trichogynes, accompanied by spermogonia and spermatia, represent
fertilization, or the probability of fertilization, then the process may
be assumed to take place in a fairly large and widely distributed series
of lichens. Copulation between the spermatium and the trichogyne has
been seen by Stahl[650], Baur[651] and by F. Bachmann[652] in _Collema_.
In _Physcia pulverulenta_ Darbishire[653] could not prove copulation
in the earlier stages, but he found what he took to be the remains of
emptied spermatia adhering to the tips of old trichogynes. Changes in the
trichogyne following on presumed copulation have been demonstrated by
several workers in the Collemaceae, and open communication as a result of
fertilization between the cells of the ascogonium has been described in
two species. This coenocytic condition of the ascogonium (or archicarp),
considered by Darbishire and others as an evidence of fertilization, has
been demonstrated by Fitzpatrick[654] in the fungus _Rhizina undulata_.
The walls between the cells of the archicarp in that Ascomycete became
more or less open, so that the ascogenous hyphae growing from the central
cells were able easily to draw nutrition from the whole coenocyte, but
no process of fertilization in _Rhizina_ preceded the breaking down of
the septa and no fusion of nuclei was observed until the stage of ascus
formation.
The real distinction between fertile and vegetative hyphae lies,
according to Harper[655], in the relative size of the nuclei. F. Bachmann
speaks of one large nucleus in the spermatium of _Collema pulposum_ which
would indicate sexual function. There is however very little nuclear
history of lichens known at any stage until the beginning of ascus
formation, when fusion of two nuclei certainly take place as in fungi to
form the definitive nucleus of the ascus.
The whole matter may be summed up in Fünfstück’s[656] statement that:
“though research has proved as very probable that fertilization takes
place, it is an undoubted fact that no one has observed any such process.”
F. FINAL STAGES OF APOTHECIAL DEVELOPMENT
The emergence of the lichen apothecium from the thallus, and the form it
takes, are of special interest, as, though it is essentially fungal in
structure, it is subject to various modifications entailed by symbiosis.
_a._ OPEN OR CLOSED APOTHECIA. Schwendener[657] drew attention to two
types of apothecia directly influenced by the thallus: those that are
closed at first and only open gradually, and those which are, as he says,
open from the first. The former occur in genera and species in which the
thallus has a stoutish cortex, as, for instance, in _Lobaria_ where the
young fructification has all the appearance of an opening perithecium.
The open apothecia (_primitus aperta_) are found in non-corticate
lichens, in which case the pioneer paraphyses arrive at the surface
easily and without any converging growth. Similar apothecia are borne
directly on the hypothallus at the periphery, or between the thalline
areolae, and they are also characteristic of thin or slender thalli as in
_Coenogonium_.
In both types of apothecium, the paraphyses pierce the cortex (Fig. 100)
and secure the emergence of the developing ascomata.
[Illustration: Fig. 100. _Physcia ciliaris_ DC. Vertical section
of apothecium still covered by the cortex. _a_, paraphyses; _b_,
hypothecium; _c_, gonidia of thallus and amphithecium. × 150 (after
Baur).]
_b._ EMERGENCE OF THE ASCOCARP. Hue[658] has taken up this subject in
recent years and has described the process by which the vegetative hyphae
surrounding the fruit primordium, excited to active growth by contact
with the generative system, take part in the later stages of fruit
formation. The primordium generally occupies a position near to, or just
within, the upper medulla, and the hyphae in contact with it soon begin
to branch freely in a vertical direction, surrounding the developing
fruit and carrying it upwards generally to a superficial position. The
different methods of the final emergence give two very distinct types of
mature apothecium: the _lecideine_ in which the gonidial zone takes no
part in the upward growth, and the _lecanorine_ into which the gonidia
enter as an integral part.
In the lecideine series (Fig. 101) the encircling hyphae from the upper
medulla rise as a compact column through the gonidial zone to the surface
of the thallus; they then spread radially before curving up to form
the outer wall or “proper margin” round the spore-bearing disc. The
branching of the hyphae is fastigiate with compact shorter branches at
the exterior. In such an apothecium gonidia are absent both below the
hypothecium and in the margins.
[Illustration: Fig. 101. _Lecidea parasema_ Ach. Vertical section of
thallus and apothecium with proper margin only × ca. 50.]
In lecanorine development the ascending hyphae from the medulla, in some
cases, carry with them algal cells which multiply and spread as a second
gonidial layer under the hypothecium (Fig. 102). These hyphae may also
spread in a radial direction while still within the thallus and give
rise to an “immersed” apothecium which is lecanorine as it encloses
gonidia within its special tissues, for example, in _Acarospora_ and
_Solorina_. But in most cases the lecanorine fruit is superficial and
not unfrequently it is raised on a short stalk (_Usnea_, etc.); both the
primary gonidial zone of the thallus and the outer cortex are associated
with the medullary column of hyphae from the first and grow up along
with it, thus providing the outer part of the apothecium, an additional
“thalline margin” continuous with the thallus itself. It is an advanced
type of development peculiar to lichens, and it provides for fertility
of long continuance which is in striking contrast with the fugitive
ascocarps of the Discomycetes.
[Illustration: Fig. 102. _Lecanora tartarea_ Ach. Vertical section of
apothecium. _a_, hymenium; _b_, proper margin or parathecium; _c_,
thalline margin or amphithecium. × 30 (after Reinke).]
The distinction between lecideine and lecanorine apothecia is of great
value in classification, but it is not always easily demonstrable; it
is occasionally necessary to examine the early stages, as in the more
advanced the thalline margin may be pushed aside by the turgid disc and
become practically obliterated.
The “proper margin” reaches its highest development in the lecideine
and graphideine types. It is less prominent or often almost entirely
replaced when the thalline margin is superadded, except in genera such as
_Thelotrema_ and _Diploschistes_ which have distinct “double margins.”
There is an unusual type of apothecium in the genus _Gyrophora_.
The fruit is lecideine, the thalline gonidia taking no part in the
development. The growth of the initial ascogenous tissue, according to
Lindau[659], is constantly towards the periphery of the disc so that a
weak spot arises in the centre which is promptly filled by a vigorous
sterile growth of paraphyses. This process is repeated from new centres
again and again, resulting in the irregularly concentric lines of sterile
and fertile areas of the “gyrose” fruit (Fig. 103). The paraphyses soon
become black at the tips. Asci are not formed until the ascogenous layer
has acquired a certain degree of stability, and spores are accordingly
present only in advanced stages of growth.
[Illustration: Fig. 103. Apothecial gyrose discs of _Gyrophora
cylindrica_ Ach. × 12 (after Lindau).]
G. LICHEN ASCI AND SPORES
_a_. HISTORICAL. The presence of spores, as such, in the lichen fruit was
first established by Hedwig[660] in _Anaptychia_ (_Physcia_) _ciliaris_.
He rightly judged the minute bodies to be the “semina” of the plant. In
that species they are fairly large, measuring about 50µ, long and 24µ
thick, and as they are very dark in colour when mature, they stand out
conspicuously from the surrounding colourless tissue of the hymenium.
Acharius[661] also took note of these “semina” and happily replaced the
term by that of “spores.” They may be produced, he states, in a compact
nucleus (_Sphaerophoron_), in a naked disc (_Calicium_), or they may be
embedded in the disc (_Opegrapha_ and _Lecidea_). Sprengel[662] opined
that the spores—which he figures—were true seeds, though he allows
that there had been no record of their development into new plants.
Luyken[663] made a further contribution to the subject by dividing
lichens into gymnocarpous and angiocarpous forms, according as the
spores, enclosed in cells or vesicles (thecae), were borne in an open
disc or in a closed perithecium.
In his _Systema_ of lichen genera Eschweiler[664], some years later,
described and figured the spores as “thecae” enclosed in cylindrical
asci. Fée[665] in contemporary works gave special prominence to the
colour and form of the spores in all the lichens dealt with.
_b_. DEVELOPMENT OF THE ASCUS. The first attempt to trace the origin
and development of lichen asci and spores was made by Mohl[666]. He
describes the mother-cell (the ascus) as filled at first with a clouded
granular substance changing later into a definite number—usually eight—of
simple or septate spores. Dangeard[667] included the lichens _Borrera_
(_Physcia_) _ciliaris_ and _Endocarpon_ (_Dermatocarpon_) _miniatam_
among the plants that he studied for ascus and spore development. He
found that in lichens, as in fungi, the ascus arose usually from the
penultimate cell of a crooked hypha (Fig. 104) and that it contained
at first two nuclei derived from adjoining cells. These nuclei are
similar in size to those of the vegetative hyphae, and in each there is
a large nucleolus with chromatin material massed on one side. Fusion
takes place, as in fungi, between the two nuclei, and the secondary or
definitive nucleus thus formed divides successively to form the eight
spore-nuclei. Baur[668] and Nienburg[669] have confirmed Dangeard’s
results as regards lichens, and René Maire[670] has also contributed
important cytological details on the development of the spores. In
_Anaptychia_ (_Physcia_) _ciliaris_ he found that the fused nucleus
became larger and that a synapsis stage supervened during which the long
slender chromatin filaments became paired, and at the same time shorter
and thicker. The nuclear membrane disappeared as the chromatin filaments
were united in masses joined together by linin threads which also
disappeared later. At the most advanced stage observed by Maire there
was visible a nucleolus embedded in a condensed plasma and surrounded
by eight medianly constricted filaments destined to form the equatorial
plate. A few isolated observations were also made on the cytology of the
ascus in _Peltigera canina_, in which lichen the preceding ascogonial
development is wholly vegetative. The secondary nucleus was seen to
contain a chromatin mass and a large nucleolus; in addition two angular
bodies of uncertain signification were associated with the nucleolus,
each with a central vacuole. The nucleolus disappeared in the prophase of
the first division and four double chromosomes were then plainly visible.
The succeeding phases of the first and the second nuclear division were
not seen, but in the prophase of the third it was possible to distinguish
four chromatin masses outside the nucleolus. The slow growth of the
lichen plant renders continuous observation extremely difficult.
[Illustration: Fig. 104. Developing asci of _Physcia ciliaris_ DC. × 800
(after Baur).]
F. Bachmann[671] was able to make important cytological observations in
her study of _Collema pulposum_. As regards the vegetative and ascogonial
nuclei, five or perhaps six chromosomes appeared on the spindle when the
nucleus divided. In the asci, the usual paired nuclei were present in the
early stages and did not fuse until the ascus had elongated considerably.
After fusion the definitive nucleus enlarged with the growth of the ascus
and did not divide until the ascus had attained full size. The nucleolus
was large, and usually excentric, and there were at first a number of
chromatin masses on an irregular spirem. In synapsis the spirem was drawn
into a compact mass, but after synapsis, “the chromatin is again in the
form of a knotty spirem.” In late prophases the chromosomes, small ovoid
bodies, were scattered on the spindle; later they were aggregated in the
centre, and, in the early metaphase, about twelve were counted now split
longitudinally. There were thus twice as many chromosomes in the first
division in the ascus as in nuclear divisions of the vegetative hyphae.
F. Bachmann failed to see the second division; there were at least five
chromosomes in the third division.
Considerable importance is given to the number of the chromosomes in
the successive divisions in the ascus since they are considered to be
proof of a previous double fusion—in the ascogonium and again in the
ascus—necessitating, therefore, a double reduction division to arrive at
the gametophytic or vegetative number of five or six chromosomes in the
third division in the ascus. There have been too few observations to draw
any general conclusions.
_c._ DEVELOPMENT OF SPORES. The spore wall begins to form, as in
Ascomycetes, at the apex of the nucleus with the curving over of the
astral threads, the nucleus at that stage presenting the figure of
a flask the neck of which is occupied by the centrosome. The final
spore-nucleus, as observed by Maire, divides once again in _Anaptychia_
and division is followed by the formation of a median septum, the mature
spore being two-celled. In _Peltigera_ the spore is at first ovoid,
but both nucleus and spore gradually elongate. The fully formed spore
is narrowly fusiform and by repeated nuclear division and subsequent
cross-septation it becomes 4- or even 5-6-celled.
The spores of lichens are wholly fungoid, and, in many cases, form a
parallel series with the spores of the Ascomycetes. Markings of the
epispore, such as reticulations, spines, etc., are rarely present
(_Solorina spongiosa_), though thickening of the wall occurs in many
species (_Pertusariae_, etc.), a peculiarity which was first pointed out
by Mohl[672] who contrasted the spore walls with the delicate membranes
of other lichen cells. Some spores, described as “halonate,” have an
outer gelatinous covering which probably prevents the spore from drying
up and thus prolongs the period of possible germination. Both asci and
spores are, as a rule, more sparingly produced than in fungi; in many
instances some or all of the spores in the ascus are imperfectly formed,
and the full complement is frequently lacking, possibly owing to some
occurrence of adverse conditions during the long slow development of the
apothecium. In the larger number of genera and species the spores are
small bodies, but in some, as for instance in the _Pertusariae_ and in
some _Pyrenocarpeae_, they exceed in size all known fungus spores. In
_Varicellaria microsticta_, a rare crustaceous lichen of high mountains,
the solitary 1-septate spore measures up to 350 µ, in length and 115 µ
in width. Most spores contain reserve material in the form of fat, etc.,
many are dark-coloured; Zukal[673] has suggested that the colour may be
protective.
Their ejection from the ascus at maturity is caused by the twofold
pressure of the paraphyses and the marginal hyphae on the addition of
moisture. The spores may be shot up at least 1 cm. from the disc[674].
_d._ SPORE GERMINATION. Meyer[675] was the first who cultivated lichen
spores and the dendritic formation which he obtained by growing them on
a smooth surface was undoubtedly the prothallus (or hypothallus) of the
lichen. Actual germination was however not observed till Holle[676] in
1846 watched and figured the process as it occurs in _Physcia ciliaris_.
Spores divided by transverse septa into two or more cells, as well
as those that have become “muriform” by transverse and longitudinal
septation, may germinate from each cell.
[Illustration: Fig. 105. Multinucleate spore of _Lecidea_ (_Mycoblastus_)
_sanguinaria_ Ach. × 540 (after Zopf).]
[Illustration: Fig. 106. Germination of multinucleate spore of
_Ochrolechia pallescens_ Koerb. × 390 (after de Bary).]
_e._ MULTINUCLEATE SPORES. These spores, which are all very large, occur
in several genera or subgenera: in _Lecidea_ subg. _Mycoblastus_ (Fig.
105), _Lecanora_ subg. _Ochrolechia_ and in Pertusariaceae. Tulasne[677]
in his experiments with germinating spores found that in _Lecanora
parella (Ochrolechia pallescens?)_ germinating tubes were produced all
over the surface of the spore (Fig. 106). De Bary[678] verified his
observations in that and other species and added considerable detail:
about twenty-four hours after sowing spores of _Ochrolechia pallescens_,
numerous little warts arose on the surface of the spore which gradually
grew out into delicate hyphae. All these spores contain fat globules and
finely granular protoplasm with a very large number of minute nuclei; the
presence of the latter has been demonstrated by Haberlandt[679] and later
by Zopf[680] who reckoned about 200 to 300 in the spore of _Mycoblastus
sanguinarius_. These nuclei had continued to multiply during the ripening
of the spore while it was still contained in the ascus[680]. Owing to the
presence of the large fat globules the plasma is confined to an external
layer close to the spore wall; the nuclei are embedded in the plasma and
are connected by strands of protoplasm. The epispore in some of these
large spores is extremely developed: in some _Pertusariae_ it measures
4-5 µ in thickness.
_f._ POLARIBILOCULAR SPORES. The most peculiar of all lichen spores
are those termed _polaribilocular_—signifying a two-celled spore of
which the median septum has become so thickened that the cell-cavities
with their contents are relegated to the two poles of the spore, an
open canal frequently connecting the two cell-spaces (Fig. 107). Other
terms have been suggested and used by various writers to describe this
unusual character such as blasteniospore[681], orculiform[682] and
placodiomorph[683] or more simply polarilocular.
The polarilocular colourless spore is found in a connected series of
lichens—crustaceous, foliose and fruticose (_Placodium_, _Xanthoria_,
_Teloschistes_). In another series with a darker thallus (_Rinodina
and Physcia_) the spore is brown-coloured, and the median septum cuts
across the plasma-connection. In other respects the brown spore is
similar to the colourless one and possesses a thickened wall with reduced
cell-cavities.
[Illustration: Fig. 107. Polarilocular spores. _a_, _Xanthoria parietina_
Th. Fr.; _b_, _Rinodina roboris_ Th. Fr.; _c_, _Physcia pulverulenta_
Nyl.; _d_, _Physcia ciliaris_ DC. × 600.]
The method of cell-division in these spores resembles that known as
“cleavage by constriction,” in which the cross wall arises by an
ingrowth from all sides of the cell; in time the centre is reached
and the wall is complete, or an open pore is left between the divided
cells. Cell “cleavage” occurs frequently among Thallophytes, though it
is unknown among the higher plants. Among Algae it is the normal form
of cell-division in _Cladophora_ and also in _Spirogyra_, though in the
latter the wall passes right across and cuts through the connecting
plasma threads. Harper[684] found “cleavage by constriction” in two
instances among fungi: the conidia of _Erysiphe_ and the gametes of
_Sporodinia_ are cut off by a septum which originates as a circular
ingrowth of the outer wall, comparable, he considers, with the
cell-division of _Cladophora_.
The development of the thickened wall of polarilocular spores has been
studied by Hue[685], who contends however that there is no true septation
in the colourless spores so long as the central canal remains open.
According to his observations the wall of the young spore is formed
of a thin tegument, everywhere equal in thickness, and consisting of
concentric layers. This tegument becomes continually thicker at the
equator of the spore by the addition of new layers from the interior, and
the protoplasmic contents are compressed into a gradually diminishing
space. In the end the wall almost touches at the centre, and the
spore consists of two polar cell-cavities with a narrow open passage
between. A median line pierced by the canal is frequently seen. In a few
species there is a second constriction cleavage and the spore becomes
quadrilocular.
Hue insists that this spore should be regarded as only one-celled; for
though the walls may touch at the centre, he says they never coalesce.
He has unfortunately given no cytological observations as to whether the
spore is uni- or binucleate.
In _Xanthoria parietina_, one of the species with characteristic
polaribilocular spores, germination, it would seem, takes place mostly at
one end only of the spore, though a germinating tube issues at both ends
frequently enough to suggest that the spore is binucleate and two-celled.
The absence of germination from one or other of the cells only may
probably be due to the drain on their small resources. Hue has cited the
rarity of such instances of double germination in support of his view
of the one-celled nature of the spore. He instances that out of fifteen
spores, Tulasne[686] has figured only three that have germinated at each
end; Bornet[687] figures one in seven with the double germination and
Bonnier[688] one in sixteen spores.
Further evidence is wanted as to the nuclear history of these hyaline
spores. In the case of the brown spores, which show the same thickening
of the wall and restricted cell-cavity, though with a distinct median
septum, nuclear division was observed by René Maire[689] before septation
in one such species, _Anaptychia ciliaris_.
II. SECONDARY SPORES
A. REPRODUCTION BY OIDIA
In certain conditions of nutrition, fungal hyphae break up into separate
cells, each of which functions as a reproductive _conidium_ or _oidium_,
which on germination forms new hyphae. Neubner[690] has demonstrated a
similar process in the hyphae of the Caliciaceae and compares it with the
oidial formation described by Brefeld[691] in the Basidiomycetes.
The thallus of this family of lichens is granular or furfuraceous; it
never goes beyond the _Lepra_ stage of development[692]. In some species
it is scanty, in others it is abundant and spreads over large areas of
the trunks of old trees. It is only when growth is especially luxuriant
that oidia are formed. Neubner was able to recognize the oidial condition
by the more opaque appearance of the granules, and under the microscope
he observed the hyphae surrounding the gonidia gradually fall away and
break up into minute cylindrical cells somewhat like spermatia in size
and form. There was no question of abnormal or unhealthy conditions, as
the oidia were formed in a freely fruiting thallus.
The gonidia associated with the oidial hyphae also showed unusual
vitality and active division took place as they were set free by the
breaking up of the encircling hyphae. The germination of the oidia
provides an abundance of hyphal filaments for the rapidly increasing
algal cells, and there follows a wide-spread development of the lichen
thallus.
Oidial formation has not been observed in any other family of lichens.
B. REPRODUCTION BY CONIDIA
_a._ INSTANCES OF CONIDIAL FORMATION. It is remarkable that the type of
asexual reproduction so abundantly represented in fungi by the large
and varied group of the Hyphomycetes is practically absent in lichens.
An exception is to be found in a minute gelatinous lichen that grows
on soil. It was discovered by Bornet[693] and called by him _Arnoldia_
(_Physma_) _minutula_. From the thallus rise up simple or sparingly
branched colourless conidiophores which bear at the tips globose brown
conidia (Fig. 108). Bornet[694] obtained these conidia by keeping very
thin sections of the thallus in a drop of water[693].
[Illustration: Fig. 108. Conidia developed from thallus of _Arnoldia
minutula_ Born. × 950 (after Bornet).]
Yet another instance of conidial growth is given by Steiner[695]. He
had observed that the apothecia on plants of _Caloplaca aurantia_
var. _callopisma_ Stein. differed from those of normal appearance in
the warted unevenness of the disc and also in being more swollen and
convex, the thalline margin being almost obliterated. He found, on
microscopical examination, that the hymenium was occupied by paraphyses
and by occasional asci, the latter seldom containing spores, and being
usually more or less collapsed. The component parts of the apothecium
were entirely normal and healthy, but the paraphyses and the few asci
were crushed aside by the intrusion of numerous slender unbranched
septate conidiophores. Several of these might spring from one base and
the hypha from which they originated could be traced some distance into
the ascogenous layer, though a connection with that cell-system could not
be demonstrated. While still embedded in the hymenium, an ellipsoid or
obovate swelling began to form at the apex of the conidiophore; it became
separated from the stalk by a septum and later divided into a two-celled
conidium. The conidiophore increased in length by intercalary growth and
finally emerged above the disc; the mature conidium was pyriform and
measured 15-20 µ × 9-11 µ.
Steiner regarded these conidia as entirely abnormal; pycnidia with
stylospores are unknown in the genus and they were not, he alleges, the
product of any parasitic growth.
_b._ COMPARISON WITH HYPHOMYCETES. The conidial form of fructification
in fungi, known as a Hyphomycete, is generally a stage in the life-cycle
of some Ascomycete; it represents the rapid summer form of asexual
reproduction. The ascospore of the resting fruit-form in many species
germinates on any suitable matrix and may at once produce conidiophores
and conidia, which in turn germinate, and either continue the conidial
generation or proceed to the formation of the perfect fruiting form with
asci and ascospores.
Such a form of transient reproduction is almost impossible in lichens,
as the hypha produced by the germinating lichen ascospore has little
vitality without the algal symbiont. In natural conditions development
practically ceases in the absence of symbiosis. When union between the
symbionts takes place, and growth becomes active, thallus construction at
once commences. But in certain conditions of shade and moisture, only the
rudiments of a lichen thallus are formed, known as a leprose or sorediose
condition. Soredia also arise in the normal life of many lichens. As
the individual granules or soredia may each give rise to a complete
lichen plant, they may well be considered as replacing the lost conidial
fructification.
C. CAMPYLIDIUM AND ORTHIDIUM
Müller[696] has described under the name _Campylidium_ a supposed new
type of asexual fructification which he found on the thallus of tropical
species of _Gyalecta_, _Lopadium_, etc., and which he considered
analogous to pycnidia and spermogonia. Wainio[697] has however recognized
the cup-like structure as a fungus, _Cyphella aeruginascens_ Karst.,
which grows on the bark of trees and occasionally is parasitic on the
crustaceous thallus of lichens. Wainio has also identified the plant,
_Lecidea irregularis_, first described by Fée[698], as also synonymous
with the fungus.
Another name _Orthidium_ was proposed by Müller[699] for a type of
fructification he found in Brazil which he contrasts or associates with
_Campylidium_. It has an open marginate disc with sporophores bearing
acrogenous spores. He found it growing in connection with a thin lichen
thallus on leaves and considered it to be a form of lichen reproduction.
Possibly _Orthidium_ is also a _Cyphella_.
III. SPERMOGONIA OR PYCNIDIA
A. HISTORICAL ACCOUNT OF SPERMOGONIA
The name spermogonium was given by Tulasne[700] to the “punctiform
conceptacles” that are so plentifully produced on many lichen thalli, on
the assumption that they were the male organs of the plant, and that the
spore-like bodies borne in them were non-motile male cells or spermatia.
The first record of their association with lichens was made by
Dillenius[701], who indicates the presence of black tubercles on the
thallus of _Physcia ciliaris_. He figures them also on several species
of _Cladonia_, on _Ramalina_ and on _Dermatocarpon_, but without any
suggestion as to their function. Hedwig’s[702] study of the reproductive
organs of the Linnaean Cryptogams included lichens. He examined _Physcia
ciliaris_, a species that not only is quite common but is generally
found in a fruiting condition and with very prominent spermogonia, and
has been therefore a favourite lichen for purposes of examination and
study. Hedwig describes and figures not only the apothecia but also those
other bodies which he designates as “punctula mascula,” or again as
“puncta floris masculi.” In his later work he gives a drawing of _Lichen_
(_Gyrophora_) _proboscideus_, with two of the spermogonia in section.
Acharius[703] included them among the lichen structures which he called
“cephalodia”: he described them as very minute tubercles rising up
from the substance of the thallus and projecting somewhat above it. He
also figures a section through two “cephalodia” of _Physcia ciliaris_.
Fries[704] looked on them as being mostly “anamorphoses of apothecia,
the presence of abortive fruits transforming the angiocarpous lichen to
the appearance of a gymnocarpous form.” Wallroth[705] assigned the small
black fruits to the comprehensive fungus genus _Sphaeria_ or classified
lichens bearing spermogonia only as distinct genera and species
(_Pyrenothea_ and _Thrombium_). Later students of lichens—Schaerer[706],
Flotow[707], and others—accepted Wallroth’s interpretation of their
relation to the thallus, or they ignored them altogether in their
descriptions of species.
B. SPERMOGONIA AS MALE ORGANS
Interest in these minute “tubercles” and their enclosed “corpuscles” was
revived by Itzigsohn[708] who examined them with an improved microscope.
He macerated in water during a few days that part of the thallus on which
they were developed, and, at the end of the time, discovered that the
solution contained large numbers of motile bodies which he naturally
took to be the corpuscles from the broken down tubercles. He claimed to
have established their function as male motile cells or spermatozoa.
The discovery seemed not only to prove their sexual nature, but to link
up the reproduction of lichens with that of the higher cryptogams. The
tubercles in which the “spermatozoa” were produced he designated as
antheridia. More prolonged maceration of the tissue to the very verge
of decay yielded still larger numbers of the “spermatozoa” which we now
recognize to have been motile bacilli.
Tulasne[709] next took up the subject, and failing to find the motile
cells, he wrongly insisted that Itzigsohn had been misled by mere
Brownian movement, but at the same time he accepted the theory that
the minute conceptacles were spermogonia or male organs of lichens.
He also pointed out that their constant occurrence on the thallus of
practically every species of lichen, and their definite form, though
with considerable variation, rendered it impossible to regard them as
accidental or of no importance to the life of the plant. He compared them
with fungal pycnidia such as _Phyllosticta_ or _Septoria_ which outwardly
they resembled, but whereas the pycnidial spores germinated freely,
the spermatia of the spermogonia, as far as his experience went, were
incapable of germination.
C. OCCURRENCE AND DISTRIBUTION
_a._ RELATION TO THALLUS AND APOTHECIA. We owe to Tulasne[710] the first
comparative study of lichen spermogonia. He described not only their
outward form, but their minute structure, in a considerable number of
representative species. A few years later Lindsay[711] published a
memoir dealing with the spermogonia of the larger foliose and fruticose
lichens, and, in a second paper, he embodied the results of his study of
an equally extensive selection of crustaceous species. Lindsay’s work is
unfortunately somewhat damaged by faulty determination of the lichens he
examined, and by lack of the necessary discrimination between one thallus
and another of associated and intermingled species. Both memoirs contain,
however, much valuable information as to the forms of spermogonia, with
their spermatiophores and spermatia, and as to their distribution over
the lichen thallus.
Though spermogonia are mostly found associated with apothecia, yet in
some lichens, such as _Cerania_ (_Thamnolia_) _vermicularis_, they are
the only sporiferous organs known. Not unfrequently crustaceous thalli
bear spermogonia only, and in some _Cladoniae_, more especially in
ascyphous species, spermogonia are produced abundantly at the tips of
the podetial branches (Fig. 109), while apothecia are exceedingly rare.
Usually they occur in scattered or crowded groups, more rarely they are
solitary. Very often they are developed and the contents dispersed before
the apothecia reach the surface of the thallus; hence the difficulty
in relating these organisms, since the mature apothecium is mostly of
extreme importance in determining the species.
[Illustration: Fig. 109. _Cladonia furcata_ Schrad. Branched podetium
with spermogonia at the tips (after Krabbe).]
[Illustration: Fig. 110. _Physcia hispida_ Tuckerm. Ciliate frond. _a_,
spermogonia; _b_, apothecia. × ca. 5 (after Lindsay).]
In a very large number of lichens, both crustaceous and foliose, the
spermogonia are scattered over the entire thallus (Fig. 110), covering it
more or less thickly with minute black dots, as in _Parmelia conspersa_.
In other instances, they are to some extent confined to the peripheral
areas as in _Parmelia physodes_; or they occur on the extreme edge of the
thallus as in the crustaceous species _Lecanora glaucoma_ (_sordida_). In
_Pyrenula nitida_ they grow on the marginal hypothallus, usually on the
dark line of demarcation between two thalli.
They tend to congregate on, and indeed are practically restricted to
the better lighted portions of the thallus. On the fronds of foliose
forms, they appear, for instance, on the swollen pustules of _Umbilicaria
pustulata_, while in _Lobaria pulmonaria_, they are mostly lodged in
the ridges that surround the depressions in the thallus. In _Parmelia
conspersa_, _Urceolaria_ (_Diploschistes_) _scruposa_ and some others,
they occasionally invade the margins of the apothecium or even the
apothecial disc as in _Lichina_. Forssell[712] found that a spermogonium
had developed among cells of _Gloeocapsa_ that covered the disc of a
spent apothecium of _Pyrenopsis haematopis_.
In fruticose lichens such as _Usnea_, _Ramalina_, etc. they occur near
the apex of the fronds, and in _Cladonia_ they occupy the tips of the
ascyphous podetia or the margins of the scyphi. In some _Cladoniae_,
however, spermogonia are produced on the basal squamules, more rarely on
the squamules that clothe the podetia.
_b._ FORM AND SIZE. Spermogonia are specifically constant in form, the
same type being found on the same lichen species all over the globe. The
larger number are entirely immersed and are ovoid or roundish (Fig. 111 A)
or occasionally somewhat flattened bodies (_Nephromium laevigatum_), or
again, but more rarely, they are irregular in outline with an infolding
of the walls that gives the interior a chambered form (Fig. 111 B)
(_Lichina pygmaea_); but all of these are only visible as minute points
on the thallus.
[Illustration: Fig. 111. Immersed spermogonia. A, globose in _Parmelia
acetabulum_ Dub. × 600; B, with infolded walls in _Lecidea_ (_Psora_)
_testacea_ Ach. × 144 (after Glück).]
A second series, also immersed, are borne in small protuberances of the
thallus. These very prominent forms are rarely found in crustaceous
lichens, but they are characteristic of such well-known species as
_Ramalina fraxinea_, _Xanthoria parietina_, _Ricasolia amplissima_,
_Baeomyces roseus_, etc. Other spermogonia project slightly above the
level of the thallus, as in _Cladonia papillaria_ and _Lecidea lurida_;
while in a few instances they are practically free, these last strikingly
exemplified in _Cetraria islandica_ where they occupy the small
projections or cilia (Fig. 112) that fringe the margins of the lobes;
they are free also in most species of _Cladonia_.
In size they vary from such minute bodies as those in _Parmelia
exasperata_ which measure 25-35 µ in diam., up to nearly 1 mm. in Lobaria
_laetevirens_. As a rule, they range from about 150 µ to 400 µ across
the widest part, and are generally rather longer than broad. They open
above by a small slit or pore called the ostiole about 20 µ to 100 µ wide
which is frequently dark in colour. In one instance, in _Icmadophila
aeruginosa_, Nienburg[713] has described a spermogonium with a wide
opening, the spermatiophores being massed in palisade formation along the
bottom of a cup-like structure.
_c._ COLOUR OF SPERMOGONIA. Though usually the ostiole is visible as a
darker point than the surrounding tissue, spermogonia are often difficult
to locate unless the thallus is first wetted, when they become visible to
slight magnification. They appear as black points in many _Parmeliae_,
_Physciae_, _Roccellae_, etc., though even in these cases they are often
brown when moistened. They are distinctly brown in some _Cladoniae_, in
_Nephromium_, and in some _Physciae_; orange-red or yellow in _Placodium_
and concolorous with the thallus in _Usnea_, _Ramalina_, _Stereocaulon_,
etc.
[Illustration: Fig. 112. Free spermogonia in spinous cilia of _Cetraria
islandica_ Ach. A, part of frond; B, cilia. × 10.]
D. STRUCTURE
_a._ ORIGIN AND GROWTH. The spermogonia (or pycnidia) of lichens when
mature are more or less hollow structures provided with a distinct wall
or “perithecium,” sometimes only one cell thick and then not easily
demonstrable, as in _Physcia speciosa_, _Opegrapha vulgata_, _Pyrenula
nitida_, etc. More generally the “perithecium” is composed of a layer of
several cells with stoutish walls which are sometimes colourless, but
usually some shade of yellow to dark-brown, with a darker ostiole. The
latter, a small slit or pore, arises by the breaking down of some of the
cells at the apex. After the expulsion of the spermatia, a new tissue is
formed which completely blocks up the empty spermogonium. In filamentous
lichens such as _Usnea_ a dangerous local weakening of the thallus is
thus avoided.
Spermogonia originate from hyphae in or near the gonidial zone. The
earliest stages have not been seen, but Möller[714] noted as the first
recognizable appearance or primordium of the “pycnidia” in cultures of
_Calicium trachelinum_ a ball or coil of delicate yellowish-coloured
hyphae. At a more advanced stage the sporophores (or spermatiophores)
could be traced as outgrowths from the peripheral hyphae, directed in
palisade formation towards the centre of the hyphal coil about 20-30 µ
long and very slender and colourless. They begin to bud off spermatia
almost immediately, as it has been found that these are present in
abundance while the developing spermogonium is still wholly immersed in
the thallus. Meanwhile there is gradually formed on the outside a layer
of plectenchyma which forms the wall. Additional spermatiophores arise
from the wall tissue and push their way inwards between the ranks of the
first formed series. The spermogonium slowly enlarges and stretches and
as the spermatiophores do not grow any longer a central hollow arises
which becomes packed with spermatia (or spores) before the ostiole is
open.
A somewhat similar process of development is described by Sturgis[715]
in the spermogonia of _Ricasolia amplissima_, in which species the
primordium arises by a profuse branching of the medullary hyphae in
certain areas close to the gonidial zone. The cells of these branching
hyphae are filled with oily matter and gradually they build up a dense,
somewhat cylindrical body which narrows above to a neck-like form. The
growth is upwards through the gonidial layer, and the structure widens to
a more spherical outline. It finally reaches the outer cortex when some
of the apical cell membranes are absorbed and a minute pore is formed.
The central part becomes hollow, also by absorption, and the space thus
left is lined and almost filled with multicellular branches of the hyphae
forming the wall; from the cells of this new tissue the spermatia are
abstricted.
_b._ FORMS AND TYPES OF SPERMATIOPHORES. The variations in form of
the fertile hyphae in the spermogonium were first pointed out by
Nylander[716] who described them as sterigmata[717]. He considered the
differences in branching, etc. as of high diagnostic value, dividing
them into two groups: simple “sterigmata” (or spermatiophores), with
non-septate hyphae, and arthrosterigmata, with jointed or septate hyphae.
Simple “sterigmata” comprise those in which the spore or spermatium is
borne at the end of a secondary branch or sterigma, the latter having
arisen from a cell of the upright spermatiophore or from a simple basal
cell. The arthrosterigmata consist of “short cells almost as broad as
they are long, much pressed together, and appearing almost agglutinate
especially toward the base; they fill almost the whole cavity of the
spermogonium.” The arthrosterigmata may grow out into the centre of the
cavity as a single cell-row, as a loose branching network, or, as in
_Endocarpon_, they may form a tissue filling the whole interior. Each
cell of this tissue that borders on a cavity may bud off a spermatium
either directly or from the end of a short process.
[Illustration: Fig. 113 A. Types of lichen “sporophores” and
pycnidiospores. 1, _Peltigera rufescens_ Hoffm. × 910; 2,_Lecidea (Psora)
testacea_ Ach. × 1200; 3,_Cladonia cariosa_ Spreng. × 1000; 4, _Pyrenula
nitida_ Ach. × 1130; 5, _Parmelia tristis_ Nyl. × 700; 6, _Lobaria
pulmonaria_ Hoffm. × 1000 (after Glück).]
The most important contributions on the subject of spermogonia in recent
years are those of Glück[718] and Steiner[719]. Glück, who insisted on
the “pycnidial” non-sexual character of the organs, recognized eight
types of “sporophores” differing in the complexity of their branching or
in the form of the “spores” (Fig. 113 A):
1. The _Peltigera_ type: the sporophores consist of a basal cell bearing
one or more long sterigmata and rather stoutish ellipsoid spores. (These
are true pycnidia.)
2. The _Psora_ type: a more elongate simple sporophore with sterigmata
and oblong spores.
3. The _Cladonia_ type: a branching sporophore, each branch with
sterigmata and oblong spores.
4. The _Squamaria_ type (called by Glück _Placodium_): also a branching
sporophore but with long sickle-like bent spores.
5. The _Parmelia_ type: a more complicated system of branching and
anastomosing of the sporophores, with oblong spores.
6, and 7. The _Sticta_ and _Physcia_ types: in both of these the
sporophores are multi-septate; they consist of a series of radiately
arranged hyphae rising from a basal tissue all round the pycnidium. They
anastomose to form a network and bud off “spermatia” from the free cells
or rather from minute sterigmata. In the _Physcia_ type there is more
general anastomosis of the sporophores and frequently masses of sterile
cells along with the fertile members occupy the centre of the pycnidium.
The spermatia of these and the following _Endocarpon_ type are short
cylindrical bodies (Fig. 113 B).
[Illustration: Fig. 113 B. 7, _Physcia ciliaris_ DC. × 600; 8,
_Endocarpon_ sp. × 600 (after Glück).]
8. _Endocarpon_ type: the pycnidium is filled by a tissue of short broad
cells, with irregular hollow spaces lined by fertile cells similar to
those of the _Sticta_ and _Physcia_ types.
The three last named types of sporophores represent Nylander’s section
of arthrosterigmata. Steiner has followed Nylander in also arranging the
various forms into two leading groups. The first, characterized by the
secondary branch or “sterigma,” he designates “exobasidial”; the second,
comprising the three last types in which the spores are borne directly on
the cells of the sporophore or on very short processes, he describes as
“endobasidial.” Steiner also introduces a new term, _fulcrum_, for the
sporophore.
The pycnidia in which these different sporophores occur are not, as a
rule, characteristic of one family. _Peltigera_ type is found only in
one family and the Cladonia type is fairly constant in _Cladoniae_,
but “_Psora_” pycnidia are found on very varying lichens among the
Lecideaceae, Verrucariaceae and others. The _Squamaria_ type with long
bent spores is found not only in _Squamaria_ (Glück’s _Placodium_) but
also in _Lecidea_, _Roccella_, _Pyrenula_, etc. _Parmelia_ type is
characteristic of many _Parmeliae_ and also of species of _Evernia_,
_Alectoria_, _Platysma_ and _Cetraria_. The _Sticta_ type occurs in
_Gyrophora_, _Umbilicaria_, _Nephromium_ and _Lecanora_ as well as in
_Sticta_ and in one species at least of _Collema_. To the _Physcia_ type
belong the pycnidia of most _Physciaceae_ and of various _Parmeliae_, and
to the closely related _Endocarpon_ type the pycnidia of _Endocarpon_ and
of _Xanthoria parietina_.
[Illustration: Fig. 114. Sterile filaments in spermogonia of _Lecidea
fuscoatra_ Ach. much magnified (after Lindsay).]
_c._ PERIPHYSES AND STERILE FILAMENTS. In a few species, _Roccella
tinctoria_, _Pertusaria globulifera_, etc., short one-celled sterile
hyphae are formed within the spermogonium near the ostiole, towards
which they converge. They correspond to the periphyses in the perithecia
of some Pyrenolichens, Verrucaria, etc. (described by Gibelli[720] as
spermatiophores); they are also present in some of the Pyrenomycetes
(_Sordaria_, etc.), and in many cases replace the paraphyses in function
when these have broken down. Sterile hyphae also occur, towards the
base, mingled with the fertile spermatiophores (Fig. 114). These latter
were first described and figured by Tulasne[721] in the spermogonia
of _Ramalina fraxinea_ as stoutish branching filaments, rising from
the same base as the spermatiophores but much longer, and frequently
anastomosing with each other. They have been noted also in _Usnea
barbata_ and in several species of _Parmelia_, and have been compared by
Nylander[722] to paraphyses. They are usually colourless, but, in the
_Parmeliae_, are often brownish and thus easily distinguished from the
spermatiophores. It has been stated that these filaments are sometimes
fertile. Similar sterile hyphae have been recorded in the pycnidia of
fungi, in _Sporocladus_ (_Hendersonia_) _lichenicola_ (Sphaeropsideae) by
Corda[723] who described them as paraphyses, and also in _Steganosporium
cellulosum_ (Melanconieae). These observations have been confirmed by
Allescher[724] in his recent work on _Fungi Imperfecti_. Keiszler[725]
has described a _Phoma_-like, pycnidium parasitic on the leprose thallus
of _Haematomma elatinum_. It contains short slender sporophores and,
mixed with these, long branched sterile hyphae which reach to the ostiole
and evidently function as paraphyses, though Keiszler suggests that they
may be a second form of sporophore that has become sterile. On account of
their presence he placed the fungus in a new genus _Lichenophoma_.
E. SPERMATIA OR PYCNIDIOSPORES
_a._ ORIGIN AND FORM OF SPERMATIA. Lichen spermatia arise at the tips of
the sterigmata either through simple abstriction or by budding. In the
former case—as in the _Squamaria_ type—a delicate cross-wall is formed
by which the spermatium is separated off. When they arise by budding,
there is first a small clavate sac-like swelling of the end of the short
process or sterigma which gradually grows out into a spermatium on a very
narrow base. This latter formation occurs in the _Sticta_, _Physcia_ and
_Endocarpon_ types.
Nylander[726] has distinguished the following forms of spermatia:
1. Ob-clavate, the broad end attached to the sterigma as in _Usneae_,
_Cetraria glauca_ and _C. juniperina_.
2. Acicular and minute but slightly swollen at each end, somewhat
dumb-bell like, in _Cetraria nivalis_, _C. cucullata_, _Alectoria_,
_Evernia_ and some _Parmeliae_, frequently borne on “arthrosterigmata.”
3. Acicular, cylindrical and straight, the most common form; these
occur in most of the _Lecanorae_, _Cladoniae_, _Lecideae_, Graphideae,
Pyrenocarpeae and occasionally they are budded off from arthrosterigmata.
4. Acicular, cylindrical, bent; sometimes these are very long, measuring
up to 40 µ; they are found in various _Lecideae_, _Lecanorae_,
Graphideae, Pyrenocarpeae, and also in _Roccella_, _Pilophorus_ and
species of _Stereocaulon_.
5. Ellipsoid or oblong and generally very minute; they are borne on
simple sterigmata and are characteristic of the genera _Calicium_,
_Chaenotheca_, _Lichina_, _Ephebe_, of the small genus _Glypholecia_ and
of a few species of _Lecanora_ and _Lecidea_.
In many instances there is more or less variation of form and of size in
the species or even in the individual. There are no spherical spermatia.
_b._ SIZE AND STRUCTURE. The shortest spermatia in any of our British
lichens are those of _Lichina pygmaea_ which are about 1·4 µ in length
and the longest are those of _Lecanora crassa_ which measure up to 39
µ. In width they vary from about 0·5 µ to 2 µ. The mature spermogonium
is filled with spermatia and, generally, with a mass of mucilage that
swells with moisture and secures their expulsion.
The spermatia of lichens are colourless and are provided with a
cell-wall and a nucleus. The presence of a nucleus was demonstrated by
Möller[727] in the spermatia of _Calicium parietinum_, _Opegrapha atra_,
_Collema microphyllum_, _C. pulposum_ and _C. Hildenbrandii_, and by
Istvanffi[728] in those of _Buellia punctiformis_ (_B. myriocarpa_),
_Opegrapha subsiderella_, _Collema Hildenbrandii_, _Calicium
trachelinum_, _Pertusaria communis_ and _Arthonia communis_ (_A.
astroidea_). Istvanffi made use of fresh material, fixing the spermatia
with osmic acid, and in all of these very minute bodies he demonstrated
the presence of a nucleus which stained readily with haematoxylin and
which he has figured in the spermatia of _Buellia punctiformis_ as
an extremely small dot-like structure in the centre of the cell. On
germination, as in the cell-multiplication of other plants, the nucleus
leads the way. Germination is preceded by nuclear division, and each new
hyphal cell of the growing mycelium receives a nucleus.
_c._ GERMINATION OF SPERMATIA (pycnidiospores). The strongest argument
in favour of regarding the spermatia of lichens as male cells had always
been the impossibility of inducing their germination. That difficulty
had at length been overcome by Möller[727] who cultivated them in
artificial solutions, and by that means obtained germination in nine
different lichen species. He therefore rejected the commonly employed
terms spermatia and spermogonia and substituted pycnoconidium and
pycnidia. Pycnidiospore has been however preferred as more in accordance
with modern fungal terminology. His first experiment was with the
“spermatia” of _Buellia punctiformis_ (_B. myriocarpa_) which measure
about 8-10 µ in length and about 3 µ in width, and are borne directly
on the septate spermatiophores (arthrosterigmata). In a culture drop,
the spore had swelled to about double its size by the second or third
day, and germination had taken place at both ends, the membrane of the
spore being continuous with that of the germinating tube. In a short time
cross septa were formed in the hyphae which at first were very close
to each other. While apical growth advanced these first formed cells
increased in width to twice the original size and, in consequence, became
slightly constricted at the septa. In fourteen days a circular patch of
mycelium had been formed about 280 µ in diameter. The development exactly
resembled that obtained from the ascospores of the same species grown in
the absence of gonidia. The largest thallus obtained in either case was
about 2 mm. in diameter after three months’ growth. The older hyphae had
a tendency to become brownish in colour; those at the periphery remained
colourless. In _Opegrapha subsiderella_ the development, though equally
successful, was very much slower. The pycnidiospores (or spermatia) have
the form of minute bent rods measuring 5·7 µ × 1·5 µ. Each end of the
spore produced slender hyphae about the fifth or sixth day after sowing.
In four weeks, the whole length of the filament with the spore in the
middle was 300 µ. In four months a patch of mycelium was formed 2 mm.
in diameter. Growth was even more sluggish with the pycnidiospores of
_Opegrapha atra_. In that species they are rod-shaped and 5-6 µ long.
Germination took place on the fifth or sixth day and in fourteen days a
germination tube was produced about five times the length of the spore.
In four weeks the first branching was noticed and was followed by a
second branching in the seventh week. In three months the mycelial growth
measured 200-300 µ across.
Germination was also observed in a species of _Arthonia_, the spores of
which had begun to grow while still in the pycnidium. The most complete
results were obtained in species of _Calicium_: in _C. parietinum_ the
spores, which are ovoid, slightly bent, and brownish in colour, swelled
to an almost globose shape and then germinated by a minute point at the
junction of spore and sterigma, and also at the opposite end; very rarely
a third germinating tube was formed. Growth was fairly rapid, so that
in four weeks there was a loose felt of mycelium measuring about 2 cm.
× 1 cm. and 1 mm. in depth. Parallel cultures were carried out with the
ascospores and the results in both cases were the same; in five or six
weeks small black points appeared, which gradually developed to pycnidia
with mature pycnidiospores from which further cultures were obtained.
On _C. trachelinum_, which has a thin greyish-white thallus spreading
over old trunks of trees, the pycnidia are usually abundant. Lindsay had
noted two different kinds and his observation was confirmed by Möller.
The spores in one pycnidium are ovoid, measuring 2·5-3 µ × 1·5-2 µ;
in the other rarer form, they are rod-shaped and 5-7 µ long. In the
artificial cultures they both swelled, the rod-like spores to double
their width before germination, and sometimes several tubes were put
forth. Growth was slow, but of exactly the same kind from these two types
of spores as from the ascospores. At the end of the second month pycnidia
appeared on all the cultures, in each case producing the ovoid type of
spore.
In a second paper Möller[729] recorded the partially successful
germination of the “spermatia” of _Collema_ (_Leptogium_) _microphyllum_,
the species in which Stahl had demonstrated sexual reproduction. Growth
was extraordinarily slow: after a month in the culture solution the first
swelling of the spermatium prior to germination took place, and some time
later small processes were formed in two or three directions. In the
fourth month a branched filament was formed.
Möller’s experiments with ascospores and pycnidiospores were primarily
undertaken to prove that the lichen hyphae were purely fungal and
parasitic on the algae. A series of cultures were made by Hedlund[730] in
order to demonstrate that the pycnidiospores were asexual reproductive
bodies; they were grown in association with the lichen alga and their
germination was followed up to the subsequent formation of a lichen
thallus.
_d._ VARIATION IN PYCNIDIA. On the thallus of _Catillaria denigrata_
(_Biatorina synothea_) Hedlund found that there were constantly present
two types of pycnidia: the one with short slightly bent spores 4-8
µ × 1·5 µ, the other with much longer bent spores 10-20 µ × 1·5 µ;
there were numerous transition forms between the two kinds of spores.
Germination took place by the prolongation of the spore; the hypha
produced became septate and branches were soon formed. Hedlund found that
frequently germination had already begun in the spores expelled from the
spermogonium. In newly formed thalline areolae it was possible to trace
back the mycelium to innumerable germinating spores of both types, long
and short.
Lindsay had recorded more than one form of spermogonium on the same
lichen thallus, the spermatia varying considerably in size; but he was
most probably dealing with the mixed growth of more than one species. The
observations of Möller and Hedlund on this point are more exact, but the
limits of variation would very well include the two forms found by Möller
in _Calicium trachelinum_; and in the different pycnidia of _Catillaria
denigrata_ Hedlund not only observed transition stages between the two
kinds of spores, but the longer pycnidiospores, as he himself allows,
indicated the elongation prior to germination: there is no good evidence
of more than one form in any species.
F. PYCNIDIA WITH MACROSPORES
Tulasne[731] records the presence on the lichen thallus of “pycnidia”
as well as of “spermogonia”; the former producing stylospores, larger
bodies than spermatia, occasionally septate and containing oil-drops or
guttulae. These spores are pyriform or ovoid in shape and are always
borne at the tips of simple sporophores. He compared the pycnidia with
the fungus genera _Cytospora_, _Septoria_, etc. As a rule they occur on
lichens with a poorly developed thallus, on some species of _Lecanora_,
_Lecanactis_, _Calicium_, _Porina_, in the family Strigulaceae and in
_Peltigera_.
There is no morphological difference between pycnidia and spermogonia
except that the spermatia of the latter are narrower; but the difference
is so slight that, as Steiner has pointed out, these organs found on
_Lecanora piniperda_, _L. Sambuci_ and _L. effusa_ have been described at
one time as containing microconidia (spermatia), at another macroconidia
(stylospores). He also regards as macrospores those of the pycnidia
of _Calicium trachelinum_ which Möller was able to germinate so
successfully, and all the more so as they were brownish in colour, true
microspores or spermatia being colourless.
Müller[732] has recorded some observations on the pycnidia and
stylospores of the Strigulaceae, a family of tropical lichens inhabiting
the leaves of the higher plants. On the thallus of _Strigula elegans_
var. _tremula_ from Madagascar and from India, he found pycnidia with
stylospores of abnormal dimensions measuring 18-26 µ in length and 3 µ
in width, and with 1 to 7 cross septa. In _Strigula complanata_ var.
_genuina_ the stylospores were 2-8-septate and varied from 7-65 µ, in
length, some of the spores being thus ten times longer than others, while
the width remained the same. Müller considers that in these cases the
stylospore has already grown to a septate hypha while in the pycnidium.
As in the pycnidiospores, described later by Hedlund, the spores had
germinated by increase in length followed by septation.
The spermogonia of _Strigula_, which are exactly similar to the pycnidia
in size and structure, produce spermatia, measuring about 3 µ × 2 µ,
and it is suggested by Müller that the stylospores may represent merely
an advanced stage of development of these spermatia. Both organs were
constantly associated on the same thallus; but whereas the spermogonia
were abundant on the younger part of the thallus at the periphery, they
were almost entirely replaced by pycnidia on the older portions near the
centre, only a very few spermogonia (presumably younger pycnidial stages)
being found in that region.
Lindsay[733] has described a great many different lichen pycnidia,
but in many instances he must have been dealing with species of the
“Fungi imperfecti” that were growing in association with the scattered
granules of crustaceous lichens. There are many fungi—Discomycetes
and Pyrenomycetes—parasitic on lichen thalli, and he has, in some
cases, undoubtedly been describing their secondary pycnidial form
of fruit, which indeed may appear far more frequently than the more
perfect ascigerous form, and might easily be mistaken for the pycnidial
fructification of the lichen.
G. GENERAL SURVEY
_a._ SEXUAL OR ASEXUAL. It has been necessary to give the preceding
detailed account of these various structures—pycnidia or spermogonia—in
view of the extreme importance attached to them as the possible male
organs of the lichen plant, and, in giving the results obtained by
different workers, the terminology employed by each one has been adopted
as far as possible: those who consider them to be sexual structures call
them spermogonia; those who refuse to accept that view write of them as
pycnidia.
Tulasne, Nylander and others unhesitatingly accepted them as male
organs without any knowledge of the female cell or of any method of
fertilization. Stahl’s discovery of the trichogyne seemed to settle the
whole question; but though he had evidence of copulation between the
spermatium and the receptive cell or trichogyne he had no real record of
any sexual process.
Many modern lichenologists reject the view that they are sexual; they
regard them as secondary organs of fructification analogous to the
pycnidia so abundant in the related groups of fungi. One would naturally
expect these pycnidia to reappear in lichens, and it might be considered
somewhat arbitrary to classify pycnidia in Sphaeropsideae as asexual
reproductive organs, and then to regard the very similar structures in
lichens as sexual spermogonia. It has also been pointed out that when
undoubted pycnidia do occur on the lichen thallus, as in _Calicium_,
_Strigula_, _Peltigera_, etc., they in no way differ from structures
regarded as spermogonia except in the size of the pycnidiospores—and,
even among these, there are transition forms. The different types of
spermatia can be paralleled among the fungal pycnidiospores and the same
is also true as regards the sporophores generally. Those described as
arthrosterigmata by Nylander—as endosporous by Steiner—were supposed
to be peculiar to lichens; but recently Laubert[734] has described a
fungal pycnidium which grew on the trunk of an apple tree and in which
the spores are not borne on upright sporophores but are budded off from
the cells of the plectenchyma lining the pycnidium. It may be that
future research will discover other such instances, though that type of
sporophore is evidently of very rare occurrence among fungi.
_b._ COMPARISON WITH FUNGI. The most obvious spermogonia among fungi
with which to compare those of lichens occur in the Uredineae where they
are associated with the life-cycle of a large number of rust species.
They are small flask-shaped structures very much like the simpler forms
of pycnidia and they produce innumerable spermatia which are budded off
from the tips of simple spermatiophores. The mature spermatium has a
delicate cell-wall and contains a thin layer of cytoplasm with a dense
nucleus which occupies almost the whole cavity, cytological characters
which, as Blackman[735] has pointed out, are characteristic of male
cells and are not found in any asexual reproductive spores. If we accept
Istvanffi’s[736] description and figures of the lichen spermatia as
correct, their structure is wholly different: there being a very small
nucleus in the centre of the cell comparable in size with those of the
vegetative hyphae (Fig. 115).
[Illustration: Fig. 115. _a_, spermatia; _b_, hypha produced from
spermatium of _Buellia punctiformis_ Th. Fr. × 950 (after Istvanffi).]
Lichen “spermatia” also differ very strikingly from the male cells of any
given group of plants in their very great diversity of form and size;
but the chief argument adduced by the opponents of the sexual theory
is the capacity of germination that has been proved to exist in a fair
number of species. It is true that germination has been induced in the
spermatia of the Uredines by several research workers—by Plowright[737],
Sappin-Trouffy[738] and by Brefeld[739]—who employed artificial nutritive
solutions (sugar or honey), but the results obtained were not much
more than the budding process of yeast cells. Brefeld also succeeded
in germinating the “spermatia” of a pyrenomycetous fungus, _Polystigma
rubrum_, one of the germinating tubes reaching a length four times that
of the spore; but it is now known that all of these fungal spermatia are
non-functional, either sexually or asexually, and degenerate soon after
their expulsion, or even while still in the spermogonium.
_c._ INFLUENCE OF SYMBIOSIS. In any consideration of lichens it is
constantly necessary to hark back to their origin as symbiotic organisms,
and to bear in mind the influence of the composite life on their
development. After germination from the spore, the lichen hypha is so
dependant on its association with the alga, that, in natural conditions,
though it persists without the gonidia for a time, it attains to only a
rather feeble growth of mycelial filaments. In nutritive cultures, as
Möller has proved, the absence of the alga is partly compensated by the
artificial food supply, and a scanty thalline growth is formed up to the
stage of pycnidial fruits. Not only in pycnidia but in all the fruiting
bodies of lichens, symbiosis has entailed a distinct retrogression in the
reproductive importance of the spores, as compared with fungi.
In Ascomycetes, the asci constitute the overwhelming bulk of the
hymenium; in most lichens, there are serried ranks of paraphyses with
comparatively few asci, and the spores are often imperfectly developed.
It would not therefore be surprising if the bodies claimed by Möller and
others as pycnidiospores had also lost even to a considerable extent
their reproductive capacity.
_d._ VALUE IN DIAGNOSIS. Lichen spermogonia have once and again been
found of value in deciding the affinity of related plants, and though
there are a number of lichens in which we have no record of their
occurrence, they are so constant in others, that they cannot be ignored
in any true estimation of species. Nylander laid undue stress on
spermogonial characters, considering them of almost higher diagnostic
value than the much more important ascosporous fruit. They are, after
all, subsidiary organs, and often—especially in crustaceous species—they
are absent, or their relation to the species under examination is
doubtful.