BIONOMICS
A. GROWTH AND DURATION
Lichens are perennial plants mostly of slow growth and of long
continuance; there can therefore only be approximate calculations either
as to their rate of increase in dimensions or as to their duration in
time. A series of somewhat disconnected observations have however been
made that bear directly on the question, and they are of considerable
interest.
Meyer[896] was among the first to be attracted by this aspect of lichen
life, and after long study he came to the conclusion that growth varied
in rapidity according to the prevailing conditions of the atmosphere and
the nature of the substratum; but that nearly all species were very slow
growers. He enumerates several,—_Lichen_ (_Xanthoria_) _parietinus_,
_L._ (_Parmelia_) _tiliaceus_, _L._ (_Rhizocarpon_) _geographicus_,
_L._ (_Haematomma_) _ventosus_, and _L._ (_Lecanora_) _saxicolus_,—all
species with a well-defined outline, which, after having attained some
considerable size, remained practically unchanged for six and a half
years, though, in some small specimens of foliose lichens, he noted,
during the same period, an increase of one-fourth to one-third of their
size in diameter. In one of the above crustaceous species, _L. ventosus_,
the specimen had not perceptibly enlarged in sixteen years, though during
that time the centre of the thallus had been broken up by weathering and
had again been regenerated.
Meyer also records the results of culture experiments made in the open,
possibly with soredia or with thalline scraps: he obtained a growth
of _Xanthoria parietina_ (on wrought iron kept well moistened), which
fruited in the second year, and in five years had attained a width of
5-6 lines (about 1 cm.); _Lecanora saxicola_ growing on a moist rock
facing south grew 4-7 lines in six and a half years, and bore very minute
apothecia.
Lindsay[897] quotes a statement that a specimen of _Lobaria pulmonaria_
had been observed to occupy the same area of a tree after the lapse
of half a century. Berkeley[898] records that a plant of _Rhizocarpon
geographicum_ remained in much the same condition of development during
a period of twenty-five years. The latter is a slow grower and, in
ordinary circumstances, it does not fruit till about fifteen years after
the thallus has begun to form. Weddell[899], also commenting on the long
continuance of lichens, says there are crustaceous species occupying on
the rock a space that might be covered by a five-franc piece, that have
taken a century to attain that size.
Phillips[900] on the other hand argues against the very great age of
lichens, and suggests 20 years as a sufficient time for small plants
to establish themselves on hard rocks and attain full development. He
had observed a small vigorous plant of _Xanthoria parietina_ that in the
course of five years had extended outwards to double its original size.
The centre then began to break up and the whole plant finally disappeared.
Exact measurements of growth have been made by several observers.
Scott Elliot[901] found that a _Pertusaria_ had increased about half a
millimetre from the 1st February to the end of September. Vallot[902]
kept under observation at first three then five different plants of
_Parmelia saxatilis_ during a period of eight years: the yearly increase
of the thallus was half a centimetre, so that specimens of twenty
centimetres in breadth must have been growing from forty to fifty years.
Bitter’s[903] observations on _Parmelia physodes_ agree in the main with
those of Vallot: the increase of the upper lobes during the year was 3-4
mm. In a more favourable climate Heere found that _Parmelia caperata_
(Fig. 49) on a trunk of _Aesculus_ in California had grown longitudinally
1·5 cm. and transversely 1 cm. The measurements extended over a period
of seven winter months, five of them being wet and therefore the most
favourable season of growth. In warm regions lichens attain a much
greater size than in temperate or northern countries, and growth must be
more rapid.
A series of measurements was also made by Heere[904] on _Ramalina
reticulata_ (Fig. 64), a rapid growing tree-lichen, and one of
the largest American species. The shorter lobes were selected for
observation, and were tested during a period of seven months from
September to May, five of the months being in the wet season. There was
great variation between the different lobes but the average increase
during that period was 41 per cent.
Krabbe[905] took notes of the colonization of _Cladonia rangiferina_
(Fig. 127) on burnt soil: in ten years the podetia had reached a height
of 3 to 5 cm., giving an annual growth of about 3-5 mm. It is not unusual
to find specimens in northern latitudes 18 inches long (50 cm.), which,
on that computation, must have been 100 to 160 years old; but while
increase goes on at the apex of the podetia, there is constant perishing
at the base of at least as much as half the added length and these plants
would therefore be 200 or 300 years old. Reinke[906] indeed has declared
that apical growth in these _Cladina_ species may go on for centuries,
given the necessary conditions of good light and undisturbed habitat.
Other data as to rate of growth are furnished by Bonnier[907] in the
account of his synthetic cultures which developed apothecia only after
two to three years. The culture experiments of Darbishire[908] and
Tobler[909] with _Cladonia soredia_ are also instructive, the former with
synthetic spore- and alga-cultures having obtained a growth of soredia
in about seven months; the latter, starting with soredia, had a growth of
well-formed squamules in nine months.
It has been frequently observed that abundance of moisture facilitates
growth, and this is nowhere better exemplified than in crustaceous
soil-lichens. Meyer found that on lime-clay soil which had been thrown
up from a ditch in autumn, lichens such as _Gyalecta geoica_ were fully
developed the following summer. He gives an account also of another soil
species, _Verrucaria_ (_Thrombium_) _epigaea_, which attained maturity
during the winter half of the year. Stahl[910] tells us that _Thelidium
minutulum_, a pyrenocarpous soil-lichen, with a primitive and scanty
thallus, was cultivated by him from spore to spore in the space of
three months. Such lichens retain more of the characteristics of fungi
than do those with a better developed thallus. Rapid colonization by a
soil-lichen was also observed in Epping Forest by Paulson[911]. In autumn
an extensive growth of _Lecidea uliginosa_ covered as if with a dark
stain patches of soil that had been worn bare during the previous spring.
The lichen had reached full development and was well fruited.
These facts are quite in harmony with other observations on growth made
on Epping Forest lichens. The writers[912] of the report record the
finding of “fruiting lichens overspreading decaying leaves which can
scarcely have lain on the ground more than two or three years; others
growing on old boots or on dung and fruiting freely; others overspreading
growing mosses.” They also cite a definite instance of a mass of concrete
laid down in 1903 round a surface-water drain which in 1910—seven years
later—was covered with _Lecanora galactina_ in abundant fruit; and of
another case of a Portland stone garden-ornament, new in 1904, and, in
1910, covered with patches of a fruiting _Verrucaria_ (probably _V.
nigrescens_). Both these species, they add, have a scanty thallus and
generally fruit very freely.
A series of observations referring to growth and “ecesis” or the
spreading of lichens have been made by Bruce Fink[913] over a period of
eight years. His aim was mainly to determine the time required for a
lichen to re-establish itself on areas from which it had been previously
removed. Thus a quadrat of limestone was scraped bare of moss and of
_Leptogium lacerum_, except for bits of the moss and particles of the
lichen which adhered to the rock, especially in depressions of the
surface. After four years, the moss was colonizing many small areas on
which grew patches of the lichen 2 to 10 mm. across. Very little change
occurred during the next four years.
Numerous results are also recorded as to the rate of growth, the average
being 1 cm. per year or somewhat under. The greatest rate seems to have
been recorded for a plant of _Peltigera canina_ growing on “a mossy rock
along a brook in a low moist wood, well-shaded.” A plant, measuring 10 by
14 cm., was deprived of several large apothecia. The lobes all pointed
in the same direction, and the plant increased 1·75 cm. in one year.
Two other plants, deprived of their lobes, regenerated and increased
from 2 and 5 cm. respectively to 3·5 and 6 cm. No other measurements are
quite so high as these, though a plant of _Parmelia caperata_ (sterile),
measuring from 1 to 2 cm. across, reached in eight years a dimension of
10 by 13 cm. Other plants of the same species gave much slower rates
of increase. A section of railing was marked bearing minute scattered
squamules of _Cladonia pityrea_. After two years the squamules had
attained normal size and podetia were formed 2 to 4 mm. long.
Several areas of _Verrucaria muralis_ were marked and after ten months
were again measured; the largest plants, measuring 2·12 by 2·4 cm.
across, had somewhat altered in dimensions and gave the measurements 2·2
by 3 cm. Some crustose species became established and produced thalli and
apothecia in two to eight years. Foliose lichens increased in diameter
from 0·3 to 3·5 cm. per year. So far as external appearance goes,
apothecia are produced in one to eight years; it is concluded that they
require four to eight years to attain maturity in their natural habitats.
B. SEASON OF FRUIT FORMATION
The presence of apothecia (or perithecia) in lichens does not always
imply the presence of spores. In many instances they are barren, the
spores having been scattered or not yet matured; the disc in these cases
is composed of paraphyses only, with possible traces of asci. In any
month of the year, however, some lichens may be found in fruit.
Baur[914] found, for instance, that _Parmelia acetabulum_ developed
carpogonia the whole year round, though somewhat more abundantly in
spring and autumn. _Pertusaria communis_ similarly has a maximum period
of fruit-formation at these two seasons. This is probably true of
tree-lichens generally: in summer the shade of the foliage would inhibit
the formation of fruits, as would the extreme cold of winter; but were
these conditions relaxed spore-bearing fruits might be expected at any
season though perhaps not continuously on the same specimen.
An exception has been noted by Baur in _Pyrenula nitida_, a crustaceous
tree Pyrenolichen. He found carpogonia only in February and April, and
the perithecia matured in a few weeks, presumably at a date before the
trees were in full leaf; but even specimens of _Pyrenula_ are not unusual
in full spore-bearing conditions in the autumn of the year.
To arrive at any true knowledge as to the date and duration of spore
production, it would be necessary to keep under observation a series of
one species, examining them microscopically at intervals of a few weeks
or months and noting any conditions that might affect favourably or
unfavourably the reproductive organs. A comparison between corticolous
and saxicolous species would also be of great interest to determine the
influence of the substratum as well as of light and shade. But in any
case it is profitable to collect and examine lichens at all seasons of
the year, as even when the bulk of the spores is shed, there may remain
belated apothecia with a few asci still intact.
C. DISPERSAL AND INCREASE
The natural increase of lichen plants may primarily be sought for in
the dispersal of the spores produced in the fruiting-bodies. These are
ejected, as in fungi, by the pressure of the paraphyses on the mature
ascus. The spores are then carried away by wind, water, insects, etc.
In a few lichens gonidia are enclosed in the hymenium and are ejected
along with the spores, but, in most, the necessary encounter with the
alga is as fortuitous, and generally as certain, as the pollination
of anemophilous flowers. A case of dispersal in _Sagedia microspora_
has been described by Miyoshi[915] in which entire fruits, small round
perithecia, were dislodged and carried away by the wind. The addition
of water caused them to swell enormously and brought about the ejection
of the spores. Areas covered by the thallus are also being continually
enlarged by the spreading growth of the hypothallus.
_a._ DISPERSAL OF CRUSTACEOUS LICHENS. These lichens are distributed
fairly equally on trees or wood (corticolous) and on rocks (saxicolous).
Some species inhabit both substrata. As regards corticolous lichens that
live on smooth bark such as hazel or mountain-ash, the vegetative body
or thallus is generally embedded beneath the epidermis of the host.
Soredia are absent and the thallus is protected from dispersal. In these
lichens there is rather an abundant and constant formation of apothecia
or perithecia.
Other species that affect rugged bark and are more superficial are less
dependent on spore production. The thallus is either loosely granular, or
is broken up into areolae. The areolae are each a centre of growth, and
with an accession of moisture they swell up and exert pressure on each
other. Parts of the thallus thus become loosened and are dislodged and
carried away. If anchored on a suitable substratum they grow again to
a complete lichen plant. Sorediate lichens are dependent almost wholly
on these bud-like portions for increase in number; soredia are easily
separated from the parent plant, and easily scattered. Darbishire[916]
noted frequently that small _Poduridae_ in moving over the surface of
_Pertusaria amara_ became powdered with soredia and very evidently took a
considerable part in the dissemination of the species.
Crustaceous rock lichens are rarely sorediate, but they secure vegetative
propagation[917] by the dispersal of small portions of the thallus. The
thalli most securely attached are cracked into small areolae which, by
unequal growth, become very soon lop-sided, or, by intercalary increase,
form little warts and excrescences on their surface. These irregularities
of development give rise to more or less tension which induces a
loosening of the thallus from the substratum. Weather changes act
similarly and gradually the areolae are broken off. Loosening influence
is also exercised by the developing fruits, the expanding growth of which
pushes aside the neighbouring tissues. Wind or water then carries away
the thalline particles which become new centres of growth if a suitable
substratum is reached.
_b._ DISPERSAL OF FOLIOSE LICHENS. It is a matter of common observation
that, in foliose lichens where fruits are abundant, there are few or
no soredia and _vice versa_. In either case propagation is ensured. In
addition to these obvious methods of increase many lichens form isidia,
outgrowths from the thallus which are easily detached. Bitter[918]
considers for instance that the coralloid branchlets, which occur in
compact tufts on the thallus of _Umbilicaria pustulata_, are of immense
service as organs of propagation. Apothecia and pycnidia are rarely
present in that species, and the plant thus falls back on vegetative
production. Slender crisp thalline outgrowths, easily separable, occur
also on the edges of lobes, as in species of _Peltigera_, _Platysma_, etc.
Owing to the gelatinous character of lichen hyphae, the thallus quickly
becomes soft with moisture and is then easily torn and distributed by
wind, animals, etc. The action of lichens on rocks has been shown to be
of a constantly disintegrating character, and the destruction of the
supporting rock finally entails the scattering of the plant. This cause
of dispersal is common to both crustaceous and foliose species. The older
central parts of a lichen may thus have disappeared while the areolae on
lobes of the circumference are still intact and in full vigour.
As in crustaceous lichens the increase in the area of growth may take
place by means of the lichen mycelium which, originating from the
rhizinae in contact with the substratum, spreads as a hypothallus under
the shelter of the lobes and far beyond them. When algae are encountered
a new lobe begins to form. The process can be seen perhaps most
favourably in lichens on decaying wood which harbours moisture and thus
enables the wandering hyphae to retain life.
_c._ DISPERSAL OF FRUTICOSE LICHENS. Many of these lichens are abundantly
fruited; in others soralia are as constantly developed. Species of
_Usnea_, _Alectoria_, _Ramalina_ and many _Cladoniae_ are mainly
propagated by soredia. They are all peculiarly liable to be broken and
portions of the thallus scattered by the combined action of wind and rain.
Peirce[919] found that _Ramalina reticulata_ (Fig. 65), of which the
fronds are an open network, was mainly distributed by the tearing of the
lichen in high wind. This takes place during the winter rains, when not
only the lichen is wet and soft in texture, but when the deciduous trees
are bare of leaves, at a season, therefore, when the drifting thalline
scraps can again catch on to branch or stem. A series of observations on
the dispersal of forms of long pendulous Usneas was made by Schrenk[920].
In the Middle and North Atlantic States of America these filamentous
species rarely bear apothecia. The high winds break and disperse them
when they are in a wet condition. They generally grow on Spruces and
Firs, because the drifting filaments are more easily caught and entangled
on short needles. The successive wetting and drying causes them to coil
and uncoil, resulting in a tangle impossible to unravel, which holds them
securely anchored to the support.
D. ERRATIC LICHENS
In certain lichens, there is a tendency for the thallus to develop
excrescences of nodular form which easily become free and drift about
in the wind while still living and growing. They are carried sometimes
very long distances, and fall in thick deposits over localities far
from their place of origin. The most famous instance is the “manna
lichen,” _Lecanora esculenta_, which has been scientifically examined
and described by Elenkin[921]. He distinguishes seven different forms
of the species: f. _esculenta_, f. _affinis_, f. _alpina_, and f.
_fruticulosa-foliacea_ which are Alpine lichens, the remainder, f.
_desertoides_, f. _foliacea_ and f. _esculenta-tarquina_, grow on the
steppes or in the desert[922].
Elenkin[921] adds to the list of erratic lichens a variety of _Parmelia
molliuscula_ along with _P. ryssolea_ from S. Russia, from the Asiatic
steppes and from Alpine regions. Mereschkovsky[923] has also recorded
from the Crimea _Parmelia vagans_, probably derived from _P. conspersa_
f. _vaga_ (f. nov.). It drifts about in small rather flattened bits, and,
like other erratics, it never fruits.
Meyer[924] long ago described the development of wandering lichens:
scraps that were torn from the parent thallus continued to grow if there
were sufficient moisture, but at the same time undergoing considerable
change in appearance. The dark colour of the under surface disappears
in the frequently altered position, as the lobes grow out into narrow
intermingling fronds forming a more or less compact spherical mass;
the rhizoids also become modified and, if near the edge, grow out into
thread-like structures which bind the mass together. Meyer says that
“wanderers” have been noted as belonging to _Parmelia acetabulum_,
_Platysma glaucum_ and _Anaptychia ciliaris_.
[Illustration: Fig. 121. _Parmelia revoluta_ var. _concentrica_ Cromb.
_a_, plant on flint with detached fragment; _b_, upper surface of three
specimens; _c_, three specimens as found on chalk downs; _d_, specimens
in section showing central cavity (S. H., _Photo._).]
The most notable instance in Britain of the “erratic” habit is that of
_Parmelia revoluta_ var. _concentrica_ (Fig. 121), first found on Melbury
Hill near Shaftesbury, Dorset, and described as “a spherical unattached
lichen which rolls on the exposed downs.” It has recently been observed
on the downs near Seaford in Sussex, where, however, it seems to be
confined to a small area about eight acres in extent which is exposed to
south-west winds. The lichen is freely distributed over this locality.
To R. Paulson and Somerville Hastings[925] we owe an account of the
occurrence and origin of the _revoluta_ wanderers. The specimens vary
considerably in shape and size, and measure from 1 to 7 cm. in longest
diameter. Very few are truly spherical, some are more or less flattened
and many are quite irregular. The revolute edges of the overlapping
lobes give a rough exterior to the balls, which thereby become entangled
amongst the grass, etc., and movement is impeded or prevented, except
in very high winds. Crombie[926] had suggested that the concentric
plant originated from a corticolous habitat, but no trees are near the
Seaford locality. Eventually specimens were found growing on flints in
the immediate neighbourhood. While still on the stone the lichen tends
to become panniform, a felt of intermingling imbricate lobes is formed,
portions of which, in time, become crowded out and dislodged. When
scattered over the ground, these are liable to be trampled on by sheep or
other animals and so are broken up; each separate piece then forms the
nucleus of new concentric growth.
Crombie[926] observed at Braemar, drifting about on the detritus of
Morrone, an analogous structure in _Parmelia omphalodes_. He concluded
that nodular excrescences of the thallus had become detached from the
rocks on which the lichen grew; while still attached to the substratum
_Parmelia omphalodes_ and the allied species, _P. saxatilis_, form dense
cushion-like masses.
E. PARASITISM
_a._ GENERAL STATEMENT. The parasitism of _Strigula complanata_, an
exotic lichen found on the leaves of evergreen trees, has been already
described[927]; Dufrenoy[928] records an instance of hyphae from a
_Parmelia_ thallus piercing pine-needles through the stomata and causing
considerable injury. Lichen hyphae have attacked and destroyed the
protonemata of mosses. Cases have also been recorded of _Usnea_ and
_Ramalina_ penetrating to the living tissue of the tree on which they
grew, and there may be other similar parasitisms; but these exceptions
serve to emphasize the independent symbiotic growth of lichens.
There are however some lichens belonging to widely diverse genera that
have retained, or reverted to, the saprophytic or parasitic habit of
their fungal ancestors, though the cases that occur are generally of
lichens preying on other lichens. The conditions have been described as
those of “antagonistic symbiosis” when one lichen is hurtful or fatal
in its action on the other, and as “parasymbiosis” when the association
does little or no injury to the host. The parasitism of fungi on lichens,
though falling under a different category, in many instances exhibits
features akin to parasymbiosis.
The parasitism of fungus on fungus is not unusual; there are instances of
its occurrence in all the different classes. In the Phycomycetes there
are genera wholly parasitic on other fungi such as _Woronina_ and other
Chytridiaceae; _Piptocephalus_, one of the Mucorini, is another instance.
_Cicinnobolus_, one of the Sphaeropsideae, preys on _Perisporiae_;
a species of _Cordyceps_ is found on _Elaphomyces_, and _Orbilia
coccinella_ on _Polyporus_; while among Basidiomycetes, _Nyctalis_, an
agaric, grows always on _Russula_.
There are few instances of lichens finding a foothold on fungi, for the
simple reason that the latter are too short lived. On the perennial
_Polyporeae_ a few have been recorded by Arnold[929], but these are
not described as doing damage to the host. They are mostly species of
_Lecidea_ or of allied genera. Kupfer[930] has also listed some 15
different lichens that he found on _Lenzites_ sp.
_b._ ANTAGONISTIC SYMBIOSIS. In discussing the nutrition of lichens[931]
note has been taken of the extent to which some species by means of
enzymes destroy the thallus of other lichens in their vicinity and
then prey on the dead tissues. A constantly cited[932] example is that
of _Lecanora atriseda_ which in its early stages lives on the thallus
of _Rhizocarpon geographicum_ inhabiting mountain rocks. A detailed
examination of the relationship between these two plants was made by
Malme and later by Bitter[933]. Both writers found that the _Lecanora_
thallus as it advanced caused a blackening of the _Rhizocarpon_ areolae,
the tissues of which were killed by the burrowing slender filaments of
the _Lecanora_, easily recognized by their longer cells. The invader
thereafter gradually formed its own medulla, gonidial layer and cortex
right over the surface of the destroyed thallus. _Lecidea insularis_
(_L. intumescens_) similarly takes possession of and destroys the
thallus of Lecanora glaucoma and Malme[932] strongly suspects that
_Buellia verruculosa_ and _B. aethalea_ may be living on the thallus of
_Rhizocarpon distinctum_ with which they are constantly associated.
Other cases of facultative parasitism have been studied by Hofmann[934],
more especially three different species, _Lecanora dispersa_, _Lecanora_
sp. and _Parmelia hyperopta_, which were found growing on the thick
foliose thallus of _Dermatocarpon miniatum_. These grew, at first
independently, on a wall along with many examples of _Endocarpon_ on
to which they spread as opportunity offered. The thallus of the latter
was in all cases distorted, the area occupied by the invaders being
finally killed. The attacking lichens had benefited materially by the
more nutritive substratum: their apothecia were more abundant and their
thallus more luxuriant. The gonidia especially had profited; they were
larger, more brightly coloured, and they increased more freely. Hoffmann
offers the explanation that the strain on the algae of providing organic
food for the hyphal symbiont was relaxed for the time, hence their more
vigorous appearance.
_Arthonia subvarians_ is always parasitic on the apothecia of _Lecanora
galactina_, and Almquist[935] discovered that the hymenium of the host
alone is injured, the hypothecium and excipulum being left intact.
The “parasitism” of _Pertusaria globulifera_ on _Parmelia perlata_ and
_P. physodes_, as described by Bitter[936], may also be included under
antagonistic symbiosis. The hyphae pierce the _Parmelia_ thallus, break
it up and gradually absorb it. Chemical as well as mechanical influences
are concerned in the work of destruction as both the fungus and the alga
of the victim are dissolved. _Lecanora tartarea_ already dealt with as a
marauding lichen[937] over decaying vegetation may spread also to living
lichens. Fruticose soil species, such as _Cetraria aculeata_ and others,
die from the base and the _Lecanora_ gains entrance to their tissues at
the decaying end which is open.
Arnold[938] speaks of these facultative parasites that have merely
changed their substratum as pseudo-parasites, and he gives a list of
instances of such change. In many cases it is rather the older thalli
that are taken possession of, and, in nearly every case, the invader
is some crustaceous species. The plants attacked are generally ground
lichens or more particularly those that inhabit damp localities, such
as _Peltigera_ or _Cladonia_ or certain bark lichens. Drifting soredia
or particles of a lichen would easily take hold of the host thallus and
develop in suitable conditions. To give a few of the instances observed,
there have been found, by Arnold, Crombie and others:
on _Peltigera canina_: _Callopisma cerina_, _Rinodina turfacea_ var.,
_Bilimbia obscurata_ and _Lecanora aurella_;
on _Peltigera aphthosa_: _Lecidea decolorans_;
on _Cladoniae_: _Bilimbia microcarpa_, _Bacidia Beckhausii_ and
_Urceolaria scruposa_, etc.
_Urceolaria_ (_Diploschistes_) has a somewhat bulky crustaceous thallus
which may be almost evanescent in its semi-parasitic condition, the only
gonidia retained being in the margin of the apothecia. Nylander[939]
found isolated apothecia growing vigorously on _Cladonia_ squamules.
Hue[940] describes _Lecanora aspidophora_ f. _errabunda_, an Antarctic
lichen, as not only a wanderer but as a “shameless robber.” It is to be
seen everywhere on and about other lichens, settling small glomeruli of
apothecia here and there on the thallus of _Umbilicariae_ or between
the areolae of _Buelliae_, and always too vigorous to be ousted from its
position.
_Bacidia flavovirescens_ has been regarded by some lichenologists[941] as
a parasite on _Baeomyces_, but recent work by Tobler[942] seems to have
proved that the bright green thallus is that of the _Bacidia_.
_c._ PARASYMBIOSIS. There are certain lichens that are obligative
parasites and pass their whole existence on an alien thallus. They may
possibly have degenerated from the condition of facultative parasitism
as the universal history of parasitism is one of increased dependence
on the host, and of growing atrophy of the parasite, but, in the case
of lichens, there is always the peculiar symbiotic condition to be
considered: the parasite produces its own vigorous hyphae and normal
healthy fruits, it often claims only a share of the carbohydrates
manufactured by the gonidia. The host lichen is not destroyed by this
parasymbiosis though the tissues are very often excited to abnormal
growth by the presence of the invading organism.
Lauder Lindsay[943] was one of the first to study these “microlichens”
as he called them, and he published descriptions of those he had himself
observed on various hosts. He failed however to discriminate between
lichens and parasitic fungi. It is only by careful research in each
case that the affinity to fungi or to lichens can be determined; very
frequently the whole of them, as possessing no visible thallus, have been
classified with fungi, but that view ignores the symbiosis that exists
between the hyphae of the parasite and the gonidia of the host.
Parasitic lichens are rather rare on gelatinous thalli; but even among
these, a few instances have been recorded. Winter[944] has described a
species of _Leptoraphis_, the perithecia of which are immersed in the
thallus of _Physma franconicum_. The host is wholly unaffected by the
presence of the parasite except for a swelling where it is situated.
The foreign hyphae are easily distinguishable; they wander through the
thallus of the host with their free ends in the mucilage of the gonidial
groups from which they evidently extract nourishment. Species of the
lichen genus _Obryzum_ are also parasitic on gelatinous lichens.
The parasitic genus _Abrothallus_[945] has been the subject of frequent
study. There are a number of species which occur as little black discs
on various thalli of the large foliose lichens. They were first of all
described as parasitic fungi, later Tulasne[946] affirmed their lichenoid
nature as proved by the structure, consistence and long duration of the
apothecia. Lindsay[947] wrote a monograph of the genus dealing chiefly
with _Abrothallus Smithii_ (_Buellia Parmeliarum_) and _A. oxysporus_,
with their varieties and forms that occur on several different hosts.
In some instances the thallus is apparently quite unaffected by the
presence of _Abrothallus_, in others, as in _Cetraria glauca_, there
is considerable hypertrophy produced, the portion of the thallus on
which the parasites are situated showing abnormal growth in the form
of swellings or pustules which may be regarded as gall-formations.
Crombie[948] points this out in a note on _C. glauca_ var. _ampullacea_,
figured first by Dillenius, which is merely a swollen condition due to
the presence of _Abrothallus_.
The internal structure and behaviour of _Abrothallus_ has more
recently been followed in detail by Kotte[949]. He recognized a number
of different species growing on various thalli of _Parmelia_ and
_Cetraria_, but _Abrothallus Cetrariae_ was the only one that produced
gall-formation. The mycelium of the parasite in this instance penetrates
to the medulla of the host lichen as a loose weft of hyphae which are
divided into more or less elongate cells. These send out side branches,
which grow towards the algal cells, and by their short-celled filaments
clasp them exactly in the same way as do the normal lichen hyphae. Thus
in the neighbourhood of the parasite an algal cell may be surrounded by
the hyphae not only of the host, but also by those of _Abrothallus_. The
two different hyphae can generally be distinguished by their reaction
to iodine: in some cases _Abrothallus_ hyphae take the stain, in others
the host hyphae. In addition to apothecia, spermogonia or pycnidia are
produced, but in one of the species examined by Kotte, _Abrothallus
Peyritschii_ on _Cetraria caperata_, there was no spermogonial wall
formed. The hyphae also penetrate the host soredia or isidia, so that
on the dispersal of these vegetative bodies the perpetuation of both
organisms is secured in the new growth.
_Abrothallus_ draws its organic food from the gonidia in the same way as
the host species, and possibly the parasitic hyphae obtain also water and
inorganic food along with the host hyphae. They have been traced down
to the rhizinae and may even reach the hypothallus, but no injury to
the host has been detected. It is a case of joint symbiosis and not of
parasitism. Microscopic research has therefore justified the inclusion of
these and other forms among lichens.
_d._ PARASYMBIOSIS OF FUNGI. There occur on lichens, certain parasites
classed as fungi which at an early stage are more or less parasymbionts
of the host; as growth advances they may become parasitic and cause
serious damage, killing the tissues on which they have settled.
Zopf[950] found several instances of such parasymbiosis in his study
of fungal parasites, such as _Rhymbocarpus punctiformis_, a minute
Discomycete which inhabits the thallus of _Rhizocarpon geographicum_.
By means of staining reagents he was able to trace the course of the
parasitic hyphae, and found that they travelled towards the gonidia and
clasped them lichen-wise without damaging them, since these remained
green and capable of division. At no stage was any harm caused to the
host by the alien organism. Another instance he observed was that of
_Conida rubescens_ on the thallus of _Rhizocarpon epipolium_. By means
of fine sections through the apothecia of _Conida_ and the thallus of
the host, he proved the presence of numerous gonidia in the subhymenial
tissue, these being closely surrounded by the hyphae of the parasite,
and entirely undamaged: they retained their green colour, and in size
and form were unchanged. Zopf[951] at first described these parasites as
fungi though later[951] he allows that they may represent lower forms of
lichens.
Tobler[952] has added two more of these parasymbiotic species on the
border line between lichens and fungi, similar to those described
by Zopf. One of these, _Phacopsis vulpina_, belonging to the fungus
family Celidiaceae, is parasitic on _Letharia vulpina_. The fronds of
the host plant are considerably altered in form by its presence, being
more branched and curly. Where the parasite settles a swelling arises
filled with its hyphae, and the host gonidia almost disappear from the
immediate neighbourhood, only a few “nests” being found and these very
mucilaginous. These nests as well as single gonidia are surrounded by
_Phacopsis_ hyphae which have gradually displaced those of the _Letharia_
thallus. The gonidia are excited to division and increase in number on
contact with either lichen or fungus hyphae, but in the latter case the
increase is more abundant owing doubtless to a more powerful chemical
irritant in the fungus. As development advances, the _Phacopsis_ hyphae
multiply to the exclusion of both lichen hyphae and gonidia from the area
of invasion. Finally the host cortex is split, the fungus bursts through,
and the tissue beneath the parasite becomes brown and dead. _Phacopsis_
begins as a “parasymbiont,” then becomes parasitic, and is at last
saprophytic on the dead cells. The hyphae travel down into the medulla of
the host and also into the soredial outgrowths, and are dispersed along
with the host. The effect of _Verrucula_ on the host thallus may also be
cited[953].
Tobler gives the results of his examination of still another fungus,
_Karschia destructans_. It becomes established on the thallus of
_Chaenotheca chrysocephala_ and its hyphae gradually penetrate down to
the underlying bark (larch). The lichen thallus beneath the fungus is
killed, but gonidia in the vicinity are sometimes clasped: _Karschia_
also is thus a parasymbiont, then a parasite, and finally a saprophyte.
Elenkin[954] describes certain fungi which to some extent are
parasymbionts. One of these, _Conidella urceolata_ n. sp., grew on forms
of _Lecanora esculenta_. The other, a stroma-forming species, had invaded
the thallus of _Parmelia molliuscula_, where it caused gall-formation.
As the growth of the gall was due to the co-operation of the lichen
gonidia, the fungus must at first have been a parasymbiont. Only dead
gonidia were present in the stroma; probably they had been digested by
the parasite. Because of the stroma Elenkin placed the fungus in a new
genus, _Trematosphaeriopsis_.
_e._ FUNGI PARASITIC ON LICHENS. A solution or extract of lichen thallus
is a very advantageous medium in which to grow fungi. It is therefore
not surprising that lichens are a favourite habitat for parasitic fungi.
Stahl[955] has noted that the lichens themselves flourish best where
there is frequent moistening by rain or dew with equally frequent drying
which effectively prevents the growth of fungi. Species of _Peltigera_
are however able to live in damp conditions: without being injured, they
have been observed to maintain their vigour when cultivated in a very
moist hothouse while all the other forms experimented with were attacked
and finally destroyed by various fungi.
Lindsay[956] devoted a great deal of attention to the microscopic study
of the minute fruiting bodies so frequently present on lichen thalli and
published descriptions of microlichens, microfungi and spermogonia. He
and others naturally considered these parasitic organisms to be in many
cases either the spermogonia or pycnidia of the lichen itself. It is
often not easy to determine their relationship or their exact systematic
position; many of them are still doubtful forms.
There exists however a very large number of fully recognized parasitic
microfungi belonging to various genera. Lindsay discovered many of
them. Zopf[957] has given exact descriptions of a series of forms, with
special reference to their effect on the host thallus. In an early
paper he described a species, _Pleospora collematum_, that he found on
_Physma compactum_ and other Collemaceae. The hyphae of the parasite
differed from those of the host in being of a yellow colour; they did
not penetrate or spread far, being restricted to rhizoid-like filaments
at the base of their fruiting bodies (perithecia and pycnidia). Their
presence caused a slight protuberance but otherwise did no harm to
the host; the _Nostoc_ cells in their immediate vicinity were even
more brightly coloured than in other parts of the thallus. In another
paper[958] he gives an instance of gall-formation in _Collema pulposum_
induced by the presence of the fungus _Didymosphaeria pulposi_. Small
protuberances were formed on the margins of the apothecia, more rarely
on the lobes of the thallus, each one the seat of a perithecium of the
fungus. No damage was done to either constituent of the thallus.
_Agyrium flavescens_ grows parasitically on the under surface of
_Peltigera polydactyla_. M. and Mme Moreau[959] found that the hyphae
of the fungus spread between the medullary filaments of the lichen;
no haustoria were observed. The mature fruiting body had no distinct
excipulum, but was surrounded by a layer of dead lichen cells.
It is not easy to determine the difference between parasites that are
of fungal nature and those that are lichenoid; but as a general rule
the fungi may be recognized by their more transient character, very
frequently by their effect on the host thallus, which is more harmful
than that produced by lichens, and generally by their affinity to fungi
rather than to lichens. Opinions differ and will continue to differ on
this very difficult question.
The number of such fungi determined and classified has gradually
increased, and now extends to a very long list. Even as far back as
1896 Zopf reckoned up 800 instances of parasitism of 400 species of
fungi on about 350 different lichens and many more have been added. Abbé
Vouaux[960] is the latest writer on the subject, but his work is mostly a
compilation of species already known. He finds representatives of these
parasites in nine families of Pyrenomycetes and six of Discomycetes.
He leaves out of account the much debated Coniocarps, but he includes
with fungi all those that have been proved to be parasymbiotic, such as
_Abrothallus_.
A number of fungus genera, such as _Conida_, etc., are parasitic only on
lichens. Most of them have one host only; others, such as _Tichothecium
pygmaeum_, live on a number of different thalli. Crustaceous species are
often selected by the parasites, and no great damage, if any, is caused
to these hosts, except when the fungus is seated on the disc of the
apothecium, so that the spore-bearing capacity is lessened or destroyed.
In some of the larger lichens, however, harmful effects are more
visible. In _Lobaria pulmonaria_, the fruits of which are attacked by
the Discomycete, _Celidium Stictarum_[961], there is at first induced
an increased and unusual formation of lichen apothecia. These apothecia
are normally seated for the most part on the margins of the lobes or
pustules, but when they are invaded by the fungus, they appear also in
the hollows between the pustules and even on the under surface of the
thallus. In the large majority of cases the fungus is partly or entirely
embedded in the thallus; the gonidia in the vicinity may remain green
and healthy, or all the tissues in the immediate neighbourhood of the
parasite may be killed.
_f._ MYCETOZOA PARASITIC ON LICHENS. Mycetozoa live mostly on decayed
wood, leaves, humus, etc. One minute species, _Listerella paradoxa_,
always inhabits the podetia of _Cladonia rangiferina_. Another species,
_Hymenobolina parasitica_, was first detected and described by Zukal[962]
as a true parasite on the thallus of Physciaceae; it has since been
recorded in the British Islands on _Parmeliae_[963]. This peculiar
organism differs from other mycetozoa in that the spores on germination
produce amoebae. These unite to form a rose-red plasmodium which slowly
burrows into the lichen thallus and feeds on the living hyphae. It is a
minute species, but when abundant the plasmodia can just be detected with
the naked eye as rosy specks scattered over the surface of the lichen.
Later the grey sporangia are produced on the same areas.
F. DISEASES OF LICHENS
_a._ CAUSED BY PARASITISM. Zopf[964] has stated that of all plants,
lichens are the most subject to disease, reckoning as diseases all the
instances of parasitism by fungi or by other lichens. There are however
only rare instances in which total destruction or indeed any permanent
harm to the host is the result of such parasitism. At worst the trouble
is localized and does not affect the organism as a whole. Some of
these cases have been already noted under antagonistic symbiosis or
parasymbiosis. Several instances have however been recorded where real
injury has been caused by the penetration of some undetermined fungus
mycelium. Zukal[965] records two such observed by him in _Parmelia
encausta_ and _Physcia villosa_: the thallus of the former was dwarfed
and deformed by the presence of the alien mycelium, the latter was
excited to abnormal proliferation.
_b._ CAUSED BY CROWDING. Lichens suffer frequently from being overgrown
by other lichens; they may also be crowded out by other plants. My
attention was called by Mr P. Thompson to a burnt plot of ground in
Epping Forest, which, after the fire, had been colonized by _Peltigera
spuria_. In the course of a few years, other vegetation had followed,
depriving the lichen of space and light and gradually driving it out.
When last examined only a few miserable specimens remained, and these
were reduced in vitality by an attack of the lichen parasite _Illosporium
carneum_.
_c._ CAUSED BY ADVERSE CONDITIONS. Zukal considers as pathological, at
least in origin, the cracking of the thallus so frequent in crustaceous
lichens as well as in the more highly developed forms. As the cracks are
beneficial in the aeration of the plant, they can hardly be regarded as
symptoms of a diseased condition. The more evident ringed breaks in the
cortex of _Usneae_, due probably to wind action, have more reason to be
so regarded; they are most pronounced in _Usnea articulata_, where the
portions bounded by the rings are contracted and swollen, and a hollow
space is formed between the cortex and the central axis. The swellings
that are produced on lichen thalli, such as those of _Umbilicaria_ and
some species of _Gyrophora_, due to intercalary growth are normal to
the plant, though occasionally the swollen weaker portions may become
ruptured and the cortex be thrown off. As pathological also must be
regarded the loss of cortex sometimes occasioned by excessive soredial
formation at the margins of the lobes: the upper cortex may be rolled
back and eventually torn away; the gonidial layer is exposed and
transformed into soredia which are swept away by the wind and rain, till
finally only traces of the lower cortex are left.
Zukal[966] has instanced, as a case of diseased condition observed by
him, the undue thickening of the cortex in _Pertusaria communis_ whereby
the formation of the fruiting bodies is inhibited and even vegetative
development is rendered impossible. There arrives finally a stage when
splitting takes place and the whole thallus breaks down and disappears.
As a rule however there need be no limit to the age of the lichen plant.
There is no vital point or area in the thallus; injury of one part leaves
the rest unhurt, and any fragment in growing condition, if it combines
both symbionts, can carry on the life of the plant, the constant renewal
of gonidia preventing either decay or death. Barring accidents many
lichens might exist as long as the world endures.
G. HARMFUL EFFECT OF LICHENS
One lichen only, _Strigula complanata_, a tropical species, has been
proved to be truly and constantly parasitic. It grows on the surface of
thick leathery leaves such as those of _Camellia_[967], etc. and the alga
and fungus both penetrate the epidermis and burrow beneath the cuticle
and outer cells, causing them to become brown. It undoubtedly injures the
leaves.
Friedrich[968] has given an isolated instance of the hold-fast hyphae
of _Usnea_ piercing through the cortex to the living tissue of the
host, and not only destroying the middle lamella by absorption, but
entering the cells. The _Usnea_ plant was characterized by exceptionally
vigorous growth. Practically all corticolous lichens are epiphytic and
the injury they cause is of an accidental nature. Crustaceous species on
the outer bark occupy the dead cortical layers and seem to be entirely
harmless[969]. The larger foliose and fruticose forms are not so
innocuous: by their abundant enveloping growth they hinder the entrance
of air and moisture, and thus impede the life of the higher plant.
Gleditsch[970], one of the earliest writers on Forestry, first indicated
the possibly harmful effect of lichens especially on young trees and
“in addition,” he says, “they serve as cover for large numbers of small
insects which are hurtful in many ways to the trees.” Lindau[971] pointed
out the damage done to pine-needles by _Xanthoria parietina_ which grew
round them like a cuff and probably choked the stomata, the leaves so
clothed being mostly withered. Dufrenoy[972] states that he found the
hyphae of a _Parmelia_ entering a pine-needle by the stomata, and that
the starch disappeared from the neighbouring parenchyma the cells of
which tended to disintegrate.
It is no uncommon sight to see neglected fruit trees with their branches
crowded with various lichens, _Evernia prunastri_, _Ramalina farinacea_,
etc. Such lichens often find the lenticels a convenient opening for
their hold-fasts and exercise a smothering effect on the trees. Lilian
Porter[973] distinctly states that _Ramalinae_ by their penetrating bases
damage the tissues of the trees. The presence of lichens is however
generally due to unhealthy conditions already at work. Friedrich[974]
reported of a forest which he examined, in which the atmospheric moisture
was very high, with the soil water scarce, that those trees that were
best supplied with soil water were free from lichens, while those with
little water at the base bore dead branches which gave foothold to a rich
growth of the epiphytes.
Experiments to free fruit trees from their coating of lichens were made
by Waite[975]. With a whitewash brush he painted over the infested
branches with solutions of Bordeaux mixture of varying strength, and
found that this solution, commonly in use as a fungicide, was entirely
successful. The trees were washed down about the middle of March, and
some three weeks later the lichens were all dead, the fruticose and
foliose forms had changed in colour to a yellowish or brownish tint and
were drooping and shrivelled.
Waite was of opinion that the lichens did considerable damage to the
trees, but it has been held by others that in very cold climates they
may provide protection against severe frost. Instances of damage are
however asserted by Bouly de Lesdain[976]. The bark of willows he found
was a favourite habitat of numerous lichens: certain species, such as
_Xanthoria parietina_, completely surrounded the branches, closing the
stomata; others, such as _Physcia ascendens_, by the mechanical strain of
the rhizoids, first wet and then dry, gradually loosened the outer bark
and gave entry to fungi which completed the work of destruction.
H. GALL-FORMATION
Several instances of gall-formation to a limited extent have been already
noted as caused by parasitic fungi or lichens. Greater abnormality
of development is induced in a few species by the presence of minute
animals, mites, wood-lice, etc. Zopf[977] noted these deformations of
the thallus in specimens of _Ramalina Kullensis_ collected on the coasts
of Sweden. The fronds were frequently swollen in a sausage-like manner,
and branching was hindered or altogether prevented; apothecia were
rarely formed, though pycnidia were abundant. Here and there, on the
swollen portions of the thallus, small holes could be detected and other
larger openings of elliptical outline, about 1-1-1/2 mm. in diameter,
the margins of which had a nibbled appearance. Three types of small
articulated animals were found within the openings: species of mites,
spiders and wood-lice. Mites were the most constant and were more or
less abundant in all the deformations; frequently a minute Diplopodon
belonging to the genus _Polyxenus_ was also met with.
Zopf came to the conclusion that the gall-formation was mainly due to
the mites: they eat out the medulla and possibly through some chemical
irritation excite the algal zone and cortex to more active growth, so
that an extensive tangential development takes place. The small spiders
may exercise the same power; evidently the larger holes were formed by
them.
Later Zopf added to gall-deformed plants _Ramalina scopulorum_ var.
_incrassata_ and _R. cuspidata_ var. _crassa_. He found in the hollow
swollen fronds abundant evidence of mites, but whether identical with
those that attacked _R. Kullensis_ could not be determined. These two
_Ramalinae_ are maritime species; they are morphologically identical,
as are also the deformed varieties, and the presence of mites, excreta,
etc., are plainly visible in our British specimens.
Bouly de Lesdain[978] found evidence of mite action in _Ramalina
farinacea_ collected from _Pinus sylvestris_ on the dunes near Dunkirk.
The cortex had been eaten off either by mites or by a small mollusc
(_Pupa muscorum_) and the fronds had collapsed to a more or less convex
compact mass. Somewhat similar deformations, though less pronounced, were
observed in other _Ramalinae_.
In _Cladonia sylvatica_ and also in _Cl. rangiformis_ Lesdain has
indicated ff. _abortiva_ Harm. as evidently the result of insect attack.
In both cases the tips of the podetia are swollen, brown, bent and
shrivelled.
One of the most curious and constant effects, also worked out by Lesdain,
occurs in _Physcia hispida_ (_Ph. stellaris_ var. _tenella_). In that
lichen the gonidia at the tips of the fronds are scooped out and eaten by
mites, so that the upper cortex becomes separated from the lower part of
the thallus. As the hyphae of the cortex continue to develop, an arched
hood is formed of a whitish shell-like appearance and powdery inside.
Sometimes the mites penetrate at one point only, at other times the
attack is at several places which may ultimately coalesce into one large
cavity. In a crustaceous species, _Caloplaca_ (_Placodium_) _citrina_ he
found constant evidence of the disturbing effect of the small creatures,
which by their action caused the areolae of the thallus to grow into
minute adherent squamules. A pathological variety, which he calls var.
_sorediosa_, is distinguished by the presence of cup-like hollows which
are scooped out by Acarinae and are filled by yellowish soredia. In
another form, var. _maritima_, the margins of the areolae, occasionally
the whole surface, become powdery with a citrine yellow efflorescence as
a result of their nibbling.
Zukal[979] adds to the deformations due to organic agents, the
hypertrophies and abnormalities caused by climatic conditions. He finds
such irregularities of structure more especially developed in countries
with a very limited rainfall, as in certain districts of Chili, Australia
and Africa, where changes in cortex and rhizoids and proliferations of
the thallus testify to the disturbance of normal development.