is with _Lecidea_ and not with any
fungus: where the thallus is evanescent or scanty, its lack is due to
retrogressive rather than to primitive characters.
_bb._ _LECANORACEAE._ A number of genera have arisen in this large
family, but they are distinguished mainly if not entirely by spore
characters, and by some systematists have all been included in the
one genus _Lecanora_, since the changes have taken place within the
developing apothecium.
There is one genus, _Harpidium_, which is based on thalline characters,
represented by one species, _H. rutilans_, common enough on the
Continent, but not yet found in our country. It has a thin crustaceous
homoiomerous thallus, the component hyphae of which are divided into
short cells closely packed together and forming a kind of cellular
tissue in which the algae are interspersed. The dorsiventral stratose
arrangement prevails however in the other genera and a more or less
amorphous “decomposed” cortex is frequently present. The medulla rests on
the substratum.
With the stouter thallus, there is slightly more variety of crustaceous
form than in Lecideaceae: there occurs occasionally an outgrowth of the
thalline granules as in _Haematomma ventosum_ which marks the beginning
of fruticulose structure. Of a more advanced structure is the thallus of
_Lecanora esculenta_, a desert lichen which becomes detached and erratic,
and which in some of its forms is almost coralline, owing to the apical
growth of the original granules or branches: a more or less radiate
arrangement of the tissues is thus acquired.
The squamulose type is well represented in _Lecanora_, and the species
with that form of thallus have frequently been placed in a separate
genus, _Squamaria_. These squamules are never very large; they possess an
upper, somewhat amorphous, cortex; the medulla rests on the substratum,
except in such a species as _Lecanora lentigera_, where they are free, a
sort of fibrous cortex being formed of hyphae which grow in a direction
parallel with the surface. In none of them are rhizinae developed.
_cc._ _PARMELIACEAE._ The chief advance, apart from size, of the
squamulose to the foliose type is the acquirement of a lower cortex along
with definite organs of attachment which in Parmeliaceae are invariably
rhizoidal and are composed of compact strands of hyphae extending from
the cells of the lower cortex.
In the genus _Parmelia_ rhizinae are almost a constant character, though
in a few species, such as _Parmelia physodes_, they are scanty or
practically absent. It is not possible, however, to consider that these
species form a lower group, as in other respects they are highly evolved,
and rhizinae may be found at points on the lower surface where there
is irritation by friction. Soredia and isidia occur frequently and, in
several species, almost entirely replace reproduction by spores. In one
or two northern or Alpine species, _P. stygia_ and _P. pubescens_, the
lobes are linear or almost filamentous. They are retained in _Parmelia_
because the apothecia are superficial on the fronds which are partly
dorsiventral, and because rhizinae have occasionally been found. Some of
the _Parmeliae_ attain to a considerable size; growth is centrifugal and
long continued.
Two monotypic genera classified under Parmeliaceae, _Physcidia_ and
_Heterodea_, are of considerable interest as they indicate the bases of
parallel development in _Parmelia_ and _Cetraria_. The former, a small
lichen, is corticate only on the upper surface, and without rhizinae;
and from the description, the cortex is of a fastigiate character. The
solitary species grows on bark in Cuba; it is related to _Parmelia_,
as the apothecia are superficial on the lobes. The second, _Heterodea
Mülleri_, a soil-lichen from Australasia, is more akin to _Cetraria_ in
that the apothecia are terminal. The upper surface is corticate with
marginal cilia, the lower surface naked or only protected by a weft of
brownish hyphae amongst which cyphellae are formed; pseudocyphellae
appear in _Cetraria_.
The genus _Cetraria_ contains very highly developed thalline forms,
either horizontal (subgenus _Platysma_), or upright (_Eucetraria_).
Rhizinae are scanty or absent, but marginal cilia in some upright species
act as haptera. _Cetraria aculeata_ is truly fruticose with a radiate
structure.
An extraordinary development of the under cortex characterizes the genera
_Anzia_[1011] and _Pannoparmelia_: rhizinae-like strands formed from the
cortical cells branch and anastomose with others till a wide mesh of
a spongy nature is formed. They are mostly tropical or subtropical or
Australasian, and possibly the spongy mass may be of service in retaining
moisture. A species of _Anzia_ has been recorded by Darbishire[1012] from
Tierra del Fuego.
_dd._ _USNEACEAE._ As we have seen, the change to fruticose structure
has arisen as an ultimate development in a number of groups; it reaches
however its highest and most varied form in this family. Not only are
there strap-shaped thalli, but a new form, the filamentous and pendulous,
appears; it attains to a great length, and is fitted to withstand severe
strain. The various adaptations of structure in these two types of
thallus have already been described[1013].
In _Parmelia_ itself there are indications of this line of development
in _P. stygia_, with short stiff upright branching fronds, and in _P.
pubescens_, with its tufts of filaments, but these two species are more
or less dorsiventral in structure and do not rise from the substratum.
In _Cetraria_ also there is a tendency towards upright growth and in _C.
aculeata_ even to radiate structure. But advance in these directions has
stopped short, the true line of evolution passing through species like
_Parmelia physodes_ with raised, and in some varieties, tubular fronds,
and the somewhat similar species _P. Kamtschadalis_ with straggling
strap-like lobes, to _Evernia_. That genus is a true link between foliose
and fruticose forms and has been classified now with one series, now with
the other.
In _Evernia furfuracea_, the lobes are free from the substratum except
when friction causes the development of a hold-fast and the branching out
of new lobes from that point. It is however dorsiventral in structure,
the under surface is black and the gonidial zone lies under the upper
cortex. _Evernia prunastri_ is white below and is more fruticose in
habit, the long fronds all rising from one base. They are thin and limp,
no strengthening tissue has been evolved, and they tend to lie over on
one side; both surfaces are corticate and gonidia sometimes travel round
the edge, becoming frequently lodged here and there along the under side.
The extreme of strap-shaped fruticose development is reached in the genus
_Ramalina_. In less advanced species such as _R. evernioides_ there is a
thin flat expansion anchored to the substratum at one point and alike on
both surfaces. In _R. fraxinea_ the fronds may reach considerable width
(var. _ampliata_), but in that and in most species there is a provision
of sclerotic strands to support and strengthen the fronds. One of those
best fitted to resist bending strains is _R. scopulorum_ (_siliquosa_)
which grows by preference on sea-cliffs and safely withstands the maximum
of exposure to wind or weather.
The filamentous structure appears abruptly, unless we consider it as
foreshadowed by _Parmelia pubescens_. The base is secured by strong
sheaths of enduring character; tensile strains are provided for either
by a chondroid axis, as in _Usnea_, or by cortical development, as in
_Alectoria_; the former method of securing strength seems to be the
most advantageous to the plant as a whole, since it leaves the outer
structures more free to develop, and there is therefore in _Usnea_ a
greater variety of branching and greater growth in length, which are less
possible with the thickened cortex of _Alectoria_.
_ee._ _PHYSCIACEAE._ There remains still an important phylum of
Lecanorales well defined by the polarilocular spores[1014]. It also
arises from a _Biatora_ species and forms a parallel development. Even
in this phylum there are two series: one with colourless spores and
mostly yellow or reddish either in thallus or apothecium, and the other
with brown spores and with cinereous-grey or brown thalli. The dark
spores are in many of the species typically polarilocular, though in some
the median septum is not very wide and no canal is visible. Practically
all of the lighter coloured forms contain parietin either in thallus or
apothecia or in both; it is absent in the dark-spored series.
Among the lighter coloured forms it is difficult to decide which of
these two striking characteristics developed first, the acid or the
peculiar spore. Probably the acid has the priority: there is one common
rock lichen in this country, _Placodium rupestre_ (_Lecanora irrubata_),
which gives a strong red acid reaction with potash, but in which the
spores are still simple, and the fruit structure in the biatorine stage.
Another species, _Pl. luteoalbum_, with a purplish reaction in the fruit
only, shows septate spores but with only a rather narrow septum. The
development continues through biatorine forms to lecanorine with a fully
formed thalline margin. Among these latter we encounter _Pl. nivale_
which is well provided with acid but in which the spores have become long
and fusiform with little trace of the polar cells or central canal. We
must allow here also for reversions, and wanderings from the straight
road.
From crustaceous the advance is normal and simple to squamulose forms
which in this phylum maintain a stiff regularity of thalline outline
termed “effigurate”; the squamules, developing from the centre, extend
outwards in a radiate-stellate manner. There are also foliose thalli in
the genus _Xanthoria_ and fruticose in _Teloschistes_. The cortex in the
former horizontal genus is of plectenchyma, and no peculiar structures
have emerged. In _Teloschistes_ the cortex is of compact parallel hyphae
(fibrous) which form the strengthening structure of the narrow compressed
fronds (_T. flavicans_).
In the brown-spored series there is a considerable number of species
that are crustaceous united in the genus _Rinodina_, all of which have
marginate apothecia. One of them, _Rinodina oreina_, approaches in
thalline structure the effigurate forms of _Placodium_; while in _R.
isidioides_, a rare British species, there is an isidioid squamulose
development.
Among foliose genera, the tropical genus _Pyxine_ is peculiar in its
almost lecideine fruit, a few gonidia occurring only in the early stages;
its affinity with _Physcia_ holds, however, through the one-septate brown
spores with very thick walls and the reduced lumen of the cells. The more
simple type of fruit may be merely retrogressive.
_Physcia_, the remaining genus, is mainly foliose and with dorsiventral
thallus. A few species have straggling semi-upright fronds and these
have sometimes been placed in a separate genus _Anaptychia_. Only one
“_Anaptychia_,” _Ph. intricata_, has a radiate structure with fibrous
cortex all round; in the others the upper cortex alone is fibrous—of
long parallel hyphae—but that character appears in nearly every one of
the horizontal species as well, sometimes in the upper, sometimes in the
lower cortex.
In _Physcia_ the horizontal thallus is of smaller dimensions than in
_Parmelia_, and never becomes so free from the substratum: it is attached
by rhizinae and soredia appear frequently. Very often the circular
effigurate type of development prevails.
It is difficult to trace with any certainty the origin of this series of
the phylum. Some workers have associated it with the purely lecideine
genus, _Buellia_, but the brown septate spores of the latter are of
simple structure, though occasionally approaching the _Rinodina_ type.
There are also differences in the thallus, that of _Buellia_, especially
when it is saxicolous, inclining to _Rhizocarpon_ in form. It is more
consistent with the outer and inner structure to derive Rinodina from
some crustaceous _Placodium_ form with a marginate apothecium, therefore
from a form of fairly advanced development. As the parietin content
disappeared—perhaps from the preponderance of other acids—the colouration
changed and the spores became dark-coloured.
Many genera and even families, such as Thelotremaceae, etc., have
necessarily been omitted from this survey of phylogeny in lichens, but
the tracing of the main lines of development has indicated the course of
evolution, and has demonstrated not only the close affinity between the
members of this polyphyletic class of plants, as shown in the constantly
recurring thalline types, but it has proved the extraordinary vigour
gained by both the component organisms through the symbiotic association.
The principal phyla[1015], developing on somewhat parallel lines, are
given in the appended table:
ARCHILICHENS
---------------+---------------+----------+------------+----------------
Phyla | Crustose |Squamulose| Foliose | Fruticose
---------------+---------------+----------+------------+----------------
| | | |
Pyrenolichens | Verrucariaceae| Dermatocarpaceae |
| | | |
Coniocarpineae | Caliciaceae | Sphaerophoraceae |Sphaerophoraceae
| | | |
|{Arthoniaceae | | |
Graphidineae |{Graphidaceae | | |Roccellaceae
|{Dirinaceae | | |
Cyclocarpineae | | | |
| | | |
|{ Lecideaceae Gyrophoraceae |
|{ Coenogoniaceae | |
Lecideales |{ (filamentous gonidia) | |
|{ Cladoniaceae | |
|{ (primary and secondary | |
|{ thalli) | |
| | | |
Lecanorales | Lecanoraceae |Parmeliaceae|Usneaceae
| | | |
Polariloculares| | | |
| | | |
{Colourless | | | |
{ spores | Placodium |Xanthoria |Teloschistes
{Brown spores |Rinodina, | |Physcia |Physcia
| Pyxine | | | (Anaptychia)
---------------+--------------+-----------+------------+----------------
SCHEME OF SUGGESTED PROGRESSION IN LICHEN STRUCTURE
PYRENOCARPINEAE
PYRENOCARPEAE
Pyrenothamniaceae Phyllopyreniaceae
| |
| |
Dermatocarpaceae |
| |
| |
Dermatocarpaceae |
| |
| |
| |
Verrucariaceae Pyrenulaceae
(Protococcaceae) (_Trentepohlia_)
CONIOCARPINEAE
CONIOCARPEAE
Sphaerophorus
| Pilophorus
| |
+--------------+
Pleurocybe Acroscyphus
| |
| |
Calycidium Tholurna
+-----+----+
Cyphelium Tylophoron Tylophorella
| +-----+-----+
Caliciaceae Pyrgillus
(Protococcaceae) (_Trentepohlia_)
CYCLOCARPINEAE
PHYCOLICHENS (CYANOPHILI)
--------------------^---------------
Stictaceae
| Peltigeraceae
| |
| Hydrothyria
+--+----------+
Peccania Phloeopeccania Pannariaceae
| | |
| | Leptodendriscum and |
| | Leptogidium |
Synalissa +----------+ | Heppiaceae
Thyrea | | Polychidium |
| | | | Leptogium
Paulia | | | |
| | | | Collema
+---+-+------+ | |
Pyrenopsidaceae Thermutis Physma
(_Gloeocapsa_) (_Scytonema_) (_Nostoc_)
LECIDEALES LECANORALES POLARILOCULARES
-----------^-------------
Usneae
|
Stereocaulon Eucetraria Ramalina
| | |
Ochropheae Cocciferae | | |
| | | | |
+--------+--+ | | Evernia Teloschistes Physcia
| | | | | sect.
Sphaero- | | Cetraria Parmelia Xanthoria Anaptychia
phoropsis | | (Platysma) physodes | |
| | | | | | |
Gyrophoraceae | Cladonia Pilophoron | Parme- | | Physcia
| | | | | liaceae | | |
| | +---+----+ | +----+ | |
+-------+ Baeomyces Heterodea Physcidia {Euplacodium Rinodina
| | +---------+ { | |
{Sect. Psora Gonophillus {Lecanora sect. { | {Callopisma|
{ | | { Squamaria {Placodium | |
{Sect. Eulecidea Sect. Biatora { | { | {Blastenia |
| | { Lecanora Colourless Brown
| | | spores spores
| | | +------+---------+
| | Sect. Biatora Sect. Biatora
| | | |
+----------------+--------+---------+--------------------+
Lecidea
(Protococcaceae)