uncialis_ to the fairly large foliose fronds of _Cl. foliacea_ which
extend to 5 cm. in length and about 1 cm. or more in width. It is
interesting to note that when the primary thallus is well developed, the
podetia are relatively unimportant and frequently are not formed. As a
rule the squamules are rounded or somewhat elongate in form with entire
or variously cut and crenate margins. They may be very insignificant
and sparsely scattered over the substratum, or massed in crowded swards
of leaflets which are frequently almost upright. In colour they are
bluish-grey, yellowish or brownish above, and white beneath (red in
_Cl. miniata_), frequently becoming very dark-coloured towards the
rooting base. These several characteristics are specific and are often
of considerable value in diagnosis. In certain conditions of shade
or moisture, squamules are formed on the podetium; they repeat the
characters of the basal squamules of the species.
B. TISSUES OF THE PRIMARY THALLUS
The stratose layers of tissue in the squamules of _Cladonia_ are arranged
as in other horizontal thalli.
_a._ CORTICAL TISSUE. In nearly all these squamules the cortex is of
the “decomposed” type. In a few species there is a plectenchymatous
formation—in _Cl. nana_, a Brazilian ground species, and in two New
Zealand species, _Cl. enantia_ f. _dilatata_ and _Cl. Neo-Zelandica_. The
principal growing area is situated all round the margins though generally
there is more activity at the apex. Frequently there is a gradual
perishing of the squamule at the base which counterbalances the forward
increase.
The upper surface in some species is cracked into minute areolae; the
cracks, seen in section, penetrate almost to the base of the decomposed
gelatinous cortex. They are largely due to alternate swelling and
contraction of the gelatinous surface, or to extension caused, though
rarely, by intercalary growth from the hyphae below. The surface is
subject to weathering and peeling as in other lichens; but the loss is
constantly repaired by the upward growth of the meristematic hyphae from
the gonidial zone; they push up between the older cortical filaments and
so provide for the expansion as well as for the renewal of the cortical
tissue.
_b._ GONIDIAL TISSUE. The gonidia consisting of Protococcaceous algae
form a layer immediately below the cortex. Isolated green cells are not
unfrequently carried up by the growing hyphae into the cortical region,
but they do not long survive in this compact non-aerated tissue. Their
empty membranes can however be picked out by the blue stain they take
with iodine and sulphuric acid.
Krabbe[390] has described the phases of development in the growing
region: he finds that differentiation into pith, gonidial zone and cortex
takes place some little way back from the edge. At the extreme apex the
hyphae lie fairly parallel to each other; further back, they branch
upwards to form the cortex, and to separate the masses of multiplying
gonidia, by pushing between them and so spreading them through the whole
apical tissue. The gonidia immediately below the upper cortex, where
they are well-lighted, continue to increase and gradually form into the
gonidial zone; those that lie deeper among the medullary hyphae remain
quiescent, and before long disappear altogether.
Where the squamules assume the upright position (as in _Cladonia
cervicornis_), there is a tendency for the gonidia to pass round to the
lower surface, and soredia are occasionally formed.
_c._ MEDULLARY TISSUE. The hyphae of the medulla are described by Wainio
as having long cells with narrow lumen, and as being encrusted with
granulations that may coalesce into more or less detachable granules;
in colour they are mostly white, but pale-yellow in _Cl. foliacea_ and
blood-red in _Cl. miniata_, a subtropical species. They are connected at
the base of the squamules with a filamentous hypothallus which penetrates
the substratum and attaches the plant. In a few species rhizinae are
formed, while in others the hyphae of the podetium grow downwards,
towards and into the substratum as a short stout rhizoid.
_d._ SOREDIA. Though frequent on the podetia, soredia are rare on the
squamules, and, according to Wainio[391], always originate at the growing
region, from which they spread over the under surface—rather sparsely in
_Cl. cariosa_, _Cl. squamosa_, etc., but abundantly in _Cl. digitata_
and a few others. In some instances, they develop further into small
corticate areolae on the under surface (_Cl. coccifera_, _Cl. pyxidata_
and _Cl. squamosa_).
2. RADIATE OR SECONDARY THALLUS
A. ORIGIN OF THE PODETIUM
The upright podetium, as described by Wainio[392] and by Krabbe[393], is
a secondary product of the basal granule or squamule. It is developed
from the hyphae of the gonidial zone, generally where a crack has
occurred in the cortex and rather close to the base or more rarely on
or near the edge of the squamule (_Cl. verticillata_, etc.). At these
areas, certain meristematic gonidial hyphae increase and unite to form a
strand of filaments below the upper cortex but above the gonidial layer,
the latter remaining for a time undisturbed as to the arrangement of the
algal cells.
This initial tissue—the primordium of the podetium—continues to grow
not only in width but in length: the basal portion grows downwards
and at length displaces the gonidial zone, while the upper part as a
compact cylinder forces its way through the cortex above, the cortical
tissue, however, taking no part in its formation; as it advances, the
edges of the gonidial and cortical zones bend upwards and form a sheath
distinguishable for some time round the base of the emerging podetium.
Even when the primary horizontal thallus is merely crustaceous, the
podetia take origin similarly from a subcortical weft of hyphae in an
areola or granule.
B. STRUCTURE OF THE PODETIUM
_a._ GENERAL STRUCTURE. In the early stages of development the podetium
is solid throughout, two layers of tissue being discernible—the hyphae
forming the centre of the cylinder being thick-walled and closely
compacted, and the hyphae on the exterior loosely branching with numerous
air-spaces between the filaments.
In all species, with the exception of _Cl. solida_, which remains solid
during the life of the plant, a central cavity arises while the podetium
is still quite short (about 1 to 1·5 mm. in _Cl. pyxidata_ and _Cl.
degenerans_). The first indication of the opening is a narrow split in
the internal cylinder, due to the difference in growth tension between
the more free and rapid increase of the external medullary layer and
the slower elongation of the chondroid tissue at the centre. The cavity
gradually widens and becomes more completely tubular with the upward
growth of the podetium; it is lined by the chondroid sclerotic band which
supports the whole structure (Fig. 67).
_b._ GONIDIAL TISSUE. In most species of Cladoniaceae, a layer of
gonidial tissue forms a more or less continuous outer covering of the
podetium, thus distinguishing it from the purely hyphal stalks of the
apothecia in Caliciaceae. Even in the genus _Baeomyces_, while the
podetia of some of the species are without gonidia, neighbouring species
are provided with green cells on the upright stalks clearly showing their
true affinity with the _Cladoniae_. In one British species of _Cladonia_
(_Cl. caespiticia_) the short podetium consists only of the fibrous
chondroid cylinder, and thus resembles the apothecial stalk of _Baeomyces
rufus_, but in that species also there are occasional surface gonidia
that may give rise to squamules.
[Illustration: Fig. 67. _Cladonia squamosa_ Hoffm. Vertical section of
podetium with early stage of central tube and of podetial squamules × 100
(after Krabbe).]
Krabbe[394] concluded from his observations that the podetial gonidia
of _Cladonia_ arrived from the open, conveyed by wind, water or insects
from the loose soredia that are generally so plentiful in any _Cladonia_
colony. They alighted, he held, on the growing stalks and, being
secured by the free-growing ends of the exterior hyphae, they increased
and became an integral part of the podetium. In more recent times
Baur[395] has recalled and supported Krabbe’s view, but Wainio[396],
on the contrary, claims to have proved that in the earliest stages of
the podetium the gonidia were already present, having been carried up
from the gonidial zone of the primary thallus by the primordial hyphae.
Increase of these green cells follows normally by cell-division or
sporulation.
Algal cells have been found to be common to different lichens, but in
_Cladoniae_ Chodat[397] claims to have proved by cultures that each
species tested has a special gonidium, determined by him as a species of
_Cystococcus_, which would render colonization by algae from the open
much less probable. In addition, the fungal hyphae are specific, and any
soredia (with their combined symbionts) that alighted on the podetium
could only be utilized if they originated from the same species; or, if
they were incorporated, the hyphae belonging to any other species would
of necessity die off and be replaced by those of the podetium.
_c._ CORTICAL TISSUE. In some species a cortex of the decomposed type
of thick-walled conglutinate hyphae is present, either continuous over
the whole surface of the podetium, as in _Cl. gracilis_ (Fig. 68), or in
interrupted areas or granules as in _Cl. pyxidata_ (Fig. 69) and others.
In _Cl. degenerans_, the spaces between the corticated areolae are filled
in by loose filaments without any green cells. _Cl. rangiferina_, _Cl.
sylvatica_, etc. are non-corticate, being covered all over with a loose
felt of intricate hyphae.
[Illustration: Fig. 68. _Cladonia gracilis_ Hoffm. (S. H., _Photo._).]
[Illustration: Fig. 69. _Cladonia pyxidata_ Hoffm. (S. H., _Photo._).]
In the section _Clathrinae_ (_Cl. retepora_, etc.) the cortex is formed
of longitudinal hyphae with thick gelatinous walls.
_d._ SOREDIA. Frequently the podetium is coated in whole or in part by
granules of a sorediate character—coarsely granular in _Cl. pyxidata_,
finely pulverulent in _Cl. fimbriata_. Though fairly constant to type
in the different species, they are subject to climatic influences, and,
when there is abundant moisture, both soredia and areolae develop into
squamules on the podetium. A considerable number of species have thus a
more or less densely squamulose “form” or “variety.”
C. DEVELOPMENT OF THE SCYPHUS
Two types of podetia occur in _Cladonia_: those that end abruptly and
are crowned when fertile by the apothecia or spermogonia (pycnidia), or
if sterile grow indefinitely tapering gradually to a point (Fig. 70);
and those that widen out into the trumpet-shaped or cup-like expansion
called the scyphus (Fig. 69). Species may be constantly scyphiferous or
as constantly ascyphous; in a few species, and even in individual tufts,
both types of podetium may be present.
[Illustration: Fig. 70. _Cladonia furcata_ Schrad. Sterile thallus (S.H.,
_Photo._).]
Wainio[398], who has studied every stage of development in the
_Cladoniae_, has described the scyphus as originating in several
different ways:
_a._ FROM ABORTIVE APOTHECIA. In certain species the apothecium appears
at a very early stage in the development of the podetium of which it
occupies the apical region. Owing to the subsequent formation of the
tubular cavity in the centre of the stalk, the base of the apothecium
may eventually lie directly over the hollow space and, therefore, out
of touch with the growing assimilating tissues; or even before the
appearance of the tube, the wide separation between the primordium of
the apothecium and the gonidia, entailing deficient nutrition, may
have produced a similar effect. In either case central degeneration of
the apothecium sets in, and the hypothecial filaments, having begun to
grow radially, continue to travel in the same direction both outwards
and upwards so that gradually a cup-shaped structure is evolved—the
amphithecium of the fruit without the thecium.
The whole or only a part of the apothecium may be abortive, and the
scyphus may therefore be entirely sterile or the fruits may survive at
the edges. The apothecia may even be entirely abortive after a fertile
commencement, but in that case also the primordial hyphae retain the
primitive impulse not only to radial direction, but also to the more
copious branching, and a scyphus is formed as in the previous case. It
must also be borne in mind that the tendency in many _Cladonia_ species
to scyphiform has become hereditary.
Baur[399], in his study of _Cl. pyxidata_, has taken the view that
the origin of the scyphus was due to a stronger apical growth of the
hyphae at the circumference than over the central tubular portion of the
podetium, and that considerable intercalary growth added to the expanding
sides of the cup.
Scyphi originating from an abortive apothecium are characteristic of
species in which the base is closed (Wainio’s Section _Clausae_), the
tissue in that case being continuous over the inside of the cup as in
_Cl. pyxidata_, _Cl. coccifera_ and many others.
_b._ FROM POLYTOMOUS BRANCHING. Another method of scyphus formation
occurs in _Cl. amaurocrea_ and a few other species in which the branching
is polytomous (several members rising from about the same level).
Concrescence of the tissues at the base of these branches produces
a scyphus; it is normally closed by a diaphragm that has spread out
from the different bases, but frequently there is a perforation due to
stretching. These species belong to the Section _Perviae_.
_c._ FROM ARRESTED GROWTH. In most cases however where the scyphus is
open as in _Cl. furcata_, _Cl. squamosa_, etc., development of the cup
follows on cessation of growth, or on perforation at the summit of the
podetium. Round this quiescent portion there rises a circle of minute
prominences which carry on the apical development. As they increase in
size, the spaces between them are bridged over by lateral growth, and the
scyphus thus formed is large or small according to the number of these
outgrowths. Apothecia or spermogonia may be produced at their tips, or
the vegetative development may continue. Scyphi formed in this manner are
also open or “pervious.”
_d._ GONIDIA OF THE SCYPHUS. Gonidia are absent in the early stages
of scyphus formation when it arises from degeneration of the apical
tissues, either fertile or vegetative; but gradually they migrate from
the podetium, from the base of young outgrowths, or by furrows at the
edge, and so spread over the surface of the cup. Soredia may possibly
alight, as Krabbe insists that they do, and may aid in colonizing the
naked area. Their presence, however, would only be accidental; they are
not essential, and scyphi are formed in many non-sorediate species such
as _Cl. verticillata_. The cortex of the scyphus becomes in the end
continuous with that of the podetium and is always similar in type.
_e._ SPECIES WITHOUT SCYPHI. In species where the whole summit of the
podetium is occupied by an apothecium, as in _Cl. bellidiflora_, no
scyphus is formed. There is also an absence of scyphi in podetia that
taper to a point. In those podetia the hyphae are parallel to the long
axis and remain in connection with the external gonidial layer so that
they are unaffected by the central cavity. Instances of tapering growth
are also to be found in species that are normally scyphiferous such as
_Cl. fimbriata_ subsp. _fibula_, and _Cl. cornuta_, as well as in species
like _Cl. rangiferina_ that are constantly ascyphous.
The scyphus is considered by Wainio[400] to represent an advanced stage
of development in the species or in the individual, and any conditions
that act unfavourably on growth, such as excessive dryness, would also
hinder the formation of this peculiar lichen structure.
D. BRANCHING OF THE PODETIUM
Though branching is a constant feature in many species, regular dichotomy
is rare: more often there is an irregular form of polytomy in which
one of the members grows more vigorously than the others and branches
again, so that a kind of sympodium arises, as in _Cl. rangiferina_, _Cl.
sylvatica_, etc.
Adventitious branches may also arise from the podetium, owing to some
disturbance of the normal growth, some undue exposure to wind or to too
great light, or owing to some external injury. They originate from the
gonidial tissue in the same way as does the podetium from the primary
thallus; the parallel hyphae of the main axis take no part in their
development.
In a number of species secondary podetia arise from the centre of
the scyphus—constantly in _Cl. verticillata_ and _Cl. cervicornis_,
etc., accidentally or rarely in _Cl. foliacea_, _Cl. pyxidata_, _Cl.
fimbriata_, etc. Wainio[401] has stated that they arise when the scyphus
is already at an advanced stage of growth and that they are to be
regarded as adventitious branches.
The proliferations from the borders of the scyphus are in a different
category. They represent the continuity of apical growth, as the edges of
the scyphus are but an enlarged apex. These marginal proliferations thus
correspond to polytomous branching. In many instances their advance is
soon stopped by the formation of an apothecium and they figure more as
fruit stalks than as podetial branches.
E. PERFORATIONS AND RETICULATION OF THE PODETIUM
Perforations in the podetial wall at the axils of the branches are
constant in certain species such as _Cl. rangiferina_, _Cl. uncialis_,
etc. They are caused by the tension of the branches as they emerge from
the main stalk. A tearing of the tissue may also arise in the base of the
scyphus, due to its increase in size, which causes the splitting of the
diaphragm at the bottom of the cup.
Among the _Cladoniae_ the reticulate condition recurs now and again. In
our native _Cladonia cariosa_ the splitting of the podetial wall is a
constant character of the species, the carious condition being caused by
unequal growth which tears apart the longitudinal fibres that surround
the central hollow.
A more advanced type of reticulation arises in the group of the
_Clathrinae_ in which there is no inner chondroid cylinder. In _Cladonia
aggregata_, in which the perforations are somewhat irregular, two types
of podetia have been described by Lindsay[402] from Falkland Island
specimens: those bearing apothecia are short and broad, fastigiately
branched upwards and with reticulate perforations, while podetia
bearing spermogonia are slender, elongate and branched, with fewer
reticulations. An imperfect network is also characteristic of _Cl.
Sullivani_, a Brazilian species. But the most marvellous and regular form
of reticulation occurs in _Cl. retepora_, an Australian lichen (Fig.
71): towards the tips of the podetia the ellipsoid meshes are small, but
they gradually become larger towards the base. In this species the outer
tissue, though of parallel hyphae, is closely interwoven and forms a
continuous growth at the edges of the perforations, giving an unbroken
smooth surface and checking any irregular tearing. The enlargement of the
walls is solely due to intercalary growth. The origin of the reticulate
structure in the _Clathrinae_ is unknown, though it is doubtless
associated with wide podetia and rendered possible by the absence of
an internal chondroid layer. The reticulate structure is marvellously
adapted for the absorption of water: _Cl. retepora_, more especially,
imbibes and holds moisture like a sponge.
[Illustration: Fig. 71. _Cladonia retepora_ Fr. From Tasmania.]
F. ROOTING STRUCTURES OF CLADONIAE
The squamules of the primary thallus are attached, as are most squamules,
to the supporting substance by strands of hyphae which may be combined
into simple or branching rhizinae and penetrate the soil or the wood on
which the lichen grows. There is frequently but one of these rooting
structures and it branches repeatedly until the ultimate branchlets
end in delicate mycelium. Generally they are grey or brown and are
not easily traced, but when they are orange-coloured, as according to
Wainio[403] they frequently are in _Cladonia miniata_ and _Cl. digitata_,
they are more readily observed, especially if the habitat be a mossy one.
In _Cl. alpicola_ it has been found that the rooting structure is
frequently as thick as the podetium itself. If the podetium originates
from the basal portion of the squamule, the hyphae from the chondroid
layer, surrounding the hollow centre, take a downward direction and
become continuous with the rhizoid. Should the point of insertion be near
the apex of the squamule, these hyphae form a nerve within the squamule
or along the under surface, and finally also unite with the rhizoid at
the base, a form of rooting characteristic of _Cl. cartilaginea_, _Cl.
digitata_ and several other species.
Mycelium may spread from the rhizinae along the surface of the substratum
and give rise to new squamules and new tufts of podetia, a method of
reproduction that is of considerable importance in species that are
generally sterile and that form no soredia.
Many species, especially those of the section _Cladina_, soon lose all
connection with the substratum, there being a continual decay of the
lower part of the podetia. As apical growth may continue for centuries,
the perishing of the base is not to be wondered at.
G. HAPTERA
The presence of haptera in _Cladoniae_ has already been alluded to. They
occur usually in the form of cilia or rhizinae[404], but differ from the
latter in their more simple regular growth being composed of conglutinate
parallel hyphae. They arise on the edges of the squamules or of the
scyphus, but in _Cl. foliacea_ and _Cl. ceratophylla_ they are formed at
the points of the podetial branches (more rarely in _Cl. cervicornis_
and _Cl. gracilis_). By the aid of these rhizinose haptera the squamule
or branch becomes attached to any substance within reach. They also aid
in the production of new individuals by anchoring some fragment of the
thallus to a support until it has grown to independent existence and has
produced new rhizinae or hold-fasts. They are a very prominent feature of
_Cl. verticillaris_ f. _penicillata_ in which they form a thick fringe on
the edges of the squamules, or frequently grow out as branched cilia from
the proliferations on the margins of the scyphus.
H. MORPHOLOGY OF THE PODETIUM
In the above account, the podetia have been treated as part of
the vegetative thallus, seeing that, partly or entirely, they are
assimilative and absorptive organs. This view does not, however, take
into account their origin and development, in consideration of which
Wainio[405] and later Krabbe[406] considered them as part of the
sporiferous organ. This view was also held by some of the earliest
lichenologists: Necker[407], for instance, constantly referred to the
upright structure as “stipes”; Persoon[408] included it, under the
term “pedunculus,” as part of the “inflorescence” of the lichen, and
Acharius[409] established the name “podetium” to describe the stalk of
the apothecium in _Baeomyces_.
Later lichenologists, such as Wallroth[410], looked on the podetia as
advanced stages of the thallus, or as forming a supplementary thallus.
Tulasne[411] described them as branching upright processes from the
horizontal form, and Koerber[412] considered them as the true thallus,
the primary squamule being merely a protothallus. By them and by
succeeding students of lichens the twofold character of the thallus
was accepted until Wainio and Krabbe by their more exact researches
discovered the endogenous origin of the podetium, which they considered
was conclusive evidence of its apothecial character: they claimed that
the primordium of the podetium was homologous with the primordium of the
apothecium. Reinke[413] and Wainio are in accord with Krabbe as to the
probable morphological significance of the podetium, but they both insist
on its modified thalline character. Wainio sums up that: “the podetium
is an apothecial stalk, that is to say an elongation of the conceptacle
most frequently transformed by metamorphosis to a vertical thallus,
though visibly retaining its stalk character.” Sättler[414], one of the
most recent students of _Cladonia_, regards the podetium as evolved with
reference to spore-dissemination, and therefore of apothecial character.
His views are described and discussed in the chapter on phylogeny.
Reinke and others sought for a solution of the problem in _Baeomyces_,
one of the more primitive genera of the Cladoniaceae. The thallus, except
in a few mostly exotic species, scarcely advances beyond the crustaceous
condition; the podetia are short and so varied in character that species
have been assigned by systematists to several different genera. In one
of them, _Baeomyces roseus_, the podetium or stalk originates according
to Nienburg[415] deep down in the medulla of a fertile granule as a
specialized weft of tissue; there is no carpogonium nor trichogyne
formed; the hyphae that grow upward and form the podetium are generative
filaments and give rise to asci and paraphyses. In a second species, _B.
rufus_ (_Sphyridium_), the gonidial zone and outer cortex of a thalline
granule swell out to form a thalline protuberance; the carpogonium arises
close to the apex, and from it branch the generative filaments. Nienburg
regards the stalk of _B. roseus_ as apothecial and as representing an
extension of the proper margin[416] (_excipulum proprium_), that of _B.
rufus_ as a typical vegetative podetium.
In the genus _Cladonia_, differentiation of the generative hyphae
may take place at a very early stage. Wainio[417] observed, in _Cl.
caespiticia_, a trichogyne in a still solid podetium only 90µ in height;
usually they appear later, and, where scyphi are formed, the carpogonium
often arises at the edge of the scyphus. Baur[418] and Wolff[419] have
furnished conclusive evidence of the late appearance of the carpogonium
in _Cl. pyxidata_, _Cl. degenerans_, _Cl. furcata_ and _Cl. gracilis_:
in all of these species carpogonia with trichogynes were observed on
the edge of well-developed scyphi. Baur draws the conclusion that the
podetium is merely a vertical thallus, citing as additional evidence that
it also bears the spermogonia (or pycnidia), though at the same time
he allows that the apothecium may have played an important part in its
phylogenetic development. He agrees also with the account of the first
appearance of the podetium as described by Krabbe, who found that it
began with the hyphae of the gonidial zone branching upwards in a quite
normal manner, only that there were more of them, and that they finally
pierced the cortex. Krabbe also asserted that in the early stages the
podetia were without gonidia and that these arrived later from the open
as colonists, in this contradicting Wainio’s statement that gonidia were
carried up from the primary thallus.
It seems probable that the podetium—as Wainio and Baur both have
stated—is homologous with the apothecial stalk, though in most cases it
is completely transformed into a vertical thallus. If the view of their
formation from the gonidial zone is accepted, then they differ widely in
origin from normal branches in which the tissues of the main axis are
repeated in the secondary structures, whereas in this vertical thallus,
hyphae from the gonidial zone alone take part in the development. It must
be admitted that Baur’s view of the podetium as essentially thalline
seems to be strengthened by the formation of podetia at the centre of
the scyphus, as in _Cl. verticillata_, which are new structures and are
not an elongation of the original conceptacular tissue. It can however
equally be argued that the acquired thalline character is complete and,
therefore, includes the possibility of giving rise to new podetia.
The relegation of the carpogonium to a position far removed from the base
or primordium of the apothecium need not necessarily interfere with the
conception of the primordial tissue as homologous with the conceptacle;
but more research is needed, as Baur dealt only with one species, _Cl.
pyxidata_, and Gertrude Wolff confined her attention to the carpogonial
stages at the edge of the scyphus.
The _Cladoniae_ require light, and inhabit by preference open moorlands,
naked clay walls, borders of ditches, exposed sand-dunes, etc. Those with
large and persistent squamules can live in arid situations, probably
because the primary thallus is able to retain moisture for a long
time[420]. When the primary thallus is small and feeble the podetia are
generally much branched and live in close colonies which retain moisture.
Sterile podetia are long-lived and grow indefinitely at the apex though
the base as continually perishes and changes into humus. Wainio[421]
cites an instance in which the bases of a tuft of _Cl. alpestris_ had
formed a gelatinous mass more than a decimetre in thickness.
I. PILOPHORUS AND STEREOCAULON
These two genera are usually included in Cladoniaceae on account of
their twofold thallus and their somewhat similar fruit formation. They
differ from _Cladonia_ in the development of the podetia which are not
endogenous in origin as in that genus, but are formed by the growth
upwards of a primary granule or squamule and correspond more nearly to
Tulasne’s conception of the podetium as a process from the horizontal
thallus. In _Pilophorus_ the primary granular thallus persists during
the life of the plants; the short podetium is unbranched, and consists
of a somewhat compact medulla of parallel hyphae surrounded by a looser
cortical tissue, such as that of the basal granule, in which are embedded
the algal cells. The black colour of the apothecium is due to the thick
dark hypothecium.
_Stereocaulon_ is also a direct growth from a short-lived primary
squamule[422]. The podetia, called “pseudopodetia” by Wainio, are usually
very much branched. They possess a central strand of hyphae not entirely
solid, and an outer layer of loose felted hyphae in which the gonidia
find place. A coating of mucilage on the outside gives a glabrous shiny
surface, or, if that is absent, the surface is tomentose as in _St.
tomentosum_. In all the species the podetia are more or less thickly
beset with small variously divided squamules similar in form to the
primary evanescent thallus. Gall-like cephalodia are associated with most
of the species and aid in the work of assimilation.
_Stereocaulon_ cannot depend on the evanescent primary thallus for
attachment to the soil. The podetia of the different species have
developed various rooting bases: in _St. ramulosum_ there is a basal
sheath formed, in _St. coralloides_ a well-developed system of
rhizoids[423].
V. STRUCTURES PECULIAR TO LICHENS
1. AERATION STRUCTURES
A. CYPHELLAE AND PSEUDOCYPHELLAE
The thallus of Stictaceae has been regarded by Nylander[424] and others
as one of the most highly organized, not only on account of the size
attained by the spreading lobes, but also because in that family are
chiefly found those very definite cup-like structures which were named
“cyphellae” by Acharius[425]. They are small hollow depressions about 1/2
mm. or more in width scattered irregularly over the under surface of the
thallus.
_a._ HISTORICAL. Cyphellae were first pointed out by the Swiss botanist,
Haller[426]. In his description of a lichen referable to _Sticta
fuliginosa_ he describes certain white circular depressions “to be found
among the short brown hairs of the under surface.” At a later date
Schreber[427] made these “white excavated points” the leading character
of his lichen genus _Sticta_.
In urceolate or proper cyphellae, the base of the depression rests on
the medulla; the margin is formed from the ruptured cortex and projects
slightly inwards over the edge of the cup. Contrasted with these are the
pseudocyphellae, somewhat roundish openings of a simpler structure which
replace the others in many of the species. They have no definite margin;
the internal hyphae have forced their way to the exterior and form a
protruding tuft slightly above the surface. Meyer[428] reckoned them all
among soredia; but he distinguished between those in which the medullary
hyphae became conglutinated to form a margin (true cyphellae) and those
in which there was a granular outburst of filaments (pseudocyphellae).
He also included a third type, represented in _Lobaria pulmonaria_ on
the under surface of which there are numerous non-corticate, angular
patches where the pith is laid bare (Fig. 72). Delise[429], writing
about the same time on the _Sticteae_, gives due attention to their
occurrence, classifying the various species of _Sticta_ as cyphellate or
non-cyphellate.
Acharius had limited the name “cyphella” to the hollow urceolate bodies
that had a well-defined margin. Nylander[430] at first included under
that term both types of structure, but later[431] he classified the
pulverulent “soredia-like” forms in another group, the pseudocyphellae.
As a rule they bear no relation to soredia, and algae are rarely
associated with the protruding filaments. Schwendener[432], and later
Wainio[433], in describing _Sticta aurata_ from Brazil, state, as
exceptional, that the citrine-yellow pseudocyphellae of that species are
sparingly sorediate.
[Illustration: Fig. 72. _Lobaria pulmonaria_ Hoffm. Showing pitted
surface. a, under surface. Reduced (S. H., _Photo._).]
[Illustration: Fig. 73. _Sticta damaecornis_ Nyl. Transverse section of
thallus with cyphella × 100.]
_b._ DEVELOPMENT OF CYPHELLAE. The cortex of both surfaces in the thallus
of _Sticta_ is a several-layered plectenchyma of thick-walled closely
packed cells, the outer layer growing out into hairs on the under surface
of most of the species. Where either cyphellae or pseudocyphellae
occur, a more or less open channel is formed between the exterior and
the internal tissues of the lichen. In the case of the cyphellae, the
medullary hyphae which line the cup are divided into short roundish
cells with comparatively thin walls (Fig. 73). They form a tissue
sharply differentiated from the loose hyphae that occupy the medulla.
The rounded cells tend to lie in vertical rows, though the arrangement
in fully formed cyphellae is generally somewhat irregular. The terminal
empty cells are loosely attached and as they are eventually abstricted
and strewn over the inside of the cup they give to it the characteristic
white powdery appearance.
According to Schwendener[434] development begins by an exuberant growth
of the medulla which raises and finally bursts the cortex; prominent
cyphellae have been thus formed in _Sticta damaecornis_ (Fig. 73). In
other species the swelling is less noticeable or entirely absent. The
opening of the cup measures usually about 1/2 mm. across, but it may
stretch to a greater width.
_c._ PSEUDOCYPHELLAE. In these no margin is formed, the cortex is simply
burst by the protruding filaments which are of the same colour—yellow or
white—as the medullary hyphae. They vary in size, from a minute point up
to 4 mm. in diameter.
_d._ OCCURRENCE AND DISTRIBUTION. The genus _Sticta_ is divided into
two sections: (1) _Eusticta_ in which the gonidia are bright-green
algae, and (2) _Stictina_ in which they are blue-green. Cyphellae and
pseudocyphellae are fairly evenly distributed between the sections; they
never occur together. Stizenberger[435] found that 36 species of the
section _Eusticta_ were cyphellate, while in 43 species pseudocyphellae
were formed. In the section _Stictina_ there were 38 of the former
and only 31 of the latter type. Both sections of the genus are widely
distributed in all countries, but they are most abundant south of the
equator, reaching their highest development in Australia and New Zealand.
In the British Isles _Sticta_ is rather poorly represented as follows:
§ _Eusticta_ (with bright-green gonidia).
Cyphellate: _S. damaecornis._
Pseudocyphellate: _S. aurata._
§ _Stictina_ (with blue-green gonidia).
Cyphellate: _S. fuliginosa_, _S. limbata_, _S. sylvatica_, _S.
Dufourei_.
Pseudocyphellate: _S. intricata_ var. _Thouarsii_, _S. crocata_.
Structures resembling cyphellae, with an overarching rim, are sprinkled
over the brown under surface of the Australian lichen, _Heterodea
Mülleri_; the thallus is without a lower cortex, the medulla being
protected by thickly woven hyphae. _Heterodea_ was at one time included
among Stictaceae, though now it is classified under Parmeliaceae.
Pseudocyphellae are also present on the non-corticate under surface of
_Nephromium tomentosum_, where they occur as little white pustules among
the brown hairs; and the white impressed spots on the under surface of
_Cetraria islandica_ and allied species, first determined as air pores by
Zukal[436], have also been described by Wainio[437] as pseudocyphellae.
There seems no doubt that the chief function of these various structures
is, as Schwendener[438] suggested, to allow a free passage of air to the
assimilating gonidial zone. Jatta[439] considers them to be analogous
to the lenticels of higher plants and of service in the interchange
of gases—expelling carbonic acid and receiving oxygen from the outer
atmosphere. It is remarkable that such serviceable organs should have
been evolved in so few lichens.
B. BREATHING-PORES
[Illustration: Fig. 74. _Parmelia exasperata_ Carroll. Vertical section
of thallus. _a_, breathing-pores; _b_, rhizoid. × 60 (after Rosendahl).]
_a._ DEFINITE BREATHING-PORES. The cyphellae and pseudocyphellae
described above are confined to the under surface of the thallus in
those lichens where they occur. Distinct breathing-pores of a totally
different structure are present on the upper surface of the tree-lichen,
_Parmelia aspidota_ (_P. exasperata_), one of the brown-coloured species.
They are somewhat thickly scattered as isidia- or cone-like warts over
the lichen thallus (Fig. 74) and give it the characteristically rough or
“exasperate” character. They are direct outgrowths from the thallus, and
Zukal[440], who discovered their peculiar nature and function, describes
them as being filled with a hyphal tissue, with abundant air-spaces,
and in direct communication with the medulla; gonidia, if present, are
confined to the basal part. The cortex covering these minute cones, he
further states, is very thin on the top, or often wanting, so that a true
pore is formed which, however, is only opened after the cortex elsewhere
has become thick and horny. Rosendahl[441], who has re-examined these
“breathing-pores,” finds that in the early stage of their growth, near
the margin or younger portion of the thallus, they are entirely covered
by the cortex. Later, the hyphae at the top become looser and more
frequently septate, and a fine network of anastomosing and intricate
filaments takes the place of the closely cohering cortical cells. These
hyphae are divided into shorter cells, but do not otherwise differ from
those of the medulla. Rosendahl was unable to detect an open pore at any
stage, though he entirely agrees with Zukal as to the breathing function
of these structures. The gonidia of the immediately underlying zone are
sparsely arranged and a few of them are found in the lower half of the
cone; the hyphae of the medulla can be traced up to the apex. Zukal[442]
claims to have found breathing-pores in _Cornicularia_ (_Parmelia_)
_tristis_ and in several other _Parmeliae_, notably in _Parmelia stygia_.
The thallus of the latter species has minute holes or openings in the
upper cortex, but they are without any definite form and may be only
fortuitous.
[Illustration: Fig. 75 A. _Ramalina fraxinea_ Ach. A, surface view of
frond. _a_, air-pores; _b_, young apothecia. × 12. B, transverse section
of part of frond. _a_, breathing-pore; _f_, strengthening fibres. × 37
(after Brandt).]
[Illustration: Fig. 75 B. _Ramalina strepsilis_ Zahlbr. Transverse
section of part of frond showing distribution of: _a_, air-pores, and
_f_, strengthening fibres. × 37 (after Brandt).]
Zukal[442] published drawings of channels of looser tissue between the
exterior and the pith in _Oropogon Loxensis_ and in _Usnea barbata_.
He considered them to be of definite service in aeration. The fronds
of _Ramalina dilacerata_ by stretching develop a series of elongate
holes. Reinke[443] found openings in _Ramalina Eckloni_ which pierced to
the centre of the thallus, and Darbishire[444] has figured a break in
the frond of another species, _R. fraxinea_ (Fig. 75 A), which he has
designated as a breathing-pore. Finally Brandt[445], in his careful study
of the anatomy of _Ramalinae_, has described as breathing-pores certain
open areas usually of ellipsoid form in the compact cortex of several
species: in _R. strepsilis_ (Fig. 75 B) and _R. Landroensis_, and in
the British species, _R. siliquosa_ and _R. fraxinea_. These openings
are however mostly rare and difficult to find or to distinguish from
holes that may be due to any accident in the life of the lichen. It is
noteworthy that Rosendahl found no further examples of breathing-pores in
the brown _Parmeliae_ that he examined in such detail. No other organs
specially adapted for aeration of the thallus have been discovered.
_b._ OTHER OPENINGS IN THE THALLUS. _Lobaria_ is the only genus of
Stictaceae in which neither cyphellae nor pseudocyphellae are formed; but
in two species, _L. scrobiculata_ and _L. pulmonaria_, the lower surface
is marked with oblong or angular bare convex patches, much larger than
cyphellae. They are exposed portions of the medulla, which at these spots
has been denuded of the covering cortex. Corresponding with these bare
spots there is a pitting of the upper surface.
A somewhat similar but reversed structure characterizes _Umbilicaria
pustulata_, which as the name implies is distinguished by the presence
of pustules, ellipsoid swellings above, with a reticulation of cavities
below. Bitter[446] in this instance has proved that they are due to
disconnected centres of intercalary growth which are more vigorous on the
upper surface and give rise to cracks in the less active tissue beneath.
These cracks gradually become enlarged; they are, as it were, accidental
in origin but are doubtless of considerable service in aeration.
In some _Parmeliae_ there are constantly formed minute round holes,
either right through the apothecia (_P. cetrata_, etc.), or through the
thallus (_P. pertusa_). Minute holes are also present in the under cortex
of _Parmelia vittata_ and of _P. enteromorpha_, species of the subgenus
_Hypogymnia_. Nylander[447], who first drew attention to these holes of
the lower cortex, described them as arising at the forking of two lobes;
but though they do occur in that position, they as frequently bear no
relation to the branching. Bitter’s[448] opinion is that they arise
by the decay of the cortical tissues in very limited areas, from some
unknown cause, and that the holes that pierce right through the thallus
in other species may be similarly explained.
Still other minute openings into the thallus occur in _Parmelia vittata_,
_P. obscurata_ and _P. farinacea_ var. _obscurascens_. In the two latter
the openings like pin-holes are terminal on the lobes and are situated
exactly on the apex, between the pith and the gonidial zone; sometimes
several holes can be detected on the end of one lobe. Further growth in
length is checked by these holes. They appear more frequently on the
darker, better illuminated plants. In _Parmelia vittata_ the terminal
holes are at the end of excessively minute adventitious branches which
arise below the gonidial zone on the margin of the primary lobes. All
these terminal holes are directed upwards and are visible from above.
Bitter does not attribute any physiological significance to these very
definite openings in the thallus. It has been generally assumed that they
aid in the aeration of the thallus; it is also possible that they may be
of service in absorption, and they might even be regarded as open water
conductors.
C. GENERAL AERATION OF THE THALLUS
Definite structures adapted to secure the aeration of the thallus
in a limited number of lichens have been described above. These are
the breathing-pores of _Parmelia exasperata_ and the cyphellae and
pseudocyphellae of the Stictaceae, with which also may be perhaps
included the circumscribed breaks in the under cortex in some members of
that family.
Though lichens are composed of two actively growing organisms, the
symbiotic plant increases very slowly. The absorption of water and
mineral salts must in many instances be of the scantiest and the
formation of carbohydrates by the deep-seated chlorophyll cells of
correspondingly small amount. Active aeration seems therefore uncalled
for though by no means excluded, and there are many indirect channels by
which air can penetrate to the deeper tissues.
In crustaceous forms, whether corticate or not, the thallus is often
deeply seamed and cracked into areolae, and thus is easily pervious to
water and air. The growing edges and growing points are also everywhere
more or less loose and open to the atmosphere. In the larger foliose and
fruticose lichens, the soredia that burst an opening in the thallus,
and the cracks that are so frequent a feature of the upper cortex, all
permit of gaseous interchange. The apical growing point of fruticose
lichens is thin and porous, and in many of them the ribs and veins of
their channelled surfaces entail a straining of the cortical tissue that
results in the formation of thinner permeable areas. Zukal[449] devoted
special attention to the question of aeration, and he finds evidence
of air-passages through empty spermogonia and through the small round
holes that are constant in the upper surface of certain foliose species.
He claims also to have proved a system of air-canals right through the
thallus of the gelatinous Collemaceae. Though his proof in this instance
is somewhat unconvincing, he establishes the abundant presence of air in
the massively developed hypothecium of _Collema_ fruits. He found that
the carpogonial complex of hyphae was always well supplied with air, and
that caused him to view with favour the suggestion that the function of
the trichogyne is to provide an air-passage. In foliose lichens, the
under surface is frequently non-corticate, in whole or in part; or the
cortex becomes seamed and scarred with increasing expansion, the growth
in the lower layers failing to keep pace with that of the overlying
tissues, as in _Umbilicaria pustulata_.
It is unquestionable that the interior of the thallus of most lichens
contains abundant empty spaces between the loose-lying hyphae, and that
these spaces are filled with air.
2. CEPHALODIA
A. HISTORICAL AND DESCRIPTIVE
The term “cephalodium” was first used by Acharius[450] to designate
certain globose apothecia (pycnidia). At a later date he applied it
to the peculiar outgrowths that grow on the thallus of _Peltigera
aphthosa_, already described by earlier writers, along with other similar
structures, as “corpuscula,” “maculae,” etc. The term is now restricted
to those purely vegetative gall-like growths which are in organic
connection with the thallus of the lichen, but which contain one or more
algae of a different type from the one present in the gonidial zone. They
are mostly rather small structures, and they take various forms according
to the lichen species on which they occur. They are only found on thalli
in which the gonidia are bright-green algae (Chlorophyceae) and, with a
few exceptions, they contain only blue-green (Myxophyceae). Cephalodia
with bright-green algae were found by Hue[451] on two _Parmeliae_ from
Chili, in addition to the usual blue-green forms; the one contained
_Urococcus_, the other _Gloeocystis_. Several with both types of algae
were detected also by Hue[451] within the thallus of _Aspicilia_ spp.
Flörke[452] in his account of German lichens described the cephalodia
that grow on the podetia of _Stereocaulon_ as fungoid bodies, “corpuscula
fungosa.” Wallroth[453], who had made a special study of lichen gonidia,
finally established that the distinguishing feature of the cephalodia was
their gonidia which differed in colour from those of the normal gonidial
zone. He considered that the outgrowths were a result of changes that had
arisen in the epidermal tissues of the lichens, and, to avoid using a
name of mixed import such as “cephalodia,” he proposed a new designation,
calling them “phymata” or warts.
Further descriptions of cephalodia were given by Th. M. Fries[454]
in his _Monograph of Stereocaulon and Pilophorus_; but the greatest
advance in the exact knowledge of these bodies is due to Forssell[455]
who made a comprehensive examination of the various types, examples
of which occurred, he found, in connection with about 100 different
lichens. Though fairly constant for the different species, they are not
universally so, and are sometimes very rare even when present, and then
difficult to find. A striking instance of variability in their occurrence
is recorded for _Ricasolia amplissima_ (_Lobaria laciniata_) (Fig. 76).
The cephalodia of that species are prominent upright branching structures
which grow in crowded tufts irregularly scattered over the surface. They
are an unfailing and conspicuous specific character of the lichens in
Europe, but are entirely wanting in North American specimens.
[Illustration: Fig. 76. _Ricasolia amplissima_ de Not. (_Lobaria
laciniata_ Wain.) on oak, reduced. The dark patches are tufts of
branching cephalodia (A. Wilson, _Photo._).]
As cephalodia contain rather dark-coloured, blue-green algae, they are
nearly always noticeably darker than the thalli on which they grow,
varying from yellowish-red or brown in those of _Lecanora gelida_ to
pale-coloured in _Lecidea consentiens_[456], a darker red in _Lecidea
panaeola_ and various shades of green, grey or brown in _Stereocaulon_,
_Lobaria_ (_Ricasolia_), etc. They form either flat expansions of varying
size on the upper surface of the thallus, rounded or wrinkled wart-like
growths, or upright branching structures. On the lower surface, where
they are not unfrequent, they take the form of small brown nodules or
swellings. In a number of species packets of blue-green algae surrounded
by hyphae are found embedded in the thallus, either in the pith or
immediately under the cortex. They are of the same nature as the
superficial excrescences and are also regarded as cephalodia.
B. CLASSIFICATION
Forssell has drawn up a classification of these structures, as follows:
I. CEPHALODIA VERA.
1. =Cephalodia epigena= (including =perigena=) developed on the upper
outer surface of the thallus, which are tuberculose, lobulate, clavate or
branched in form. These are generally corticate structures.
2. =Cephalodia hypogena= which are developed on the under surface of the
thallus; they are termed “thalloid” if they are entirely superficial,
and “immersed” when they are enclosed within the tissues. They are
non-corticate though surrounded by a weft of hyphae. Forssell further
includes here certain placodioid (lobate), granuliform and fruticose
forms which develop on the hypothallus of the lichen, and gradually
push their way up either through the host thallus, or, as in _Lecidea
panaeola_, between the thalline granules.
Nylander[457] arranged the cephalodia known to him in three groups: (1)
Ceph. epigena, (2) Ceph. hypogena and (3) Ceph. endogena. Schneider[458]
still more simply and practically describes them as Ectotrophic
(external), and Endotrophic (internal).
II. PSEUDOCEPHALODIA.
These are a small and doubtful group of cephalodia which are apparently
in very slight connection with the host thallus, and show a tendency to
independent growth. They occur as small scales on _Solorina bispora_[459]
and _S. spongiosa_ and also on _Lecidea pallida_. Forssell has suggested
that the cephalodia of _Psoroma hypnorum_ and of _Lecidea panaeola_ might
also be included under this head.
Forssell and others have found and described cephalodia in the following
families and genera:
Sphaerophoraceae.
_Sphaerophorus_ (_S. stereocauloides_).
Lecideaceae.
_Lecidea_ (_L. panaeola_, _L. consentiens_, _L. pelobotrya_,
etc.).
Cladoniaceae.
_Stereocaulon_, _Pilophorus_ and _Argopsis_.
Pannariaceae.
_Psoroma_ (_P. hypnorum_).
Peltigeraceae.
_Peltigera_ (_Peltidea_), _Nephroma_ and _Solorina_.
Stictaceae.
_Lobaria_, _Sticta_.
Lecanoraceae.
_Lecania_ (_L. lecanorina_), _Aspicilia_[460].
Physciaceae.
_Placodium bicolor_[461].
C. ALGAE THAT FORM CEPHALODIA
The algae of the cephalodia belong mostly to genera that form the normal
gonidia of other lichens. They are:
_Stigonema_,—in _Lecanora gelida_, _Stereocaulon_, _Pilophorus robustus_,
and _Lecidea pelobotrya_.
_Scytonema_,—a rare constituent of cephalodia.
_Nostoc_,—the most frequent gonidium of cephalodia. It occurs in those of
the genera _Sticta_, _Lobaria_, _Peltigera_, _Nephroma_, _Solorina_ and
_Psoroma_; occasionally in _Stereocaulon_ and in _Lecidea pallida_.
_Lyngbya_ and _Rivularia_,—rarely present, the latter in _Sticta
oregana_[462].
_Chroococcus_ and _Gloeocapsa_,—also very rare.
_Scytonema_, _Chroococcus_, _Gloeocapsa_ and _Lyngbya_ are generally
found in combination with some other cephalodia-building alga, though
Nylander[463] found _Scytonema_ alone in the lobulate cephalodia of
_Sphaerophorus stereocauloides_, a New Zealand lichen, and the only
species of that genus in which cephalodia are developed; and Hue[460]
records _Gloeocapsa_ as forming internal cephalodia in two species of
_Aspicilia_. Bornet[464] found _Lyngbya_ associated with _Scytonema_ in
the cephalodia of _Stereocaulon ramulosum_, and, in the same lichen,
Forssell[465] found, in the several cephalodia of one specimen, _Nostoc_,
_Scytonema_, and _Lyngbya_, while, in those of another, _Scytonema_ and
_Stigonema_ were present. In the latter instance these algae were living
free on the podetium. Forssell[465] also determined two different algae,
_Gloeocapsa magma_ and _Chroococcus turgidus_, present in a cephalodium
on _Lecidea panaeola_ var. _elegans_.
As a general rule only one kind of alga enters into the formation of the
cephalodia of any species or genus. A form of _Nostoc_, for instance,
is invariably the gonidial constituent of these bodies in the genera,
_Lobaria_, _Sticta_, etc. In other lichens different blue-green algae,
as noted above, may occupy the cephalodia even on the same specimen.
Forssell finds alternative algae occurring in the cephalodia of:
_Lecanora gelida_ and _Lecidea illita_ contain either _Stigonema_ or
_Nostoc_;
_Lecidea panaeola_, with _Gloeocapsa_, _Stigonema_ or _Chroococcus_;
_Lecidea pelobotrya_, with _Stigonema_ or _Nostoc_;
_Pilophorus robustus_, with _Gloeocapsa_, _Stigonema_, or _Nostoc_.
[Illustration: Fig. 77. _Lecanora gelida_ Ach. _a_, lobate cephalodia ×
12 (after Zopf).]
Riddle[466] has employed cephalodia with their enclosed algae as
diagnostic characters in the genus _Stereocaulon_. When the alga is
_Stigonema_, as in _S. paschale_, etc., the cephalodia are generally
very conspicuous, grey in colour, spherical, wrinkled or folded, though
sometimes black and fibrillose (_S. denudatum_). Those containing
_Nostoc_ are, on the contrary, minute and are coloured verdigris-green
(_S. tomentosum_ and _S. alpinum_).
Instances are recorded of algal colonies adhering to, and even
penetrating, the thallus of lichens, but as they never enter into
relationship with the lichen hyphae, they are antagonistic rather than
symbiotic and have no relation to cephalodia.
D. DEVELOPMENT OF CEPHALODIA
_a._ ECTOTROPHIC. Among the most familiar examples of external cephalodia
are the small rather dark-coloured warts or swellings that are scattered
irregularly over the surface of _Peltigera_ (_Peltidea_) _aphthosa_.
This lichen has a grey foliose thallus of rather large sparingly divided
lobes; it spreads about a hand-breadth or more over the surface of the
ground in moist upland localities. The specific name “aphthosa” was given
by Linnaeus to the plant on account of the supposed resemblance of the
dotted thallus to the infantile ailment of “thrush.” Babikoff[467] has
published an account of the formation and development of these _Peltidea_
cephalodia. He determined the algae contained in them to be _Nostoc_ by
isolating and growing them on moist sterilized soil. He observed that
the smaller, and presumably younger, excrescences were near the edges
of the lobes. The cortical cells in that position grow out into fine
septate hairs that are really the ends of growing hyphae. Among the
hairs were scattered minute colonies of _Nostoc_ cells lying loose or so
closely adhering to the hairs as to be undetachable (Fig. 78 A). In older
stages the hairs, evidently stimulated by contact with the _Nostoc_, had
increased in size and sent out branches, some of which penetrated the
gelatinous algal colony; others, spreading over its surface, gradually
formed a cortex continuous with that of the thallus. The alga also
increased, and the structure assumed a rounded or lentiform shape. The
thalline cortex immediately below broke down, and the underlying gonidial
zone almost wholly died off and became absorbed. The hyphae of the
cephalodium had meanwhile penetrated downwards as root-like filaments,
those of the thallus growing upwards into the new overlying tissue (Fig.
78 B). The foreign alga has been described as parasitic, as it draws
from the lichen hyphae the necessary inorganic food material; but it
might equally well be considered as a captive pressed into the service of
the lichen to aid in the work of assimilation or as a willing associate
giving and receiving mutual benefit.
[Illustration: Fig. 78 A. Hairs of _Peltigera aphthosa_ Willd. associated
with _Nostoc_ colony much magnified (after Babikoff).]
[Illustration: Fig. 78 B. _Peltigera aphthosa_ Willd. Vertical section of
thallus and cephalodium × 480 (after Babikoff).]
Th. M. Fries[468] had previously described the development of the
cephalodia in _Stereocaulon_ but failed to find the earliest stages. He
concluded from his observations that parasitic algae were common in the
cortical layer of the lichens, but only rarely formed the “monstrous
growths” called cephalodia.
_b._ ENDOTROPHIC. Winter[469] examined the later stages of internal
cephalodine formation in a species of _Sticta_. The alga, probably a
species of _Rivularia_, which gives origin to the cephalodia, may be
situated immediately below the upper cortex, in the medullary layer
close to the gonidial zone, or between the pith and the under cortex.
The protuberance caused by the increasing tissue, which also contains
the invading alga, arises accordingly either on the upper or the lower
surface. In some cases it was found that the normal gonidial layer had
been pushed up by the protruding cephalodium and lay like a cap over the
top. The cephalodia described by Winter are endogenous in origin, though
the mature body finally emerges from the interior and becomes either
epigenous or hypogenous. Schneider[470] has followed the development of a
somewhat similar endotrophic or endogenous type in _Sticta oregana_ due
also to the presence of a species of _Rivularia_. How the alga attained
its position in the medulla of the thallus was not observed.
[Illustration: Fig. 79. _Nephroma expallidum_ Nyl. Vertical section of
thallus. _a_, endotrophic cephalodium × 100 (after Forssell).]
Both the algal cells of internal cephalodia and the hyphae in contact
with them increase vigorously, and the newly formed tissue curving
upwards or downwards appears on the outside as a swelling or nodule
varying in size from that of a pin-head to a pea. On the upper surface
the gonidial zone partly encroaches on the nodule, but the foreign alga
remains in the centre of the structure well separated from the thalline
gonidia by a layer of hyphae. The group is internally divided into small
nests of dark-green algae surrounded by strands of hyphae (Fig. 79).
The swellings, when they occur on the lower surface of the lichen,
correspond to those of the upper in general structure, but there is no
intermixture of thalline gonidia. That _Nostoc_ cells can grow and retain
the power to form chlorophyll in adverse conditions was proved by Etard
and Bouilhac[471] who made a culture of the alga on artificial media in
the dark, when there was formed a green pigment of chlorophyll nature.
Endotrophic cephalodia occur in many groups of lichens. Hue[472]
states that he found them in twelve species of _Aspicilia_. As packets
of blue-green algae they are a constant feature in the thallus of
_Solorinae_. The species of that genus grow on mossy soil in damp places,
and must come frequently in contact with _Nostoc_ colonies. In _Solorina
crocea_ an interrupted band of blue-green algae lies below the normal
gonidial zone and sometimes replaces it—a connecting structure between
cephalodia and a true gonidial zone.
_c._ PSEUDOCEPHALODIA. Under this section have been classified those
cephalodia that are almost independent of the lichen thallus though
to some extent organically connected with it, as for instance that of
_Lecidea panaeola_ which originate on the hypothallus of the lichen and
maintain their position between the crustaceous granules.
The cephalodia of _Lecanora gelida_, as described by Sernander[473],
might also be included here. He watched their development in their
native habitat, an exposed rock-surface which was richly covered with
the lichen in all stages of growth. Two kinds of thallus, the one
containing blue-green algae (_Chroococcus_), the other bright-green,
were observed on the rock in close proximity. At the point of contact,
growth ceased, but the thallus with bright-green algae, being the more
vigorous, was able to spread round and underneath the other and so
gradually to transform it to a superficial flat cephalodium. All such
thalli encountered by the dominant lichen were successively surrounded in
the same way. The cephalodium, growing more slowly, sent root-like hyphae
into the tissue of the underlying lichen, and the two organisms thus
became organically connected. Sernander considers that the two algae are
antagonistic to each other, but that the hyphae can combine with either.
The pseudocephalodia of _Usnea_ species are abortive apothecia; they are
surrounded at the base by the gonidial zone and cortex of the thallus,
and they contain no foreign gonidia.
E. AUTOSYMBIOTIC CEPHALODIA
Bitter[474] has thus designated small scales, like miniature thalli,
that develop constantly on the upper cortex of _Peltigera lepidophora_,
a small lichen not uncommon in Finland, and first recorded by Wainio as
a variety of _Peltigera canina_. The alga contained in the scales is a
blue-green _Nostoc_ similar to the gonidia of the thallus. Bitter[475]
described the development as similar to that of the cephalodia of
_Peltigera aphthosa_, but the outgrowths, being lobate in form, are
less firmly attached and thus easily become separated and dispersed; as
the gonidia are identical with those of the parent thallus they act as
vegetative organs of reproduction.
Bitter’s work has been criticized by Linkola[476] who claims to have
discovered by means of very thin microtome sections that there is a
genetic connection between the scales and the underlying thallus, not
only with the hyphae, as in true cephalodia, but with the algae as well,
so that these outgrowths should be regarded as isidia.
In the earliest stages, according to Linkola, a small group of algae may
be observed in the cortical tissue of the _Peltigera_ apart from the
gonidial zone and near the upper surface. Gradually a protruding head
is formed which is at first covered over with a brown cortical layer
one cell thick. The head increases and becomes more lobate in form,
being attached to the thallus at the base by a very narrow neck and
more loosely at other parts of the scale. In older scales, the gonidia
are entirely separated from those of the thallus, and a dark-brown
cortex several cells in thickness covers over the top and sides; there
is a colourless layer of plectenchyma beneath. At this advanced stage
the scales are almost completely superficial and correspond with the
cephaloidal rather than with the isidial type of formation. The algae
even in the very early stages are distinct from the gonidial zone and the
whole development, if isidial, must be considered as somewhat abnormal.
3. SOREDIA
A. STRUCTURE AND ORIGIN OF SOREDIA
Soredia are minute separable parts of the lichen thallus, and are
composed of one or more gonidia which are clasped and surrounded by
the lichen hyphae (Fig. 80). They occur on the surface or margins of
the thallus of a fairly large number of lichens either in a powdery
excrescence or in a pustule-like body comprehensively termed a “soralium”
(Fig. 81). The soralia vary in form and dimensions according to the
species. Each individual soredium is capable of developing into a new
plant; it is a form of vegetative reproduction characteristic of lichens.
[Illustration: Fig. 80. Soredia. _a_, of _Physcia pulverulenta_ Nyl.;
_b_, of _Ramalina farinacea_ Ach. × 600.]
Acharius[477] gave the name “soredia” to the powdery bodies with
reference to their propagating function; he also interpreted the soredium
as an “apothecium of the second order.” But long before his time they
had been observed and commented on by succeeding botanists: first by
Malpighi[478] who judged them to be seeds, he having seen them develop
new plants; by Micheli[479] who however distinguished between the true
fruit and those seeds; and by Linnaeus[480] who considered them to be
the female organs of the plant, the apothecia being, as he then thought,
the male organs. Hedwig[481], on the other hand, regarded the apothecia
as the seed receptacles and the soredia as male bodies. Sprengel’s[482]
statement that they were “a subtile germinating powder mixed with
delicate hair-like threads which take the place of seeds” established
finally their true function. Wallroth[483], who was the first really
to investigate their structure and their relation to the parent plant,
recognized them as of the same type as the “brood-cells” or gonidia; and
as the latter, he found, could become free from the thallus and form a
green layer on trees, walls, etc., in shady situations, so the soredia
also could become free, though for a time they remained attached to the
lichen and were covered by a veil, _i.e._ by the surrounding hyphal
filaments. Koerber[484] also gave much careful study to soredia, their
nature and function. As propagating organs he found they were of more
importance than spores, especially in the larger lichens.
[Illustration: Fig. 81. Vertical section of young soralium of _Evernia
furfuracea_ var. _soralifera_ Bitter × 60 (after Bitter).]
According to Schwendener[485], the formation of soredia is due to
increased and almost abnormal activity of division in the gonidial cell;
the hyphal filament attached to it also becomes active and sends out
branches from the cell immediately below the point of contact which force
their way between the newly divided gonidia and finally surround them.
A soredial “head” of smaller or larger size is thus gradually built up
on the stalk filament or filaments, and is ultimately detached by the
breaking down of the slender support.
_a._ SCATTERED SOREDIA. The simplest example of soredial formation may
be seen on the bark of trees or on palings when the green coating of
algal cells is gradually assuming a greyish hue caused by the invasion
of hyphal lichenoid growth. This condition is generally referred to as
“leprose” and has even been classified as a distinct genus, _Lepra_ or
_Lepraria_. Somewhat similar soredial growth is also associated with
many species of _Cladonia_, the turfy soil in the neighbourhood of the
upright podetia being often powdered with white granules. Such soredia
are especially abundant in that genus, so much so, that Meyer[486],
Krabbe[487] and others have maintained that the spores take little part
in the propagation of species. The under side of the primary thallus, but
more frequently the upright podetia, are often covered with a coating of
soredia, either finely furfuraceous, or of larger growth and coarsely
granular, the size of the soredia depending on the number of gonidia
enclosed in each “head.”
Soredia are only occasionally present on the apothecial margins:
the rather swollen rims in _Lobaria scrobiculata_ are sometimes
powdery-grey, and Bitter[488] has observed soredia, or rather soralia,
on the apothecial margins of _Parmelia vittata_; they are very rare,
however, and are probably to be explained by excess of moisture in the
surroundings.
_b._ ISIDIAL SOREDIA. In a few lichens soredia arise by the breaking down
of the cortex at the tips of the thalline outgrowths termed “isidia.”
In _Parmelia verruculifera_, for instance, where the coralloid isidia
grow in closely packed groups or warts, the upper part of the isidium
frequently becomes soredial. In that lichen the younger parts of the
upper cortex bear hairs or trichomes, and the individual soredia are also
adorned with hairs. The somewhat short warted isidia of _P. subaurifera_
may become entirely sorediose, and in _P. farinacea_ the whole thallus
is covered with isidia transformed into soralia. The transformation is
constant and is a distinct specific character. Bitter[488] considers that
it proves that no sharp distinction exists between isidia and soralia, at
least in their initial stages.
[Illustration: Fig. 82. _Usnea barbata_ Web. Longitudinal section of
filament and base of “soredial” branch × 40 (after Schwendener).]
_c._ SOREDIA AS BUDS. Schwendener[489] has described soredia in the
genus _Usnea_ which give rise to new branches. Many of the species in
that genus are plentifully sprinkled with the white powdery bodies. A
short way back from the apex of the filament the separate soredia show a
tendency to apical growth and might be regarded as groups of young plants
still attached to the parent branch. One of these developing more quickly
pushes the others aside and by continued growth fills up the soredial
opening in the cortex with a plug of tissue; finally it forms a complete
lateral branch. Schwendener calls them “soredial” branches (Fig. 82)
to distinguish them from the others formed in the course of the normal
development.
B. SORALIA
In lichens of foliose and fruticose structure, and in a few crustaceous
forms, the soredia are massed together into the compact bodies called
soralia, and thus are confined to certain areas of the plant surface.
The simpler soralia arise from the gonidial zone below the cortex by the
active division of some of the algal cells. The hyphae, interlaced with
the green cells, are thin-walled and are, as stated by Wainio[490], still
in a meristematic condition; they are thus able readily to branch and to
form new filaments which clasp the continually multiplying gonidia. This
growth is in an upward or outward direction away from the medulla, and
strong mechanical pressure is exerted by the increasing tissue on the
overlying cortical layers. Finally the soredia force their way through
to the surface at definite points. The cortex is thrown back and forms a
margin round the soralium, though shreds of epidermal tissue remain for a
time mixed with the powdery granules.
_a._ FORM AND OCCURRENCE OF SORALIA. The term “soralium” was first
applied only to the highly developed soredial structures considered
by Acharius to be secondary apothecia; it is now employed for any
circumscribed group of soredia.[491] The soralia vary in size and
form and in position, according to the species on which they occur;
these characters are constant enough to be of considerable diagnostic
value. Within the single genus _Parmelia_, they are to be found as
small round dots sprinkled over the surface of _P. dubia_; as elongate
furrows irregularly placed on _P. sulcata_; as pearly excrescences at
or near the margins of _P. perlata_, and as swollen tubercles at the
tips of the lobes of _P. physodes_ (Fig. 83). Their development is
strongly influenced and furthered by shade and moisture, and, given such
conditions in excess, they may coalesce and cover large patches of the
thallus with a powdery coating, though only in those species that would
have borne soredia in fairly normal conditions.
Soralia of definite form are of rather rare occurrence in crustaceous
lichens, with the exception of the Pertusariaceae, where they are
frequent, and some species of _Lecanora_ and _Placodium_. They are known
in only two hypophloeodal (subcortical) lichens, _Arthonia pruinosa_ and
_Xylographa spilomatica_. Among squamulose thalli they are typical of
some _Cladoniae_, and also of _Lecidea_ (_Psora_) _ostreata_, where they
are produced on the upper surface towards the apex of the squamule.
[Illustration: Fig. 83. _Parmelia physodes_ Ach. Thallus growing
horizontally; soredia on the ends of the lobes (S. H., _Photo._).]
_b._ POSITION OF SORALIFEROUS LOBES. According to observations made by
Bitter[492], the occurrence of soralia on one lobe or another may depend
to a considerable extent on the orientation of the thallus. He cites the
variability in habit of the familiar lichen, _Parmelia physodes_ and its
various forms, which grow on trees or on soil. In the horizontal thalli
there is much less tendency to soredial formation, and the soredia that
arise are generally confined to branching lobes on the older parts of the
thallus.
That type of growth is in marked contrast with the thallus obliged to
take a vertical direction as on a tree. In such a case the lobes, growing
downward from the point of origin, form soralia at their tips at an early
stage (Fig. 84). The lateral lobes, and especially those that lie close
to the substratum, are the next to become soraliate. Similar observations
have been made on the soraliferous lobes of _Cetraria pinastri_. The
cause is probably due to the greater excess of moisture draining
downwards to the lower parts of the thallus. The lobes that bear the
soralia are generally narrower than the others and are very frequently
raised from contact with the substratum. They tend to grow out from
the thallus in an upright direction and then to turn backwards at the
tip, so that the opening of the soralium is directed downwards. Bitter
says that the cause of this change in direction is not clear, though
possibly on teleological reasoning it is of advantage that the opening
of the soralium should be protected from direct rainfall. The opening
lies midway between the upper and lower cortex, and the upper tissue in
these capitate soralia continues to grow and to form an arched helmet or
hood-covering which serves further to protect the soralium.
[Illustration: Fig. 84. _Parmelia physodes_ Ach. Thallus growing
vertically; soredia chiefly on the lobes directed downwards, reduced (M.
P., _Photo._).]
Similar soralia are characteristic of _Physcia hispida_ (_Ph. stellaris_
subsp. _tenella_), the apical helmet being a specially pronounced feature
of that species, though, as Lesdain[493] has pointed out, the hooded
structures are primarily the work of insects. In vertical substrata they
occur on the lower lobes of the plant.
Apical soralia are rare in fruticose lichens, but in an Alpine variety
of _Ramalina minuscula_ they are formed at the tips of the fronds and
are protected by an extension of the upper cortical tissues. Another
instance occurs in a _Ramalina_ from New Granada referred by Nylander to
_R. calicaris_ var. _farinacea_: it presents a striking example of the
helmet tip.
_c._ DEEP-SEATED SORALIA. In the cases already described Schwendener[494]
and Nilson[495] held that the algae gave the first impulse to the
formation of the soredia; but in the Pertusariaceae[496], a family of
crustaceous lichens, there has been evolved a type of endogenous soralium
which originates with the medullary hyphae. In these, special hyphae rise
from a weft of filaments situated just above the lowest layer of the
thallus at the base of the medulla, the weft being distinguished from the
surrounding tissue by staining blue with iodine. A loose strand of hyphae
staining the usual yellow colour rises from the surface of the “blue”
weft and, traversing the medullary tissue, surrounds the gonidia on the
under side of the gonidial zone. The hyphae continue to grow upward,
pushing aside both the upper gonidial zone and the cortex, and carrying
with them the algal cells first encountered. When the summit is reached,
there follows a very active growth of both gonidia and hyphae. Each
separate soredium so produced consists finally of five to ten algal cells
surrounded by hyphae and measures 8 µ to 13 µ in diameter. The cortex
forms a well-defined wall or margin round the mass of soredia.
A slightly different development is found in _Lecanora tartarea_,
one of the “crottle” lichens, which has been placed by Darbishire in
Pertusariaceae. The hyphae destined to form soredia also start from the
weft of tissue at the base of the thallus, but they simply grow through
the gonidial zone instead of pushing it aside.
In his examination of Pertusariaceae Darbishire found that the apothecia
also originated from a similar deeply seated blue-staining tissue,
and he concluded that the soralia represented abortive apothecia and
really corresponded to Acharius’s “apothecia of the second order.”
His conclusion as to the homology of these two organs is disputed by
Bitter[497], who considers that the common point of origin is explained
by the equal demand of the hyphae in both cases for special nutrition,
and by the need of mechanical support at the base to enable the hyphae to
reach the surface and to thrust back the cortex without deviating from
their upward course through the tissues.
C. DISPERSAL AND GERMINATION OF SOREDIA
Soredia become free by the breaking down of the hyphal stalks at the
septa or otherwise. They are widely dispersed by wind or water and
soon make their appearance on any suitable exposed soil. Krabbe[498]
has stated that, in many cases, the loosely attached soredia coating
some of the _Cladonia_ podetia are of external origin, carried thither
by the air-currents. Insects too aid in the work of dissemination:
Darbishire[499] has told us how he watched small mites and other insects
moving about over the soralia of _Pertusaria amara_ and becoming
completely powdered by the white granules.
Darbishire[499] also gives an account of his experiments in the culture
of soredia. He sowed them on poplar wood about the beginning of February
in suitable conditions of moisture, etc. Long hyphal threads were at once
produced from the filaments surrounding the gonidia, and gonidia that had
become free were seen to divide repeatedly. Towards the end of August
of the same year a few soredia had increased in size to about 450µ in
diameter, and were transferred to elm bark. By September they had further
increased to a diameter of 520µ, and the gonidia showed a tendency
towards aggregation. No further differentiation or growth was noted.
More success attended Tobler’s[500] attempt to cultivate the soredia of
_Cladonia_ sp. He sowed them on soil kept suitably moist in a pot and
after about nine months he obtained fully formed squamules, at first only
an isolated one or two, but later a plentiful crop all over the surface
of the soil. Tobler also adds that soredia taken from a _Cladonia_, that
had been kept for about half a year in a dry room, grew when sown on a
damp substratum. The algae however had suffered more or less from the
prolonged desiccation, and some of them failed to develop.
A suggestion has been made by Bitter[501] that a hybrid plant might
result from the intermingling of soredia from the thallus of allied
lichens. He proposed the theory to explain the great similarity between
plants of _Parmelia physodes_ and _P. tubulosa_ growing in close
proximity. There is no proof that such mingling of the fungal elements
ever takes place.
D. EVOLUTION OF SOREDIA
Soredia have been compared to the gemmae of the Bryophytes and also to
the slips and cuttings of the higher plants. There is a certain analogy
between all forms of vegetative reproduction, but soredia are peculiar
in that they include two dissimilar organisms. In the lichen kingdom
there has been evolved this new form of propagation in order to secure
the continuance of the composite life, and, in a number of species, it
has almost entirely superseded the somewhat uncertain method of spore
germination inherited from the fungal ancestor, but which leaves more or
less to chance the encounter with the algal symbiont.
From a phylogenetic point of view we should regard the sorediate
lichens as the more highly evolved, and those which have no soredia
as phylogenetically young, though, as Lindau[502] has pointed out,
soredia are all comparatively recent. They probably did not appear until
lichens had reached a more or less advanced stage of development, and,
considering the polyphyletic origin of lichens, they must have arisen at
more than one point, and probably at first in circumstances where the
formation of apothecia was hindered by prolonged conditions of shade and
moisture.
That soredia are ontogenetic in character, and not, as Nilson[503] has
asserted, accidental products of excessively moist conditions is further
proved by the non-sorediate character of those species of crustaceous
lichens belonging to _Lecanora_, _Verrucaria_, etc. that are frequently
immersed in water. Bitter[504] found that the soredia occurring on
_Peltigera spuria_ were not formed on the lobes which were more
constantly moist, nor at the edges where the cortex was thinnest: they
always emerged on young parts of the thallus a short way back from the
edge.
Bitter[504] points out that in extremely unfavourable circumstances—in
the polluted atmosphere near towns, or in persistent shade—lichens,
that would otherwise form a normal thallus, remain in a backward
sorediose state. He considers, however, that many of these formless
crusts are autonomous growths with specific morphological and chemical
peculiarities. They hold these outposts of lichen vegetation and are not
found growing in any other localities. The proof would be to transport
them to more favourable conditions, and watch development.
4. ISIDIA
A. FORM AND STRUCTURE OF ISIDIA
Many lichens are rough and scabrous on the surface, with minute simple
or divided coral-like outgrowths of the same texture as the underlying
thallus, though sometimes they are darker in colour as in _Evernia
furfuracea_. They always contain gonidia and are covered by a cortex
continuous with that of the thallus.
This very marked condition was considered by Acharius[505] as of
generic importance and the genus, _Isidium_, was established by him,
with the diagnostic characters: “branchlets produced on the surface,
or coralloid, simple and branched.” In the genus were included the
more densely isidioid states of various crustaceous species such as
_Isidium corallinum_ and _I. Westringii_, both of which are varieties of
_Pertusariae_. Fries[506], with his accustomed insight, recognized them
as only growth forms. The genus was however still accepted in English
Floras[507] as late as 1833, though we find it dropped by Taylor[508] in
the _Flora Hibernica_ a few years later.
The development of the isidial outgrowth has been described by
Rosendahl[509] in several species of _Parmelia_. In one of them, _P.
papulosa_, which has a cortical layer one cell thick, the isidium begins
as a small swelling or wart on the upper surface of the thallus. At that
stage the cells of the cortex have already lost their normal arrangement
and show irregular division. They divide still further, as gonidia and
hyphae push their way up. The full-grown isidia in this species are
cylindrical or clavate, simple or branched. They are peculiar in that
they bear laterally here and there minute rhizoids, a development not
recorded in any other isidia. The inner tissue accords with that of the
normal thallus and there is a clearly marked cortex, gonidial zone and
pith. A somewhat analogous development takes place in the isidia of
_Parmelia proboscidea_; in that lichen they are mostly prolonged into a
dark-coloured cilium.
In _Parmelia scortea_ the cortex is several cells thick, and the
outermost rows are compressed and dead in the older parts of the thallus;
but here also the first appearance of the isidium is in the form of a
minute wart. The lower layers (4 to 6) of living cortical cells divide
actively; the gonidia also share in the new growth, and the protuberance
thus formed pushes off the outer dead cortex and emerges as an isidium
(Fig. 85). They are always rather stouter in form than those of _P.
papulosa_ and may be simple or branched. The gonidia in this case do not
form a definite zone, but are scattered through the pith of the isidium.
Here also should be included the coralloid branching isidia that
adorn the upper surface and margins of the thallus of _Umbilicaria
pustulata_. They begin as small tufts of somewhat cylindrical bodies,
but they sometimes broaden out to almost leafy expansions with crisp
edges. Most frequently they are situated on the bulging pustules where
intercalary growth is active. Owing to their continued development on
these areas, the tissue becomes slack, and the centre of the isidial tuft
may fall out, leaving a hole in the thallus which becomes still more
open by the tension of thalline expansion. New isidia sprout from the
edges of the wound and the process may again be repeated. It has been
asserted that these structures are only formed on injured parts of the
thallus—something like gall-formations—but Bitter[510] has proved that
the wound is first occasioned by the isidial growth weakening the thallus.
[Illustration: Fig. 85. Vertical section of isidia of _Parmelia scortea_
Ach. A, early stage; B. later stage. × 60 (after Rosendahl).]
B. ORIGIN AND FUNCTION OF ISIDIA
Nilson[511] (later Kajanus[512]) insists that isidia and soredia are
both products of excessive moisture. He argues that lichen species, in
the course of their development, have become adapted to a certain degree
of humidity, and, if the optimum is passed, the new conditions entail
a change in the growth of the plant. The gonidia are stimulated to
increased growth, and the mechanical pressure exerted by the multiplying
cells either results in the emergence of isidial structures where the
cortex is unbroken, or, if the cortex is weaker and easily bursts, in the
formation of soralia.
This view can hardly be accepted; isidia as well as soredia are typical
of certain species and are produced regularly and normally in ordinary
conditions; both of them are often present on the same thallus. It is
not denied, however, that their development in certain instances is
furthered by increased shade or moisture. In _Evernia furfuracea_ isidia
are more freely produced on the older more shaded parts of the thallus.
Zopf[513] has described such an instance in _Evernia olivetorina_ (_E.
furfuracea_), which grew in the high Alps on pine trees, and which was
much more isidiose when it grew on the outer ends of the branches,
where dew, rain or snow had more direct influence. He[514] quotes other
examples occurring in forms of _E. furfuracea_ which grew on the branches
of pines, larches, etc. in a damp locality in S. Tyrol. The thalli hung
in great abundance on each side of the branches, and were invariably more
isidiose near the tips, because evidently the water or snow trickled down
and was retained longer there than at the base.
Bitter[515] has given a striking instance of shade influence in
_Umbilicaria_. He found that some boulders on which the lichen grew
freely had become covered over with fallen pine needles. The result was
at first an enormous increase of the coralline isidia, though finally the
lichen was killed by the want of light.
Isidia are primarily of service to the plant in increasing the
assimilating surface. Occasionally they grow out into new thallus
lobes. The more slender are easily rubbed off, and, when scattered,
become efficient organs of propagation. This view of their function is
emphasized by Bitter who points out that both in _Evernia furfuracea_
and in _Umbilicaria pustulata_ other organs of reproduction are rare or
absent. Zopf[513] found new plants of _Evernia furfuracea_ beginning to
grow on the trunk of a tree lower down than an old isidiose specimen.
They had developed from isidia which had been detached and washed down by
rain.
VI. HYMENOLICHENS
A. SUPPOSED AFFINITY WITH OTHER PLANTS
Lichens in which the fungal elements belong to the Hymenomycetes are
confined to three tropical genera. They are associated with blue-green
algae and are most nearly related to the Thelephoraceae among fungi. The
spores are borne, as in that family, on basidia.
[Illustration: Fig. 86. _Cora Pavonia_ Fr. (after Mattirolo).]
The best known Hymenolichen, _Cora Pavonia_ (Fig. 86), was discovered
by Swartz[516] during his travels in the W. Indies (1785-87) growing on
trees in the mountains of Jamaica, and the new plant was recorded by
him as _Ulva montana_. Gmelin[517] also included it in _Ulva_ in close
association with _Ulva_ (_Padina_) _Pavonia_, but that classification was
shortly after disputed by Woodward[518] who thought its affinity was more
nearly with the fungi and suggested that it should be made the type of a
new genus near to _Boletus_ (_Polystictus_) _versicolor_. Fries[519] in
due time made the new genus _Cora_, though he included it among algae;
finally Nylander[520] established the lichenoid character of the thallus
and transferred it to the Lecanorei.
It was made the subject of more exact investigation by Mattirolo[521]
who recognized its affinity with _Thelephora_, a genus of Hymenomycetes.
Later Johow[522] went to the West Indies and studied the Hymenolichens in
their native home. The genera and species described by Johow have been
reduced to _Cora_ and _Dictyonema_; a new genus _Corella_ has since been
added by Wainio[523].
Johow found that _Cora_ grew on the mountains usually from 1000 to 2000
ft. above sea-level. As it requires for its development a cool damp
climate with strong though indirect illumination, it is found neither
in sunny situations nor in the depths of dark woods. It grows most
freely in diffuse light, on the lower trunks and branches of trees in
open situations, but high up on the stem where the vegetation is more
dense. It stands out from the tree like a small thin bracket fungus,
one specimen placed above another, with a dimidiate growth similar to
that of _Polystictus versicolor_. Both surfaces are marked by concentric
zones which give it an appearance somewhat like _Padina Pavonia_. These
zones indicate unequal intercalary growth both above and below. The whole
plant is blue-green when wet, greyish-white when dry, and of a thin
membranaceous consistency.
B. STRUCTURE OF THALLUS
There is no proper cortex in any of the genera, but in _Cora_ there
is a fastigiate branching of the hyphae in parallel lines towards the
upper surface; just at the outside they turn and lie in a horizontal
direction, and, as the branching becomes more profuse, a rather compact
cover is formed. The gonidia, which consist of blue-green _Chroococcus_
cells, lie at the base of the upward branches and they are surrounded
with thin-walled short-celled hyphae closely interwoven into a kind of
cellular tissue. The medulla of loose hyphae passes over to the lower
cortex, also of more or less loose filaments. The outermost cells of the
latter very frequently grow out into short jagged or crenate processes
(Fig. 87).
[Illustration: Fig. 87. _Cora Pavonia_ Fr. Vertical section of thallus.
_a_, upper cortex; _b_, gonidial layer; _c_, medulla and lower cortex
of crenate cells; _d_, tuft of fertile hyphae. × 160. _e_, basidia and
spores × 1000 (after Johow).]
In _Corella_, the mature lichen is squamulose or consists of small lobes;
in _Dictyonema_ there is a rather flat dimidiate expansion; in both the
alga is _Scytonema_, the trichomes of which largely retain their form and
are surrounded by parallel growths of branching hyphae. The whole tissue
is loose and spongy.
_Corella_ spreads over soil on a white hypothallus without rhizinae. In
the other two genera which live on soil, or more frequently on trees,
there is a rather extensive formation of hold-fast tissue. When the
dimidiate thallus grows on a rough bark, rhizoidal strands of hyphae
travel over it and penetrate between the cracks; if the bark is smooth,
there is a more continuous weft of hyphae. In both cases a spongy cushion
of filamentous tissue develops at the base of the lichen between the
tree and the bracket thallus. There is also in both genera an encrusting
form which Johow regarded as representing a distinct genus _Laudatea_,
but which Möller found to be merely a growth stage. Möller[524] judged
from that and from other characteristics that the same fungus enters into
the composition of both _Cora_ and _Dictyonema_ and that only the algal
constituents are different.
C. SPORIFEROUS TISSUES
As in Hymenomycetes, the spores of Hymenolichens are exogenous, and
are borne at the tips of basidia which in these lichens are produced
on the under surface of the thallus. In _Cora_ the fertile filaments
may form a continuous series of basidia over the surface, but generally
they grow out in separate though crowded tufts. As these tufts broaden
outwards, they tend to unite at the free edges, and may finally present a
continuous hymenial layer. Each basidium bears four sterigmata and spores
(Fig. 87 _e_); paraphyses exactly similar to the basidia are abundant in
the hymenium. In _Dictyonema_ the hymenium is less regular, but otherwise
it resembles that of _Cora_. No hymenium has as yet been observed in
_Corella_; it includes, so far as known, one species, _C. brasiliensis_,
which spreads over soil or rocks.