PHYSIOLOGY
I. CELLS AND CELL PRODUCTS
Any study of cells or cell-membranes in lichens should naturally include
those of both symbionts, but the algae though modified have not been
profoundly changed, and their response to the influences of the symbiotic
environment has been already described in the discussion of lichen
gonidia. The description of cells and their contents refers therefore
mainly to the fungal tissues which form the framework of the plant; they
have been transformed by symbiosis to lichenoid hyphae in some respects
differing from, in others resembling, the fungal hyphae from which they
are derived.
A. CELL-MEMBRANES
_a._ CHITIN. It was recognized by workers in the early years of the
nineteenth century that the substance forming the cell-walls of fungal
hyphae differed very markedly from the cellulose of the membranes in
other groups of plants, the blue colouration with iodine and sulphuric
acid so characteristic of cellulose being absent in most fungi. Various
explanations were suggested; but it was always held that the doubtful
substance was a cellulose containing something peculiar to fungi, this
view being strengthened by the fact that, after long treatment with
potash, a blue reaction was obtained. It was called fungus-cellulose by
De Bary[740] in order to distinguish it from true cellulose.
It was not till a much later date that any exact work was done on
the fungal cell, and that Gilson[741] by his researches was able to
prove that the membranes of fungi contained probably no cellulose, or,
“if cellulose were present, it was in a different condition from the
cellulose of other plants.” Winterstein[742] followed with the results of
his examination of fungus-cellulose: he found that it contained nitrogen
and therefore differed very considerably from typical plant cellulose.
Gilson[743] published a second paper dealing entirely with fungal tissues
in which he also established the presence of nitrogen, and added that
this nitrogenous compound resembled in various ways the chitin[744] of
animal cells. He further discovered that by heating it with potash a
substance was obtained that took a reddish-violet stain when treated with
iodine and weak sulphuric acid. This substance, called by him mycosin,
was proved later to be similar to chitosan[744], a product of chitin.
Escombe[745] analysed the hyphal membranes of _Cetraria_ and found
that they consisted mainly of a body called by him lichenin and of a
paragalactan. From _Peltigera_ he extracted a substance with physical
properties agreeing fairly well with those of chitosan, though analysis
did not give percentages reconcilable with that substance; the yield
however was very small. No lichenin was detected.
Van Wisselingh[746] examined the hyphae of lichens as well as of
fungi and experimented with a considerable number of both types of
plants. He succeeded in proving the presence of chitin in the higher
fungi (Basidiomycetes and Ascomycetes) and in lichens with one or two
exceptions (_Cladonia_ and _Cetraria_). Though in some the quantity found
was exceedingly small, in others, such as _Peltigera_, the walls of the
hyphae were extremely chitinous. More recently Wester[747] has gone
into the question as regards lichens, and he has practically confirmed
all the results previously obtained by Wisselingh. In some species, as
for instance in _Cladonia rangiferina_, _Cl. squamosa_, _Cl. gracilis_,
_Ramalina calicaris_, _Solorina crocea_ and others, he found that chitin
existed in large quantities, while in _Evernia prunastri_, _Usnea
florida_, _U. articulata_, _Sticta damaecornis_ and _Parmelia saxatilis_
very little was present. The variation in the amount present may be
very great even in the species of one genus: none for instance has been
detected in _Cetraria islandica_ nor in _C. nivalis_ while it is abundant
in other _Cetrariae_. There is also considerable variation in quantity in
different individuals of the same species, and even in different parts
of the thallus of one lichen. Factors such as habitat, age of the plant,
etc., may, however, account to a considerable extent for the differences
in the results obtained.
_b._ LICHENIN AND ALLIED CARBOHYDRATES. It has been proved, as already
stated, that chitin is present in the hyphal cell-walls of all the
lichens examined except in those of _Cetraria islandica_ (Iceland Moss),
_C. nivalis_ and, according to Wester[747], in those of _Bryopogon_
(_Alectoriae_). In these lichens another substance of purely carbohydrate
nature is the chief constituent of the cell-walls which swell up when
soaked in water to a colourless gelatinous substance.
Berzelius[748] first drew attention to the peculiar qualities of this
lichen product, and, recognizing its resemblance in many respects to
ordinary starch, he called it “lichen-starch” or “moss-starch.” More
exact observations were made later by Guérin-Varry[749] who described its
properties and showed by his experiments that it contained no admixture
of either starch or gum. He adopted the name lichenin for this organic
soluble part of Iceland Moss. An analysis of lichenin was made by
Mulder[750] who detected in addition to lichenin, which coloured yellow
with iodine, small quantities of a blue-colouring substance which could
be dissolved out from the lichenin and which he considered to be true
starch. Berg[751] also demonstrated the compound nature of lichenin:
he isolated two isomerous substances with the formula C₆H₁₀O₅. The
name “isolichenin” was given to the second blue-colouring substance by
Beilstein[752] in 1881.
More recently Escombe[753] has chemically analysed the cell-wall of
_Cetraria islandica_: after the elimination of fat, oil, colouring
matter and bitter constituents he found that there remained the compound
lichenin, an anhydride of galactose with the formula C₆H₁₀O₅, which, as
stated above, consists of two substances lichenin and isolichenin[754];
the latter is soluble in cold water and gives a blue reaction with
iodine, lichenin is only soluble in hot water and is not coloured blue.
Both are derivatives of galactose, a sugar found in a great number of
organic tissues and substances, among others in gums.
Lichenin has also been obtained by Lacour[755] from _Lecanora esculenta_,
an edible desert lichen supposed to be the manna of the Israelites.
Wisselingh[756] tested the hymenium of thirteen different lichens for
lichenin. He found it in the walls of the ascus of all those he examined
except _Graphis_. Everniin, a constituent of _Evernia prunastri_, was
isolated and described by Stüde[757]. It is soluble in water and, though
considered by Czapek[758] to be identical with lichenin, it differs,
according to Ulander[759], in being dextro-rotatory to polarized
light; lichenin on the contrary is optically inactive. Escombe[753]
also obtained a substance from _Evernia_ which he considered to be
comparable with chitosan. Usnein which has been extracted[756] from
_Usnea barbata_ may also be identical with lichenin, but that has not yet
been established. Ulander[759] examined chemically the cell-walls of a
fairly large number of lichens. _Cetraria islandica_, _C. aculeata_ and
_Usnea barbata_, designated as the “Cetraria group,” contained soluble
mucilage-forming substances similar to lichenin. A second “Cladonia
group” which included _Cl. rangiferina_ with the variety _alpestris_,
_Stereocaulon paschale_ and _Peltigera aphthosa_ yielded almost none.
After the soluble carbohydrates were removed by hot water, the insoluble
substances were hydrolysed and the “Cetraria group” was found to contain
abundant d-glucose with small quantities of d-mannose and d-galactose;
the “Cladonia group,” abundant d-mannose and d-galactose with but little
d-glucose. Hydrolysis was easier and quicker with the former group than
with the latter.
Besides these, which rank as hexosans, Ulander found small quantities
of pentosans and methyl pentosans. All these substances which are such
important constituents of the hyphal membranes of lichens are classed
by Ulander as hemicelluloses of the same nature as mannan, galactan
and dextran, or as substances between hemicellulose and the glucoses
represented by lichenin, everniin, etc. They are doubtless reserve
stores of food material, and they are chiefly located in the cell-walls
of the medullary hyphae which are often so thick as almost to obliterate
the lumen of the cells. Ulander made no test for chitin in his researches.
Ulander’s results have been confirmed by those obtained by K.
Müller[760]. In _Cladonia rangiferina_, Müller found that the
cell-membranes of the hyphae contained, as hemicelluloses, pentosans in
small quantities and galactan, but no lichenin and very little chitin.
In _Evernia prunastri_ hemicelluloses formed the chief constituents of
the thallus, and from it he was able to isolate galactan soluble in
weak hot acid, and everniin soluble in hot water, the latter with the
formula C₇H₁₅O₆, a result differing from that obtained by Stüde[761] who
has given it as C₉H₁₄O₇; chitin was also present in small quantities.
In _Ramalina fraxinea_, the soluble part of the thallus (in hot water)
differed from everniin and might probably be lichenin. _Cetraria
islandica_ was also analysed and yielded various hemicelluloses, chiefly
dextran and galactan, with less pentosan. No chitin has ever been found
in this lichen. In testing minute quantities of material for chitin,
Wisselingh[762] heated the tissue in potash to 160° C. The potash was
then gradually replaced by glycerine and distilled water; the precipitate
was placed on a slide and the preparation stained under the microscope
by potassium-iodide-iodine and weak sulphuric acid. Chitin, if present,
would have been changed by the potash to mycosin which gives a violet
colour with the staining solution.
It has been stated by Schellenberg[763] that these lichen membranes may
become lignified. He obtained a red reaction with phloroglucine test for
lignin in _Cetraria islandica_ and _Cladonia furcata_. Further research
is required.
_c._ CELLULOSE. Several workers claim to have found true cellulose in
the cell-walls of the hyphal tissues of a few lichens; but the more
careful analyses of Escombe[764], Wisselingh[762] and Wester[765] have
disproved their results. The cell-walls of all the gonidia, however, are
formed of cellulose, or according to Escombe of glauco-cellulose, except
those of _Peltigera_ in which Wester found neither cellulose nor chitin.
Czapek[766] suggests that the blue reaction with iodine characteristic
of the cell-walls in some apothecia, of the asci and of the hyphae in
cortex or medulla in a few instances, may be due to the presence of
carbohydrates of the nature of galactose. Moreau[767] in a recent paper
terms the substance that gives a blue reaction with iodine at the tips of
the asci “amyloid.” In _Peltigera_ the ascus tip is occupied by such a
plug of amyloid which at maturity is projected like a cork from the ascus
and may be found on the surface of the hymenium.
B. CONTENTS AND PRODUCTS OF THE FUNGAL CELLS
_a._ CELL-SUBSTANCES. The cells of lichen hyphae contain protoplasm and
nucleus with glucoses. It is doubtful if starch has been found in fungal
hyphae; it is replaced, in some of the tissues at least, by glycogen,
a carbohydrate (C₆H₁₀O₅) very close to, if not identical with, animal
glycogen, a substance which is soluble in water and colours reddish-brown
(wine-red) with iodine. Errera[768] first detected its presence in
Ascomycetes where it is associated with the epiplasm of the cells, more
especially of the asci, and he considered it to be physiologically
homologous with starch. He included lichens, as Ascomycetes, in his
survey of fungi and quotes, in support of his view that lichen hyphae
also contain glycogen, a statement made by Schwendener[769] that “the
contents of the ascogenous hyphae of _Coenogonium Linkii_ stain a
deep-brown with iodine.” Errera also instances the red-brown reaction
with iodine, described by de Bary[770], as characteristic of the large
spores of _Ochrolechia_ (_Lecanora_) _pallescens_, while the germinating
tubes of these spores become yellow with iodine like ordinary protoplasm.
Glycogen has been, so far, found only in the cells of the reproductive
system.
Iodine was found by Gautier[771] in the gonidia of _Parmelia_ and
_Peltigera_, _i.e._ both in bright-green and blue-green algae. The amount
was scarcely calculable.
Herissey[772] claims to have established the presence of emulsin in a
large series of lichens belonging to such widely separated genera as
_Cladonia_, _Cetraria_, _Evernia_, _Peltigera_, _Pertusaria_, _Parmelia_,
_Ramalina_, and _Usnea_. It is a ferment which acts upon amygdalin,
though its presence has been proved in plants such as lichens where no
amygdalin has been found[773]. Diastase was demonstrated in the cells of
_Roccella tinctoria_, _R. Montagnei_ and of _Dendrographa leucophaea_
by Ronceray[774] who states that, in conjunction with air and ammonia,
it forms orchil, the well-known colouring substance of these lichens.
Diastatic ferments have also been determined[775] in _Usnea florida_,
_Physcia parietina_, _Parmelia perlata_ and _Peltigera canina_.
_b._ CALCIUM OXALATE. Oxalic acid (C₂H₂O₄) is an oxidation product of
alcohol and of most carbohydrates and in combination is a frequent
constituent of plant cells. Knop[776] held that it was formed in lichens
by the reduction and splitting of lichen acids, though, as Zopf[777] has
pointed out, these are generally insoluble. Hamlet and Plowright[778]
demonstrated the presence of free oxalic acid in many families of fungi
including _Pezizae_ and _Sphaeriae_. The acid combines with calcium to
form the oxalate (CaC₂0₄), which in the crystalline form is very common
in lichens. In the higher plants the crystals are formed within the
cell, but in lichens they are always deposited on the outer surface of
the hyphal membranes, mainly of the medulla and the cortex.
Calcium oxalate was first detected in lichens by Henri Braconnot[779],
who extracted it by treating the powdered thallus of a number of species
(_Pertusaria communis_, _Diploschistes scruposus_, etc.) with different
reagents. The quantity present varies greatly in lichens: Zopf[780] found
that it was abundant in all the species inhabiting limestone, and states
that in such plants the more purely lichenic acids are relatively scarce.
Errera[781] has calculated the amount of calcium oxalate in _Lecanora
esculenta_, a desert lime-loving lichen, to be about 60 per cent. of the
whole substance of the thallus. Euler[782] gives for the same lichen
even a larger proportion, 66 per cent. of the dry weight. In _Pertusaria
communis_, a corticolous species, the oxalate occurs as irregular
crystalline masses in the medulla (Fig. 116) and has been calculated
as 47 per cent. of the whole substance. Other crustaceous species such
as _Diploschistes scruposus_, _Haematomma coccineum_, _H. ventosum_,
_Lecanora saxicola_, _Lecanora tartarea_, etc., contain large amounts
either in the form of octahedral crystals or as small granules.
[Illustration: Fig. 116. _Pertusaria communis_ DC. Vertical section of
thallus. _a_, cortex; _b_, gonidia; _c_, medulla; _d_, crystal of calcium
oxalate. × ca. 100.]
Rosendahl[783] has recently made observations as to the presence of the
oxalate in the thallus of the brown _Parmeliae_. Of the fourteen species
examined by him, eleven contained calcium oxalate as octahedral crystals
or as small prisms, often piled up in thick irregular masses. Usually the
crystals were located in the medullary part of the thallus, but in two
species, _Parmelia verruculifera_ and _P. papulosa_, they were abundant
on the surface cells of the upper cortex.
_c._ IMPORTANCE OF CALCIUM OXALATE TO THE LICHEN PLANT. It is natural to
conclude that a substance of frequent occurrence in any group of plants
is of some biological significance, and suggestions have not been lacking
as to the value of oxalic acid or of calcium oxalate in the economy of
the lichen thallus. Oxalic acid is known to be one of the most efficient
solvents of argillaceous earth and of iron oxides likely to be in the
soil. These materials are also conveyed to the thallus as air-borne
dust, and would thus, with the aid of the acid, be easily dissolved and
absorbed. As a direct proof of this, Knop[784] has stated that lichen-ash
always contains argillaceous earth. According to Kratzmann[785],
aluminium, a product of clay, is stored up in various lichens. He proved
the amount in the ash of _Umbilicaria pustulata_ to be 4·46 per cent.,
in _Usnea barbata_ 1·79 per cent., in _U. longissima_ considerable
quantities while in _Roccella tinctoria_ it occurred in great abundance.
It was also abundant in _Diploschistes scruposus_, 28·17 per cent.; it
declined in _Variolaria_ (_Pertusaria_) _dealbata_ to 7·77 per cent., in
_Cladonia rangiferina_ to 1·76-2·12 per cent. and in _Ramalina fraxinea_
to 1·8 per cent.
Calcium oxalate is directly advantageous to the thallus by virtue of
the capacity of the crystals to reduce or prevent evaporation, as has
been pointed out by Zukal[786]. A like service afforded by crystals to
the leaves of the higher plants in desert lands has been described by
Kerner[787]. These are frequently encrusted with lime crystals which
allow the copious night dews to soak underneath them to the underlying
cells, while during the day they impede, if they do not altogether check,
evaporation.
Calcium oxalate crystals are insoluble in acetic acid, soluble in
hydrochloric acid without evolution of gas; they deposit gypsum crystals
in a solution of sulphuric acid.
C. OIL-CELLS
_a._ OIL-CELLS OF ENDOLITHIC LICHENS. Calcicolous immersed lichens are
able to dissolve the lime of the substratum, and their hyphae penetrate
more or less deeply into the rock. In some forms the entire thallus
may thus be immersed, the fruits alone being visible on the surface of
the stone. In two such species, _Verrucaria calciseda_ and _Petractis_
(_Gyalecta_) _exanthematica_, Steiner[788] detected peculiar sphaeroid
or barrel-shaped cells that differed from the other hyphal cells of the
thallus, not only in their form, but in their greenish-coloured contents.
Similar cells were found by Zukal[789] in another immersed (endolithic)
lichen, _Verrucaria rupestris_ f. _rosea_. He describes them as roundish
organs crowded on the hyphae and filled with a greenish shimmering
protoplasm. He[790] found the same types of sphaeroid and other swollen
cells in the immersed thallus of several calcicolous lichens and he
finally determined the contents as fat in the form of oil. He found
also that these fat-cells, though very frequent, were not constantly
present even in the same species. His observations were confirmed by
Hulth[791] for a number of allied crustaceous lichens that grow not only
on limestone but on volcanic rocks. In them he found a like variety of
fat-cells—intercalary or torulose cells, terminal sphaeroid cells and
hyphae containing scattered oil-drops. Bachmann[792] followed with a
study of the thallus of purely calcicolous lichens. The specialized
oil-cells were fairly constant in the species he examined, and, as a
rule, they were formed either in the tissues immediately below, or at
some distance from, the gonidial zone. Fünfstück[793] has also published
an account of various oil-cells in a large series of calcicolous lichens
(Fig. 117).
[Illustration: Fig. 117. _Lecidea immersa_ Ach. A, sphaeroid fat-cells
from about 8 mm. below the surface × 550. B, oil-hyphae in process
of emptying: _a_, sphaeroid cells containing oil; _b_, cells with
oil-globules × 600 (after Fünfstück).]
The occurrence of oil-(or fat-)cells is not dependent on the presence
of any particular alga as the gonidium of the lichen. Fünfstück[794]
has described the immersed thallus of _Opegrapha saxicola_ as one of
those richest in fat-cells. The gonidia belong to the filamentous alga
_Trentepohlia umbrina_ and form a comparatively thin layer about 160µ
thick near the upper surface; isolated algal branches may grow down to
350µ into the rock, while the fungal elements descend to 11·5 mm., and
though the very lowest hyphae were without oil—as were those immediately
beneath the gonidia—the interlying filaments, he found, were crowded with
oil-cells. Sphaeroid terminal cells were not present.
Fünfstück[794] has re-examined the thallus of _Petractis exanthematica_,
an almost wholly immersed lichen with a gelatinous gonidium, a species of
_Scytonema_. The thallus is homoiomerous: the alga forms no special zone,
it intermingles with the hyphae down to the very base of the thallus; the
hyphae are extremely slender and at the base they measure only about 1µ
in width. Oil-cells are abundant in the form of intercalary cells about
3-5µ in thickness. Nearer the surface sphaeroid cells are formed on short
lateral outgrowths; they measure 14-16µ in diameter and occur in groups
of 15 to 20. The superficial part of the thallus is a mere film; the
hyphae composing it are slightly stouter and more thickly interwoven.
Bachmann[795] and Lang[796] have further described the anatomy of
endolithic thalli especially with reference to oil-cells, and have
supplemented the researches of previous workers. New methods of cutting
the rock in thin slices and of dissolving away the lime enabled them to
see the tissues in their relative positions. In these immersed lichens,
as described by them and by previous writers, and more especially in
calcicolous species, the gonidial zone of Protococcaceous algae lies
near the surface of the rock, and is mingled with delicate, thin-walled
hyphae which usually do not contain oil. The more deeply immersed layer
is formed of a weft of equally thin-walled hyphae, some of the cells
of which are swollen and filled with fat globules. These oil-cells may
occur at intervals along the hyphae or they may form an almost continuous
row. In addition, strands or bundles of hyphae (Fig. 118) containing
few or many oil globules traverse the tissue, and true sphaeroid cells
are generally present. These latter arise in great numbers on short
lateral branchlets, usually near the tip of a filament and the groups
of cells are not unlike bunches of grapes. Sometimes the oil-cells are
massed together into a complex tissue. Hyphae from this layer pierce
still deeper into the rock and constitute the rhizoidal portion of the
thallus. They also produce sphaeroid oil-cells in great abundance (Fig.
119). In the immersed thallus of _Sarcogyne_ (_Biatorella_) _pruinosa_
Lang[797] estimated the gonidial zone as 175-200µ in thickness, while the
colourless hyphae penetrated the rock to a depth of quite 15 mm.
[Illustration: Fig. 118. _Biatorella_ (_Sarcogyne_) _simplex_ Br. and
Rostr. _a_, sphaeroid oil-cells; _b_, strand of oil-hyphae from 10-15 mm.
below the surface. × 585 (after Lang).]
[Illustration: Fig. 119. _Biatorella pruinosa_ Mudd. _a_, complex
of sphaeroid oil-cells from 10 mm. below the surface; _b_, hypha of
sphaeroid cells also from inner part of the thallus. × 585 (after Lang).]
_b._ OIL-CELLS OF EPILITHIC LICHENS. The general arrangement of the
tissues and the occurrence and form of the oil-cells vary in the
different species according to the nature of the substratum. This has
been clearly demonstrated by Bachmann[798] in _Aspicilia_ (_Lecanora_)
_calcarea_, an almost exclusively calcareous lichen as the name implies.
On limestone, he found sphaeroid cells formed in great abundance on the
deeply penetrating rhizoidal hyphae (Fig. 120). On a non-calcareous brick
substratum[799], a specimen had grown which of necessity was epilithic.
The cortex and gonidial zone together were 40µ thick; immediately below
there were hyphae with irregular cells free from oil; lower still there
was formed a compact tissue of globose fat-cells. In this case the
calcareous lichen still retained the capacity to form oil-cells on the
non-calcareous impenetrable substance.
[Illustration: Fig. 120. _Lecanora_ (_Aspicilia_) _calcarea_ Sommerf.
Early stage of sphaeroid cell formation × 175 (after Bachmann).]
Very little oil is formed, as a rule, in the cells of siliceous
crustaceous lichens which are almost wholly epilithic, but Bachmann found
a tissue of oil-cells in the thallus of _Lecanora caesiocinerea_, from
Labrador, on a granite composed of quartz, orthoclase and traces of mica.
A thallus of the same species collected in the Tyrol, though of a thicker
texture, contained no oil. Bachmann[799] suggests no explanation of the
variation.
On granite, rhizoidal hyphae penetrate the rock to a slight extent
between the different crystals, but only in connection with the mica[800]
are typical sphaeroid cells formed.
More or less specialized oil-cells have been demonstrated by
Fünfstück[801] in several superficial (epilithic) lichens which grow
on a calcareous substratum, as for instance _Lecanora_ (_Placodium_)
_decipiens_, _Lecanora crassa_ and other similar species. The oil in
these lichens is usually restricted to more or less swollen or globose
cells; but it may also be present in the ordinary hyphae as globules.
Zukal[802] found that the smooth little round granules sprinkled over the
thallus of the soil-lichens, _Baeomyces roseus_ and _B. rufus_, contained
in the hyphae typical sphaeroid oil-cells and that they were specially
well developed in specimens from Alpine situations. In still another
soil-lichen, _Lecidea granulosa_, shimmering green oil was found in
short-celled torulose hyphae.
Rosendahl’s[803] researches on the brown _Parmeliae_ resulted in
the unexpected discovery of specialized oil-cells situated in the
cortices—upper and lower—of five species out of fourteen which he
examined. In one of the species, _P. papulosa_, they also occurred in
the cortex of the rhizoids. The oil-cells were thinner-walled and larger
than the neighbouring cortical cells; they were clavate or ovate in form
and sometimes formed irregular external processes. They were more or less
completely filled with oil which coloured brown with osmic acid, left a
fat stain on paper and, when extracted, burned with a shining reddish
flame. These oil-cells were never formed in the medulla nor in the
gonidial region.
_c._ SIGNIFICANCE OF OIL-FORMATION. Zukal[804] regarded the oil stored
in these specialized cells as a reserve product of service to the plant
in the strain of fruit-formation, or in times of prolonged drought or
deprivation of light. According to his observations fat was most freely
formed in lichens when periods of luxuriant growth alternated with
periods of starvation. He cites, as proof of his view, the frequent
presence of empty sphaeroid cells, and the varying production of oil
affected by the condition, habitat, etc. of the plant. Fünfstück[805],
on the other hand, considers the oil of the sphaeroid and swollen cells
as an excretion, representing the waste products of metabolism in the
active tissue, but due chiefly to the presence of an excess of carbonic
acid which, being set free by the action of the lichen acids on the
carbonate of lime, forms the basis of fat-formation. He points out that
the development of fat-cells is always greater in endolithic species in
which the gonidial layer—the assimilating tissue—is extremely reduced.
In epilithic lichens with a wide gonidial zone, the formation of oil is
insignificant. He states further that if the oil were a direct product
of assimilation, the cells in which it is stored would be found in
contact with the gonidia, and that is rarely the case, the maximum of fat
production being always at some distance.
Fünfstück tested the correctness of his views by a prolonged series
of growth experiments: he removed the gonidial layer in an endolithic
lichen, and found that fat storage continued for some time afterwards,
its production being apparently independent of assimilative activity.
The correctness of his deductions was further proved by observations on
lichens from glacier stones. In such unfavourable conditions the gonidia
were scanty or absent, having died off, but the hyphae persisted and
formed oil. He[806] also placed in the dark two quick-growing calcicolous
lichens, _Verrucaria calciseda_ and _Opegrapha saxicola_. At the end of
the experiment, he found that they had increased in size without using
up the fat. Lang[807] also is inclined to reject Zukal’s theory, seeing
that the fat is formed at a distance from the tissues—reproductive and
others—in need of food supply. He agrees with Fünfstück that the oil is
an excretion and represents a waste-product of the plant.
Considerable light is thrown on the subject of oil-formation by the
results of recent researches on the nutrition of algae and fungi.
Beijerinck[808] made comparative cultures of diatoms taken from the
soil, and he found that so long as culture conditions were favourable,
any fat that might be formed was at once assimilated. If, however, some
adverse influence checked the growth of the organism while carbonic acid
assimilation was in full vigour, fat was at once accumulated. The adverse
influence in this case was the lack of nitrogen, and Beijerinck considers
it an almost universal rule in plants and animals, that where there is
absence of nitrogen, in a culture otherwise suitable, fat-oils will be
massed in those cells which are capable of forming oil. He observed that
in two of the cultures of diatoms the one which alone was supplied with
nitrogen grew normally, while the other, deprived of nitrogen, formed
quantities of oil-drops. Wehmer[809] records the same experience in his
cultural study of _Aspergillus_. Sphaeroid fat-cells, similar to those
described by Zukal in calcicolous lichens, were formed in the hyphae of
a culture containing an overplus of calcium carbonate, and he judged,
entirely on morphological grounds, that these were not of the nature of
reserve-storage cells.
Stahel[810] has definitely established the same results in cultures
of other filamentous fungi. In an artificial culture medium in which
nitrogen was almost wholly absent, the cells of the mycelium seemed
to be entirely occupied by oil-drops, and this fatty condition he
considered to be a symptom of degeneration due to the lack of nitrogen.
These experiments enable us to understand how the hyphae of calcicolous
lichens, buried deep in the substratum, deprived of nitrogen and
overweighted with carbonic acid, may suffer from fatty degeneration as
shown by the formation of “sphaeroid-cells.” The connection between cause
and effect is more obscure in the case of lichens growing on the surface
of the soil, such as _Baeomyces roseus_, or of tree lichens such as the
brown _Parmeliae_, but the same influence—lack of sufficient nitrogenous
food—may be at work in those as well as in the endolithic species, though
to a less marked extent.
It seems probable that the capacity to form oil- or fat-cells has become
part of the inherited development of certain lichen species and persists
through changes of habitat as exemplified in _Lecanora calcarea_[811].
In considering the question of the formation and the function of fat in
plant cells, it must be remembered that the service rendered to the life
of the organism by this substance is a very variable one. In the higher
plants (in seeds, etc.) fat undoubtedly functions in the same way as
starch and other carbohydrates as a reserve food. It is evidently not
so in lichens, and in one of his early researches, Pfeffer[812] proved
that similarly oil was only an excretion in the cells of hepatics. He
grew various species in which oil-cells occurred in the dark and then
tested the cell contents. He found that after three months of conditions
in which the formation of new carbohydrates was excluded, the oil in
the cells, instead of having served as reserve material, was entirely
unchanged and must in that instance be regarded as an excretion.
D. LICHEN-ACIDS
_a._ HISTORICAL. The most distinctive and most universal of lichen
products are the so-called lichen-acids, peculiar substances found so far
only in lichens. They occur in the form of crystals or minute granules
deposited in greater or less abundance as excretory bodies on the outer
surface of the hyphal cells. Though usually so minute as scarcely to be
recognized as crystals, yet in a fairly large series their form can be
clearly seen with a high magnification. Many of them are colourless;
others are a bright yellow, orange or red, and give the clear pure tone
of colour characteristic of some of our most familiar lichens.
The first definite discovery of a lichen-acid was made towards the
beginning of the nineteenth century and is due to the researches of C.
H. Pfaff[813]. He was engaged in an examination of _Cetraria islandica_,
the Iceland Moss, which in his time was held in high repute, not only as
a food but as a tonic. He wished to determine the chemical properties
of the bitter principle contained in it, which was so much prized by
the Medical Faculty of the period, though the bitterness had to be
removed to render palatable the nutritious substance of the thallus. He
succeeded in isolating an acid which he tested and compared with other
organic acids and found that it was a new substance, nearest in chemical
properties to succinic acid. In a final note, he states that the new
“lichen-acid,” as he named it, approached still nearer to boletic acid,
a constituent of a fungus, though it was distinct from that substance
also in several particulars. The name “cetrarin” was proposed, at a
later date, by Herberger[814] who described it as a “subalkaloidal
substance, slightly soluble in cold water to which it gives a bitter
taste; soluble in hot water, but, on continued boiling, throwing down a
brown powder which is slightly soluble in alcohol and readily soluble in
ether.” Knop and Schnederman[815] found that Herberger’s “cetrarin” was
a compound substance and contained besides other substances “cetraric
acid” and lichesterinic acid. It has now been determined by Hesse[816] as
fumarprotocetraric acid (C₆₂H₅₀O₃₅), a derivative of which is cetraric
acid or triaethylprotocetraric acid with the formula C₅₄H₃₉O₂₄(OC₂H₅)₃
and not C₂₀H₁₈O₉ as had been supposed. Cetraric acid has not yet been
isolated with certainty from any lichen[817].
After this first isolation of a definite chemical substance, further
research was undertaken, and gradually a number of these peculiar acids
were recognized, the lichens examined being chiefly those that were of
real or supposed economic value either in medicine or in the arts. In
late years a wider chemical study of lichen products has been vigorously
carried on, and the results gained have been recently arranged and
published in book form by Zopf[818]. Many of the statements on the
subject included here are taken from that work. Zopf gives a description
of all the acids that had been discovered up to the date of publication,
and the methods employed for extracting each substance. The structural
formulae, the various affinities, derivatives and properties of the
acids, with their crystalline form, are set forth along with a list of
the lichens examined and the acids peculiar to each species. In many
instances outline figures of the crystals obtained by extraction are
given. For a fuller treatment of the subject, the student is referred to
the book itself, as only a general account can be attempted here.
_b._ OCCURRENCE AND EXAMINATION OF LICHEN-ACIDS. Acids have been found,
with few exceptions, in all the lichens examined. They are sometimes
brightly coloured and are then easily visible under the microscope.
Generally their presence can only be determined by reagents. Over 140
different kinds have been recognized and their formulae determined,
though many are still imperfectly known. As a rule related lichen species
contain the same acids, though in not a few cases one species may contain
several different kinds. In growing lichens, they form 1 to 8 per
cent. of the dry weight, and as they are practically, while unchanged,
insoluble in water, they are not liable to be washed out by rain, snow
or floods. Their production seems to depend largely on the presence of
oxygen, as they are always found in greatest abundance on the more freely
aerated parts of the thallus, such as the soredial hyphae, the outer
rind or the loose medullary filaments. They are also often deposited on
the exposed disc of the apothecium, on the tips of the paraphyses, and
on the wall lining the pycnidia. They are absent from the thallus of the
Collemaceae, these being extremely gelatinous lichens in which there can
be little contact of the hyphae with the atmosphere. No free acids, so
far as is known, are contained in _Sticta fuliginosa_, but a compound
substance, trimethylamin, is present in the thallus of that lichen. It
has also been affirmed that acids do not occur in any _Peltigera_ nor
in two species of _Nephromium_, but Zopf[818] has extracted a substance
peltigerin both from species of _Peltigera_ and from the section
_Peltidea_.
For purposes of careful examination freshly gathered lichens are most
serviceable, as the acids alter in herbarium or stored specimens. It is
well, when possible, to use a fairly large bulk of material, as the acids
are often present in small quantities. The lichens should be dried at a
temperature not above 40°C. for fear of changing the character of the
contained substances, and they should then be finely powdered. When only
a small quantity of material is available, it has been recommended that
reagents should be applied and the effect watched under the microscope
with a low power magnification. This method is also of great service in
determining the exact position of the acids in the thallus.
In micro-chemical examination, Senft[819] deprecates the use of
chloroform, ether, etc., seeing that their too rapid evaporation leaves
either an amorphous or crystalline mass of material which does not lend
itself to further examination. He recommends as more serviceable some oil
solution, preferably “bone oil” (neat’s-foot oil), in which a section of
the thallus should be broken up under a cover-glass and subjected to a
process of slow heating; some days must elapse before the extraction is
complete. The surplus oil is then to be drained off, the section further
bruised and the substance examined.
Acids in bulk should be extracted by ether, acetone, chloroform, benzole,
petrol-ether and lignoin or by carbon bisulphide. Such solvents as
alcohols, acetates and alkali solutions should not be used as they tend
to split up or to alter the constitution of the acids. For the same
reason, the use of chloroform is to a certain extent undesirable as it
contains a percentage of alcohol. Ether and acetone, or a mixture of
both, are the most efficient solvents, and all acids can be extracted by
their use, if the material is left to soak a sufficient length of time,
either in the cold or warmed. It is however advisable to follow with a
second solvent in case any other acid should be present in the tissues.
Concentrated sulphuric acid dissolves out all acids but often induces
colour changes in the process.
All known lichen-acids form crystals, though the crystalline form may
alter with the solution used. After filtering and distilling, the residue
will be found to contain a mixture of these crystals along with other
substances, which may be removed by washing, etc.
_c._ CHARACTER OF ACIDS. Many lichen-acids are more or less bitter
to the taste; they are usually of an acid nature though certain of
the substances are neutral, such as zeorin, a constituent of various
Lecanoraceae, Physciaceae and Cladoniaceae, stictaurin, originally
obtained from _Sticta aurata_, leiphemin, from _Haematomma coccineum_,
and others.
A large proportion are esters or alkyl salts formed by the union of an
alcohol and an acid; these are insoluble in alkaline carbonates. It
is considered probable that the fungus generates the acid, while the
alcohol arises in the metabolic processes in the alga. It has indeed
been proved that the alcohol, erythrit, is formed in at least two algae,
_Protococcus vulgaris_ and _Trentepohlia jolithus_; and the lichen-acid,
erythrin (C₂₀H₂₂O₁₀), obtained from species of _Roccella_ in which the
alga is _Trentepohlia_, is, according to Hesse, the erythrit ester of
lecanoric acid (C₁₆H₁₄O₇), a very frequent constituent of lichen thalli.
It is certain that the interaction of both symbionts is necessary for
acid production. This was strikingly demonstrated by Tobler[820] in his
cultural study of the lichen thallus. He succeeded in growing, to a
limited extent, the hyphal part of the thallus of _Xanthoria parietina_
on artificial media; but the filaments remained persistently colourless
until he added green algal cells to the culture. Almost immediately
thereafter the characteristic yellow colour appeared, proving the
presence of parietin, formerly known as chrysophanic acid. Tobler’s
observation may easily be verified in plants from natural habitats.
A depauperate form of _Placodium citrinum_ consisting mainly of a
hypothallus of felted hyphae, with minute scattered granules containing
algae, was tested with potash, and only the hyphae immediately covering
the algal granules took the stain; the hypothallus gave no reaction.
It has been suggested[821] that when a decrease of albumenoids takes
place, the quantity of lichen-acid increases, so that the excreted
substance should be regarded as a sort of waste product of the living
plant, “rather than as a product of deassimilation.” The subject is not
yet wholly understood.
_d._ CAUSES OF VARIATION IN QUANTITY AND QUALITY OF LICHEN-ACIDS. Though
it has been proved that lichen-acids are formed freely all the year round
on any soil or in any region, it happens occasionally that they are
almost or entirely lacking in growing plants. Schwarz[822] found this to
be the case in certain plants of _Lecanora tartarea_, and he suggests
that the gyrophoric acid contained in the outer cortex of that lichen had
been broken up by the ammonia of the atmosphere into carbonic acid and
orcin which is soluble in water, and would thus be washed away by rain.
It has also been shown by Schwendener[823] and others that the outer
layers of the older thallus in many lichens slowly perish, first breaking
up and then peeling off; the denuded areas would therefore have lost, for
some time at least, their particular acids. Fünfstück[824] considers that
the difference in the presence and amount of acid in the same species of
lichen may be due very often to variation in the chemical character of
the substratum, and this view tallies with the results noted by Heber
Howe[825] in his study of American _Ramalinae_. He observed that, though
all showed a pale-yellow reaction with potash, those growing on mineral
substrata gave a more pronouncedly yellow colour.
M. C. Knowles[826] found that in _Ramalina scopulorum_ the colour
reaction to potash varied extremely, being more rapid and more intense,
the more the plants were subject to the influence of the sea-spray.
Lichen-acids are peculiarly abundant in soredia, and as, in some
species, the thallus forms these outgrowths, or even becomes leprose
more freely in damp weather, the amount of acids produced may depend on
the amount of moisture in the atmosphere.
Their formation is also strongly influenced by light, as is well shown
by the varying intensity of colour in some yellow thalli. _Placodium
elegans_, always a brightly coloured lichen, changes from yellow to
sealing-wax red in situations exposed to the full blaze of the sun.
_Haematomma ventosum,_though greenish-yellow in lowland situations
is intensely yellow in the high Alps. The same variation of colour
is characteristic of _Rhizocarpon geographicum_ which is a bright
citron-yellow at high altitudes, and becomes more greenish in hue as it
nears the plains. The familiar foliose lichen _Xanthoria parietina_ is
a brilliant orange-yellow in sunny situations, but grey-green in the
shade, and then yielding only minute quantities of parietin. West[827]
and others have noted its more luxuriant growth and brighter colour when
it grows in positions where nitrogenous food is plentiful, such as the
roofs of farm-buildings, which are supplied with manure-laden dust, and
boulders by the sea-shore frequented by birds.
_e._ DISTRIBUTION OF ACIDS. Some acids, so far as is known, are only
to be found in one or at most in very few lichens, as for instance
cuspidatic acid which is present in _Ramalina cuspidata_, and scopuloric
acid, a constituent of _Ramalina scopulorum_, the acids having been held
to distinguish by their reactions the one plant from the other.
Others of these peculiar products are abundant and widely distributed.
Usninic acid, one of the commonest, has been determined in some 70
species belonging to widely diverse genera, and atranorin, a substance
first discovered in _Lecanora atra_, has been found again many times;
Zopf gives a list of about 73 species or varieties from which it has been
extracted. Another widely distributed acid is salazinic acid which has
been found by Lettau[828] in a very large number of lichens.
E. CHEMICAL GROUPING OF LICHEN-ACIDS
Most of these acids have been provisionally arranged by Zopf in groups
under the two great organic series: I. The Fat series; and II. The
Benzole or Aromatic series.
I. LICHEN-ACIDS OF THE FAT SERIES
=Group 1.= Colourless substances soluble in alkali, the solution not
coloured by iron chloride. Exs. protolichesterinic acid (C₁₉H₃₄O₄)
obtained from species of _Cetraria_, and roccellic acid (C₁₇H₃₂O₄) from
species of _Roccella_, from _Lecanora tartarea_, etc.
=Group 2.= Neutral colourless substances insoluble in alkalies, but
soluble in alcohol, the solution not coloured by iron chloride.
_Exs._ zeorin (C₅₂H₈₈O₄), a product of widely diverse lichens, such
as _Lecanora_ (_Zeora_) _sulphurea_, _Haematomma coccineum_, _Physcia
caesia_, _Cladonia deformis_, etc. and barbatin (C₉H₁₄O), a product of
_Usnea barbata_.
=Group 3.= Brightly coloured acids, yellow, orange or red, all
derivatives of pulvinic acid (C₁₈H₁₂O₅), a laboratory compound which has
not been found in nature. The group includes among others vulpinic acid
(C₁₉H₁₄O₅) from the brilliant yellow _Evernia_ (_Letharia_) _vulpina_,
stictaurin (C₃₆H₂₂O₉) deposited in orange-red crystals on the hyphae of
_Sticta aurata_, and rhizocarpic acid (C₂₆H₂₀O₆) chiefly obtained from
_Rhizocarpon geographicum_: it crystallizes out in slender citron-yellow
prisms.
=Group 4.= Only one acid, usninic (C₁₈H₁₆O₇), a derivative of
acetylacetic acid, is placed in this group. It is of very wide-spread
occurrence, having been found in at least 70 species belonging to very
different genera and families of crustaceous shrubby and leafy lichens.
Zopf himself isolated it from 48 species.
=Group 5.= The thiophaninic acid (C₁₂H₆O₉) group representing only a
small number. They are all sulphur-yellow in colour and soluble in
alcohol, the solution becoming blackish-green or dirty blue on the
addition of iron chloride, with one exception, that of subauriferin
obtained from the yellow-coloured medulla of _Parmelia subaurifera_ which
stains faintly wine-red in an iron solution. Thiophaninic acid, which
gives its name to this group, occurs in _Pertusaria lutescens_ and _P.
Wulfenii_, both of which are yellowish crustaceous lichens growing mostly
on the trunks of trees.
II. LICHEN-ACIDS OF THE BENZOLE SERIES
The larger number of lichen-acids belong to this series, of which 94 at
least are already known. They are divided into two subseries: I. Orcine
derivatives, and II. Anthracene derivatives.
SUBSERIES I. ORCINE DERIVATIVES
Zopf specially insists that the grouping of this series must be regarded
as only a provisional arrangement of the many lichen-acids that are
included therein. All of them are split up into orcine and carbonic acid
by ammonia and other alkalies. On exposure to air, the ammoniacal or
alkaline solution changes gradually into orceine, the colouring principle
and chief constituent of commercial orchil. Orcine is not found free in
nature. The orcine subseries includes five groups:
=Group 1.= The substances in this group form, with hypochlorite of lime
(“CaCl”), red-coloured compounds which yield, on splitting, orsellinic
acid. Zopf enumerates seven acids as belonging to this group, among which
is lecanoric acid (C₁₆H₁₄O₇), found in many different lichens, _e.g._
_Roccella tinctoria_, _Lecanora_ tartarea, etc.: whenever there is a
differentiated pith and cortex it occurs in the pith alone. Erythrin
(C₂₀H₂₂O₁₀), a constituent of the British marine lichen _Roccella
fuciformis_, also belongs to this orsellinic group.
=Group 2.= Substances which also form red products with CaCl, but do not
break up into orsellinic acid. Among the most noteworthy are olivetoric
acid (C₂₁H₂₆O₇), a constituent of _Evernia furfuracea_, perlatic acid
(C₂₈H₃₀O₁₀) and glabratic acid (C₂₄H₂₆O₁₁), which are obtained from
species of _Parmelia_.
=Group 3.= Contains three acids of somewhat restricted occurrence. They
do not form red products with CaCl, and they yield on splitting everninic
acid. They are: evernic acid (C₁₇H₁₆O₇), found in _Evernia prunastri_
var. _vulgaris_, ramalic acid (C₁₇H₁₆O₇) in _Ramalina pollinaria_, and
umbilicaric acid (C₂₅H₂₂O₁₁) in species of _Gyrophora_.
=Group 4.= The numerous acids of this group are not easily soluble and
have a very bitter taste. They are not coloured by CaCl; when extracted
with concentrated sulphuric acid, the solution obtained is reddish-yellow
or deep red. Among the most frequent are fumarprotocetraric acid
(C₆₂H₅₀O₃₅), the bitter principle of _Cetraria islandica_, _Cladonia
rangiferina_, etc., psoromic acid (C₂₀H₁₄O₉), obtained from _Alectoria
implexa_, _Lecanora varia_, _Cladonia pyxidata_ and many other lichens,
and salazinic acid (C₁₉H₁₄O₁₀), recorded by Zopf as occurring in
_Stereocaulon salazinum_ and in several _Parmeliae_, but now found by
Lettau[829] to be very wide-spread. He used micro-chemical methods and
detected its presence in 72 species from twelve different families. The
distribution of the acid in the thallus varies considerably.
=Group 5.= This is called the atranorin group from one of the most
important members. They are colourless substances and, like the preceding
group, are not affected by CaCl, but when split they form bodies that
colour a more or less deep red with that reagent. Atranorin (C₁₉H₁₈O₈)
is one of the most widely spread of all lichen-acids; it occurs in
Lecanoraceae, Parmeliaceae, Physciaceae and Lecideaceae. Barbatinic acid
(C₁₉H₂₀O₇), another member, is found in _Usnea ceratina_, _Alectoria
ochroleuca_ and in a variety of _Rhizocarpon geographicum_. A very large
number of acids more or less fully studied belong to this group.
SUBSERIES II. ANTHRACENE DERIVATIVES
The constituents of this subseries are derived from the carbohydrate
anthracene, and are characterized by their brilliant colours, yellow,
red, brown, red-brown or violet-brown. So far, only ten different kinds
have been isolated and studied. Parietin[830] (C₁₆H₁₂O₅), one of the best
known, has been extracted from _Xanthoria parietina_, _Placodium murorum_
and several other bright-yellow lichens; solorinic acid (C₁₅H₁₄O₅)
occurs in orange-red crystals on the hyphae of the pith and under surface
of _Solorina crocea_; nephromin (C₁₆H₁₂O₆) is found in the yellow medulla
of _Nephromium lusitanicum_; rhodocladonic acid (C₁₂H₈O₆ or C₁₄H₁₀O₇) is
the red substance in the apothecia of the red-fruited _Cladoniae_.
There are, in addition, a short series of coloured substances which
are of uncertain position. They are imperfectly known and are of rare
occurrence. An acid containing nitrogen has been extracted from _Roccella
fuciformis_, and named picroroccellin[831] (C₂₇H₂₉N₃O₅). It crystallizes
in comparatively large prisms, has an exceedingly bitter taste, and is
very sparingly soluble. It is the only lichen-acid in which nitrogen has
been detected.
One acid at least, belonging to the Fat series, vulpinic acid, which
gives the greenish-yellow colour to _Letharia vulpina_, has been prepared
synthetically by Volkard[832].
F. CHEMICAL REAGENTS AS TESTS FOR LICHENS
The employment of chemical reagents as colour tests in the determination
of lichen species was recommended by Nylander[833] in a paper published
by him in 1866. Many acids had already been extracted and examined, and
as they were proved to be constant in the different species where they
occurred, he perceived their systematic importance. As an example of
the new tests, he cited the use of hypochlorite of lime, a solution of
which, applied directly to the thallus of species of _Roccella_, produced
a bright-red “erythrinic” reaction. Caustic potash was also found to
be of service in demonstrating the presence of parietin in lichens by
a beautiful purple stain. Many lichenologists eagerly adopted the new
method, as a sure and ready means of distinguishing doubtful species; but
others have rejected the tests as unnecessary and not always to be relied
on, seeing that the acids are not always produced in sufficient abundance
to give the desired reaction, and that they tend to alter in time.
The reagents most commonly in use are caustic potash, generally indicated
by K; hypochlorite of calcium or bleaching powder by CaCl; and a solution
of iodine by I. The sign + signifies a colour reaction, while- indicates
that no change has followed the application of the test solution. Double
signs ⁺₊ or any similar variation indicate the upper or lower parts of
the thallus affected by the reagent. In some instances the reaction only
follows after the employment of two reagents represented thus: K(CaCl)+.
In such a case the potash breaks up the particular acid and compounds are
formed which become red, orange, etc., on the subsequent application of
hypochlorite of lime.
As an instance of the value of chemical tests, Zopf cites the reaction
of hypochlorite of lime on the thallus of four different species of
_Gyrophora_, the “tripe de roche”:—
Gyrophora torrefacta CaCl ⁻₊.
” polyrhiza CaCl ⁺₊.
” proboscidea CaCl ⁺₋.
” erosa CaCl ⁻₋.
It must however be borne in mind that these species are well
differentiated and can be recognized, without difficulty, by their
morphological characters. Experienced systematists like Weddell refuse
to accept the tests unless they are supported by true morphological
distinctions, as the reactions are not sufficiently constant.
G. CHEMICAL REACTIONS IN NATURE
Similar colour changes may often be observed in nature. The acids of
the exposed thallus cortex are not unfrequently split up by the gradual
action of the ammonia in the atmosphere, one of the compounds thus set
free being at the same time coloured by the alkali. Thus salazinic acid,
a constituent of several of our native _Parmeliae_, is broken up into
carbonic acid and salazininic acid, the latter taking a red colour.
Fumarprotocetraric acid is acted on somewhat similarly, and the red
colour may be seen in _Cetraria_ at the base of the thallus where contact
with soil containing ammonia has affected the outer cortex of the plant.
The same results are produced still more effectively when the lichen
comes into contact with animal excrement.
Gummy exudations from trees which are more or less ammoniacal may also
act on the thallus and form red-coloured products on contact with the
acids present. _Lecanora_ (_Aspicilia_) _cinerea_ is so easily affected
by alkalies that a thin section left exposed may become red in time owing
to the ammonia in the atmosphere.
II. GENERAL NUTRITION
A. ABSORPTION OF WATER
Lichens are capable of enduring almost complete desiccation, but though
they can exist with little injury through long periods of drought,
water is essential to active metabolism. They possess no special organs
for water conduction, but absorb moisture over their whole surface.
Several interdependent factors must therefore be taken into account in
considering the question of absorption: the type of thallus, whether
gelatinous or non-gelatinous, crustaceous, foliose or fruticose, as
also the nature of the substratum and the prevailing condition of the
atmosphere.
_a._ GELATINOUS LICHENS. The algal constituent of these lichens is some
member of the Myxophyceae and is provided with thick gelatinous walls
which have great power of imbibition and swell up enormously in damp
surroundings, becoming reservoirs of water. Species of _Collema_, for
instance, when thoroughly wet, weigh thirty-five times more than when
dry[834]. There are no interstices in the thallus and frequently no
cortex in these lichens, but the gelatinous substance itself forms on
drying an outer skin that checks evaporation so that water is retained
within the thallus for a longer period than in non-gelatinous forms.
They probably always retain some amount of moisture, as they share with
gelatinous algae the power of revival after long desiccation.
Gelatinous lichens are entirely dependent on a surface supply of water:
their hyphae—or rhizinae when present—rarely penetrate the substratum.
_b._ CRUSTACEOUS NON-GELATINOUS LICHENS. The lichens with this type
of thallus are in intimate contact with the substratum whether it be
soil, rock, tree or dead wood. The hyphae on the under surface of the
thallus function primarily as hold-fasts, but if water be retained in
the substratum, the lichen will undoubtedly benefit, and water, to some
extent, will be absorbed by the walls of the hyphae or will be drawn up
by capillary attraction. In any case, it could only be surface water
that would be available, as lichens have no means of tapping any deeper
sources of supply.
Lichens are, however, largely independent of the substratum for their
supply of water. Sievers[835], who gave attention to the subject, found
that though some few crustaceous lichens took up water from below, most
of them absorbed the necessary moisture on the surface or at the edges of
the thallus or areolae, where the tissue is looser and more permeable.
The swollen gelatinous walls of the hyphae forming the upper layers of
such lichens are admirably adapted for the reception and storage of
water, though, according to Zukal[836], less hygroscopic generally than
in the larger forms. Beckmann[837] proved this power of absorption,
possessed by the upper cortex, by placing a crustaceous lichen,
_Haematomma_ sp., in a damp chamber: he found after a while that water
had been taken up by the cortex and by the gonidial zone, while the lower
medullary hyphae had remained dry.
Herre[838] has recorded an astonishing abundance of lichens from the
desert of Reno, Nevada, and these are mostly crustaceous forms, belonging
to a limited number of species. The yearly rainfall of the region is only
about eight or ten inches, and occurs during the winter months, chiefly
as snow. It is during that period that active vegetation goes on; but
the plants still manage to exist during the long arid summer, when their
only possible water supply is that obtained from the moisture of the
atmosphere during the night, or from the surface deposit of dews.
_c._ FOLIOSE LICHENS. Though many of the leafy lichens are provided with
a tomentum of single hyphae, or with rhizinae on the under surface, the
principal function of these structures is that of attaching the thallus.
Sievers[839] tested the areas of absorption by placing pieces of the
thallus of _Parmeliae_, of _Evernia furfuracea_, and of _Cetraria glauca_
in a staining solution. After washing and cutting sections, it was seen
that the coloured fluid had penetrated by the upper surface and by the
edge of the thallus, as in crustaceous forms, but not through the lower
cortex.
By the same methods of testing, he proved that water penetrates not only
by capillarity between the closely packed hyphae, but also within the
cells. A considerable number of lichens were used for experiment, and
great variations were found to exist in the way in which water was taken
up. It has been proved that in some species of _Gyrophora_ water is
absorbed from below: in those in which rhizinae are abundant, water is
held by them and so gradually drawn up into the thallus; the upper cortex
in this genus is very thick and checks transpiration. Certain other
northern lichens such as _Cetraria islandica_, _Cladonia rangiferina_,
etc., imbibe water very slowly, and they, as well as _Gyrophora_, are
able to endure prolonged wet periods.
That foliose lichens do not normally contain much water was proved by
Jumelle[840] who compared the weight of seven different species when
freshly gathered, and after being dried; he found that the proportion
of fresh weight to dry weight showed least variation in _Parmelia
acetabulum_, as 1·14 to 1; in _Xanthoria parietina_ it was as 1·21 to 1.
_d._ FRUTICOSE LICHENS. There is no water-conducting tissue in the
elongate thallus of the shrubby or filamentous lichens, as can easily
be tested by placing the base in water: it will then be seen that the
submerged parts alone are affected. Many lichens are hygroscopic and
become water-logged when placed simply in damp surroundings. The thallus
of _Usnea_, for instance, can absorb many times its weight of water:
a mass of _Usnea_ filaments that weighed 3·8 grms. when dry increased
to 13·3 grms. after having been soaked in water for twelve hours.
Schrenk[841], who made the experiment, records in a second instance an
increase in weight from 3·97 grms. to 11·18 grms. The _Cladoniae_ retain
large quantities of water in their upright hollow podetia. The Australian
species, _Cladonia retepora_, the podetium of which is a regular network
of holes, competes with the _Sphagnum_ moss in its capacity to take up
water.
To conclude: as a rule, heteromerous, non-gelatinous lichens do not
contain large quantities of water, the weight of fresh plants being
generally about three times only that of the dry weight. Their ordinary
water content is indeed smaller than that of most other plants, though
it varies at once with a change in external conditions. It is noteworthy
that a number of lichens have their habitat on the sea-shore, constantly
subject to spray from the waves, but scarcely any can exist within the
spray of a waterfall, possibly because the latter is never-ceasing.
B. STORAGE OF WATER
The gonidial algae _Gloeocapsa_, _Scytonema_, _Nostoc_, etc. among
Myxophyceae, _Palmella_ and occasionally _Trentepohlia_ among
Chlorophyceae, have more or less gelatinous walls which act as a natural
reservoir of water for the lichens with which they are associated.
In these lichens the hyphae for the most part have thin walls, and
the plectenchyma when formed—as below the apothecium in _Collema
granuliferum_, or as a cortical layer in _Leptogium_—is a thin-walled
tissue. In lichens where, on the contrary, the alga is non-gelatinous—as
generally in Chlorophyceae—or where the gelatinous sheath is not formed
as in the altered _Nostoc_ of the _Peltigera_ thallus, the fungal hyphae
have swollen gelatinous walls both in the pith and the cortex, and not
only imbibe but store up water.
Bonnier[842] had his attention directed to this thickening of the
cell-walls as he followed the development of the lichen thallus. He made
cultures from the ascospore of _Physcia_ (_Xanthoria_) _parietina_ and
obtained a fair amount of hyphal tissue, the cell-walls of which became
thickened, but more slowly and to a much less extent than when associated
with the gonidia.
He noted also that when his cultures were kept in a continuously moist
atmosphere there was much less thickening, scarcely more than in fungi
ordinarily; it was only when they were grown under drier conditions
with necessity for storage, that any considerable swelling of the walls
took place. Further he found that the thallus of forms cultivated in
an abundance of moisture could not resist desiccation as could those
with the thicker membranes. These latter survived drying up and resumed
activity when moisture was supplied.
C. SUPPLY OF INORGANIC FOOD
As in the higher plants, mineral substances can only be taken up when
they are in a state of solution. Lichens are therefore dependent on the
substances that are contained in the water of absorption: they must
receive their inorganic nutriment by the same channels that water is
conveyed to them.
_a._ FOLIOSE AND FRUTICOSE LICHENS. These larger lichens are provided
with rhizinae or with hold-fasts, which are only absorptive to a very
limited extent; the main source of water supply is from the atmosphere
and the salts required in the metabolism of the cell must be obtained
there also—from atmospheric dust dissolved in rain, or from wind-borne
particles deposited on the surface of the thallus which may be
gradually dissolved and absorbed by the cortical and growing hyphae.
That substances received from the atmospheric environment may be all
important is shown by the exclusive habitat of some marine lichens; the
_Roccellae_, _Lichinae_, some species of _Ramalina_ and others which
grow only on rocky shores are almost as dependent on sea-water as are
the submerged algae. Other lichens, such as _Hydrothyria venosa_ and
_Lecanora lacustris_, grow in streams, or on boulders that are subject to
constant inundation, and they obtain their inorganic food mainly, if not
entirely, from an aqueous medium.
Though lichens cannot live in an atmosphere polluted by smoke, they
thrive on trees and walls by the road-side where they are liable to be
almost smothered by soil-dust. West[843] has observed that they flourish
in valleys that are swept by moisture laden winds more especially if
near to a highway, where animal excreta are mingled with the dust. The
favourite habitats of _Xanthoria parietina_ are the walls and roofs
of farm-buildings where the dust must contain a large percentage of
nitrogenous material; or stones by the sea-shore that are the haunts
of sea-birds. Sandstede[844] found on the island of Rügen that while
the perpendicular faces of the cliffs were quite bare, the tops bore
a plentiful crop of _Lecanora saxicola_, _Xanthoria lychnea_ and
_Candellariella vitellina_. He attributed their selection of habitat
to the presence of the excreta of sea-birds. As already stated the
connection of foliose and fruticose lichens with the substratum is
mainly mechanical but occasionally a kind of semiparasitism may arise.
Friedrich[845] gives an instance in a species of _Usnea_ of unusually
vigorous development. It grew on bark and the strands of hyphae,
branching from the root-base of the lichen, had reached down to the
living tissue of the tree-trunk and had penetrated between the cells by
dissolving the middle lamella. It was possible to find holes pierced
in the cell-walls of the host, but it was difficult to decide if the
hyphae had attacked living cells or were merely preying on dead material.
Lindau[846] held very strongly that lichen hyphae were non-parasitic, and
merely split apart the tissues already dead, and the instance recorded by
Friedrich is of rare occurrence[847].
That the substratum does have some indirect influence on these larger
lichens has been proved once and again. Uloth[848], a chemist as well as
a botanist, made analyses of plants of _Evernia prunastri_ taken from
birch bark and from sandstone. Qualitatively the composition of the
lichen substances was the same, but the quantities varied considerably.
Zopf[849] has, more recently, compared the acid content of a form of
_Evernia furfuracea_ on rock with that of the same species growing on
the bark of a tree. In the case of the latter, the thallus produced 4
per cent. of physodic acid and 2·2 per cent. of atranorin. In the rock
specimen, which, he adds, was a more graceful plant than the other, the
quantities were 6 per cent. of physodic acid, and 2·75 per cent. of
atranorin. In both cases there was a slight formation of furfuracinnic
acid. He found also that specimens of _Evernia prunastri_ on dead wood
contained 8·4 per cent. of lichen-acids, while in those from living trees
there was only 4·4 per cent. or even less. Other conditions, however,
might have contributed to this result, as Zopf[850] found later that this
lichen when very sorediate yielded an increased supply of atranoric acid.
Ohlert[851], who made a study of lichens in relation to their habitat,
found that though a certain number grew more or less freely on either
tree, rock or soil, none of them was entirely unaffected. _Usnea
barbata_, _Evernia prunastri_ and _Parmelia physodes_ were the most
indifferent to habitat; normally they are corticolous species, but
_Usnea_ on soil formed more slender filaments, and _Evernia_ on the same
substratum showed a tendency to horizontal growth, and became attached at
various points instead of by the usual single base.
_b._ CRUSTACEOUS LICHENS. The crustaceous forms on rocks are in a more
favourable position for obtaining inorganic salts, the lower medullary
hyphae being in direct contact with mineral substances and able to
act directly on them. Many species are largely or even exclusively
calcicolous, and there must be something in the lime that is especially
conducive to their growth. The hyphae have been traced into the limestone
to a depth of 15 mm.[852] and small depressions are frequently scooped
out of the rock by the action of the lichen, thus giving a lodgement to
the foveolate fruit.
On rocks mainly composed of silica, the lichen has a much harder
substance to deal with, and one less easily affected by acids, though
even silica may be dissolved in time. Uloth[853] concluded from his
observations that the relation of plants to the substratum was chemical
even more than physical, so far as crustaceous species were concerned.
He found that the surface of the area of rock inhabited was distinctly
marked: even such a hard substance as chalcedony was corroded by a
very luxuriant lichen flora, the border of growth being quite clearly
outlined. The corrosive action is due he considered to the carbon dioxide
liberated by the plant, though oxalic acid, so frequent a constituent
of lichens, may also share in the corrosion. Egeling[854] made similar
observations in regard to the effect of lichen growth on granite rocks;
and he further noticed that pieces of glass, over which lichens had
spread, had become clouded, the dulness of the surface being due to a
multitude of small cracks eaten out by the hyphae. Buchet[855] also
gives an instance of glass which had been corroded by the action of
lichen hyphae. It formed part of an old stained window in a chapel
that was obscured by a lichen growth which adhered tenaciously. When
the window was taken down and cleaned, it was found that the surface
of the glass was covered with small, more or less hemispherical pits
which were often confluent. The different colours in the picture were
unequally attacked, some of the figures or draperies being covered with
the minute excavations, while other parts were intact. It happened also,
occasionally, that a colour while slightly corroded in one pane would
be uninjured in another, but the suggestion is made that there might in
that case have been a difference in the length of attack by the lichen.
The selection of colours by the lichens might also be influenced by some
chemical or physical characters.
Bachmann[856] found that on granite there is equally a selection
of material by the hyphae: as a rule they avoid the acid silica
constituents; while they penetrate and traverse the grains of mica which
are dissolved by them exactly as are lime granules.
On another rock consisting mainly of muscovite and quartz he[857] found
that crystals of garnet embedded in the rock were reduced to a powder
by the action of the lichen. He concludes that the destroying action of
the hyphae is accelerated by the presence of carbon dioxide given off
by the lichen, and dissolved in the surrounding moisture. Lang[858] and
Stahlecker[859] have both come to the conclusion that even the quartz
grains are corroded by the lichen hyphae. Stahlecker finds that they
change the quartz into amorphous silicic acid, and thus bring it into
the cycle of organic life. Chalk and magnesia are extracted from the
silicates where no other plant could procure them. Lichens are generally
rare on pure quartz rocks, chiefly, however, for the mechanical reason
that the structure is of too close a grain to afford a foothold.
D. SUPPLY OF ORGANIC FOOD
_a._ FROM THE SUBSTRATUM. The Ascomycetous fungi, from which so many
of the lichens are descended, are mainly saprophytes, obtaining their
carbohydrates from dead plant material, and lichen hyphae have in some
instances undoubtedly retained their saprophytic capacity. It has been
proved that lichen hyphae, which naturally could not exist without the
algal symbiont, may be artificially cultivated on nutrient media without
the presence of gonidia, though the chief and often the only source of
carbon supply is normally through the alga with which the hyphae are
associated in symbiotic union.
A large number of crustaceous lichens grow on the bark of trees, and
their hyphae burrow among the dead cells of the outer bark using up the
material with which they come in contact. Others live on dead wood,
palings, etc. where the supply of disintegrated organic substance is even
greater; or they spread over withered mosses and soil rich in humus.
_b._ FROM OTHER LICHENS. Bitter[860] has recorded several instances
observed by him of lichens growing over other lichens and using up their
substance as food material. Some lichens are naturally more vigorous than
others, and the weaker or more slow growing succumb when an encounter
takes place. _Pertusaria globulifera_ is one of these marauding species;
its habitat is among mosses on the bark of trees, and, being a quick
grower, it easily overspreads its more sluggish neighbours. It can
scarcely be considered a parasite, as the thallus of the victim is first
killed, probably by the action of an enzyme.
_Lecanora subfusca_ and allied species which have a thin thallus are
frequently overgrown by this _Pertusaria_ and a dark line generally
precedes the invading lichen; the hyphae and the gonidia of the
_Lecanorae_ are first killed and changed to a brown structureless mass
which is then split up by the advancing hyphae of the _Pertusaria_ into
small portions. A little way back from the edge of the predatory thallus
the dead particles are no longer visible, having been dissolved and
completely used up. _Pertusaria amara_ also may overgrow _Lecanorae_,
though, generally, its onward course is checked and deflected towards a
lateral direction; if however it is in a young and vigorous condition, it
attacks the thallus in its path, and ahead of it appears the rather broad
blackish line marking the fatal effect of the enzyme, the rest of the
host thallus being unaffected. Neither _Pertusaria_ seems to profit much,
and does not grow either faster or thicker; the thallus appears indeed to
be hindered rather than helped by the encounter. _Biatora_ (_Lecidea_)
_quernea_ with a looser, more furfuraceous thallus is also killed and
dissolved by _Pertusariae_; but if the _Biatora_ is growing near to a
withering or dead lichen it, also, profits by the food material at hand,
grows over it and uses it up. Bitter has also observed lichens overgrown
by _Haematomma_ sp.; the growth of that lichen is indeed so rapid that
few others can withstand its approach.
Another common rock species, _Lecanora sordida_ (_L. glaucoma_), has
a vigorous thallus that easily ousts its neighbours. _Rhizocarpon
geographicum_, a slow-growing species, is especially liable to be
attacked; from the thallus of _L. sordida_ the hyphae in strands push
directly into the other lichen in a horizontal direction and split up the
tissues, the algae persist unharmed for some time, but eventually they
succumb and are used up; the apothecia, though more resistant than the
thallus, are also gradually undermined and hoisted up by the new growth,
till finally no trace of the original lichen is left. _Lecanora sordida_
is however in turn invaded by _Lecidea insularis_ (_L. intumescens_)
which is found forming small orbicular areas on the _Lecanora_ thallus.
It kills its host in patches and the dead material mostly drifts away. On
any strands that are left _Candellariella vitellina_ generally settles
and evidently profits by the dead nutriment. It does not spread to the
living thallus. _Lecanora polytropa_ also forms colonies on these vacant
patches, with advantage to its growth.
Even the larger lichens are attacked by these quick-growing crusts.
_Pertusaria globulifera_ spreads over _Parmelia perlata_ and _P.
physodes_, gradually dissolving and consuming the different thalline
layers; the lower cortex of the victim holds out longest and can be seen
as an undigested black substance within the _Pertusaria_ thallus for some
time. As a rule, however, the lichens with large lobes grow over the
smaller thalli in a purely mechanical fashion.
_c._ FROM OTHER VEGETATION. Zukal[861] has given instances of association
between mosses and lichens in which the latter seemed to play the part of
parasite. The terricolous species _Baeomyces rufus_ (_Sphyridium_) and
_Biatora decolorans_, as well as forms of _Lepraria_ and _Variolaria_,
he found growing over mosses and killing them. Stems and leaves of the
moss _Plagiothecium sylvaticum_ were grown through and through by the
hyphae of a _Pertusaria_, and he observed a leaf of _Polytrichum commune_
pierced by the rhizinae of a minute _Cladonia_ squamule. The cells had
been invaded and the neighbouring tissue was brown and dead.
Perhaps the most voracious consumer of organic remains is _Lecanora
tartarea_, more especially the northern form _frigida_. It is the
well-known cudbear lichen of West Scotland, and is normally a rock
species. It has an extremely vigorous thickly crustaceous and
quick-growing thallus, and spreads over everything that lies in its
path—decaying mosses, dead leaves, other lichens, etc. Kihlman[862] has
furnished a graphic description of the way it covers up the vegetation
on the high altitudes of Russian Lapland. More than any other plant it
is able to withstand the effect of the cold winds that sweep across
these inhospitable plains. Other plant groups at certain seasons or in
certain stages of growth are weakened or killed by the extreme cold of
the wind, and, immediately, a growth of the more hardy grey crust of
_Lecanora tartarea_ begins to spread over and take possession of the area
affected—very frequently a bank of mosses, of which the tips have been
destroyed, is thus covered up. In the same way the moorland _Cladoniae_,
_C. rangiferina_ (the reindeer moss) and some allied species, are
attacked. They have no continuous cortex, the outer covering of the long
branching podetia being a loose felt of hyphae; they are thus sensitive
to cold and liable to be destroyed by a high wind, and their stems,
which are blackened as decay advances, become very soon dotted with the
whitish-grey crust of the more vigorous and resistant _Lecanora_.
III. ASSIMILATION AND RESPIRATION
A. INFLUENCE OF TEMPERATURE
_a._ HIGH TEMPERATURE. It has been proved that plants without chlorophyll
are less affected by great heat than those that contain chlorophyll.
Lichens in which both types are present are more capable of enduring
high temperatures than the higher plants, but with undue heat the alga
succumbs first. In consequence, respiration, by the fungus alone, can go
on after assimilation (photosynthesis) and respiration in the alga have
ceased.
Most Phanerogams cease assimilation and respiration after being subjected
for ten minutes to a temperature of 50° C. Jumelle[863] made a series of
experiments with lichens, chiefly of the larger fruticose or foliaceous
types, with species of _Ramalina_, _Physcia_ and _Parmelia_, also with
_Evernia prunastri_ and _Cladonia rangiferina_. He found that as regards
respiration, plants which had been kept for three days at 45° C., fifteen
hours at 50°, then five hours at 60°, showed an intensity of respiration
almost equal to untreated specimens, gaseous interchange being manifested
by an absorption of oxygen and a giving up of carbon dioxide.
The power of assimilation was more quickly destroyed: as a rule it
failed after the plants had been subjected successively to a temperature
of one day at 45° C., then three hours at 50° and half-an-hour at 60°.
The assimilating green alga, being less able to resist extreme heat, as
already stated, succumbed more quickly than the fungus. Jumelle also
gives the record of an experiment with a crustaceous lichen, _Lecidea_
(_Lecanora_) _sulphurea_, a rock species. It was kept in a chamber heated
to 50° for three hours and when subsequently placed in the sunlight
respiration took place but no assimilation.
Very high temperatures may be endured by lichen plants in quite natural
conditions, when the rock or stone on which they grow becomes heated by
the sun. Zopf[864] tested the thalli of crustaceous lichens in a hot
June, under direct sunlight, and found that the thermometer registered
55° C.
_b._ LOW TEMPERATURE. Lichens support extreme cold even better than
extreme heat. In both cases it is the power of drying up and entering
at any season into a condition of lowered or latent vitality that
enables them to do so. In winter during a spell of severe cold they are
generally in a state of desiccation, though that is not always the case,
and resistance to cold is not due to their dry condition. The water of
imbibition is stored in the cell-walls and it has been found that lichens
when thus charged with moisture are able to resist low temperatures, even
down to -40° C. or -50° as well as when they are dry. Respiration in that
case was proved by Jumelle[865] to continue to -10°, but assimilation
was still possible at a temperature of -40°: _Evernia prunastri_ exposed
to that extreme degree of cold, but in the presence of light, decomposed
carbon dioxide and gave off oxygen.
B. INFLUENCE OF MOISTURE
_a._ ON VITAL FUNCTIONS. Gaseous interchange has been found to vary
according to the degree of humidity present[865]. In lichens growing in
sheltered positions, or on soil, there is less complete desiccation, and
assimilation and respiration may be only enfeebled. Lichens more exposed
to the air—those growing on trees, etc.—dry almost completely and gaseous
interchange may be no longer appreciable. In severe cold any water
present would become frozen and the same effect of desiccation would be
produced. At normal temperatures, on the addition of even a small amount
of moisture the respiratory and assimilative functions at once become
active, and to an increasing degree as the plant is further supplied with
water until a certain optimum is reached, after which the vital processes
begin somewhat to diminish.
Though able to exist with very little moisture, lichens do not endure
desiccation indefinitely, and both assimilation and respiration probably
cease entirely during very dry seasons. A specimen of _Cladonia
rangiferina_ was kept dry for three months, and then moistened:
respiration followed but it was very feeble and assimilation had almost
entirely ceased. Somewhat similar results were obtained with _Ramalina
farinacea_ and _Usnea barbata_.
In normal conditions of moisture, and with normal illumination,
assimilation in lichens predominates over respiration, more carbon
dioxide being decomposed than is given forth; and Jumelle has argued from
that fact, that the alga is well able to secure from the atmosphere all
the carbon required for the nutrition of the whole plant. The intensity
of assimilation, however, varies enormously in different lichens and is
generally more powerful in the larger forms than in the crustaceous: the
latter have often an extremely scanty thallus and they are also more in
contact with the substratum—rock, humus or wood—on which they may be
partly saprophytic, thus obtaining carbohydrates already formed, and
demanding less from the alga.
An interesting comparison might be made with fungi in regard to which
many records have been taken as to their possible duration in a dry
state, more especially on the viability of spores, _i.e._ their
persistent capacity of germination. A striking instance is reported
by Weir[866] of the regeneration of the sporophores of _Polystictus
sanguineus_, a common fungus of warm countries. The plant was collected
in Brazil and sent to Munich. After about two years in the mycological
collection of the University, the branch on which it grew was exposed
in the open among other branches in a wood while snow still lay on the
ground. In a short time the fungus revived and before the end of spring
not only had produced a new hymenium, but enlarged its hymenial surface
to about one-fourth of its original size and had also formed one entirely
new, though small, sporophore.
_b._ ON GENERAL DEVELOPMENT. Lichens are very strongly influenced by
abundance or by lack of moisture. The contour of the large majority
of species is concentric, but they become excentric owing to a more
vigorous development towards the side of damper exposure, hence the
frequent one-sided increase of monophyllous species such as _Umbilicaria
pustulata_. Wainio[867] observed that species of _Cladonia_ growing
in dry places, and exposed to full sunlight, showed a tendency not to
develop scyphi, the dry conditions hindering the full formation of the
secondary thallus. As an instance may be cited _Cl. foliacea_, in which
the primary thallus is much the most abundantly developed, its favourite
habitat being the exposed sandy soil of sea-dunes.
Too great moisture is however harmful: Nienburg[868] has recorded his
observations on _Sphyridium_ (_Baeomyces rufus_): on clay soil the
thallus was pulverulent, while on stones or other dryer substratum it was
granular—warted or even somewhat squamulose.
_Parmelia physodes_ rarely forms fruits, but when growing in an
atmosphere constantly charged with moisture[869], apothecia are more
readily developed, and the same observation has been made in connection
with other usually barren lichens. It has been suggested that, in these
lichens, the abrupt change from moist to dry conditions may have a
harmful effect on the developing ascogonium.
The perithecia of _Pyrenula nitida_ are smaller on smooth bark[870] such
as that of _Corylus_, _Carpinu_s, etc., probably because the even surface
does not retain water.
IV. ILLUMINATION OF LICHENS
A. EFFECT OF LIGHT ON THE THALLUS
As fungi possess no chlorophyll, their vegetative body has little or no
use for light and often develops in partial or total darkness. In lichens
the alga requires more or less direct illumination; the lichen fungus,
therefore, in response to that requirement has come out into the open:
it is an adaptation to the symbiotic life, though some lichens, such as
those immersed in the substratum, grow with very little light. Like other
plants they are sensitive to changes of illumination: some species are
shade plants, while others are as truly sun plants, and others again are
able to adapt themselves to varying degrees of light.
Wiesner[871] made a series of exact observations on what he has
termed the “light-use” of various plants. He took as his standard of
unity for the higher plants the amount of light required to darken
photographic paper in one second. When dealing with lichens he adopted
a more arbitrary standard, calculating as the unit the average amount
of light that lichens would receive in entirely unshaded positions. He
does not take account of the strength or duration of the light, and
the conclusions he draws, though interesting and instructive, are only
comparative.
_a._ SUN LICHENS. The illumination of the Tundra lichens is reckoned by
Wiesner as representing his unit of standard illumination. In the same
category as these are included many of our most familiar lichens, which
grow on rocks subject to the direct incidence of the sun’s rays, such as,
for instance, _Parmelia conspersa_, _P. prolixa_, etc. _Physcia tenella_
(_hispida_) is also extremely dependent on light, and was never found by
Wiesner under 1/8 of full illumination. _Dermatocarpon miniatum_, a rock
lichen with a peltate foliose thallus, is at its best from 1/3 to 1/8 of
illumination, but it grows well in situations where the light varies in
amount from 1 to 1/24. _Psora_ (_Lecidea_) _lurida_, with dark-coloured
crowded squamules, grows on calcareous soil among rocks well exposed
to the sun and has an illumination from 1 to 1/30, but with a poorer
development at the lower figure. Many crustaceous rock lichens are also
by preference sun-plants as, for instance, _Verrucaria calciseda_ which
grows immersed in calcareous rocks but with an illumination of 1 to 1/3;
in more shady situations, where the light had declined to 1/29, it was
found to be less luxuriant and less healthy.
Sun lichens continue to grow in the shade, but the thallus is then
reduced and the plant is sterile. Zukal has made a list of those which
grow best with a light-use of 1 to 1/10, though they are also found not
unfrequently in habitats where the light cannot be more than 1/50. Among
these light-loving plants are the Northern Tundra species of _Cladonia_,
_Stereocaulon_, _Cetraria_, _Parmelia_, _Umbilicaria_, and _Gyrophora_,
as also _Xanthoria parietina_, _Placodium elegans_, _P. murorum_, etc.,
with some crustaceous species such as _Lecanora atra_, _Haematomma
ventosum_, _Diploschistes scruposus_, many species of Lecideaceae, some
Collemaceae and some Pyrenolichens.
Wiesner’s conclusion is that the need of light increases with the
lowering of the temperature, and that full illumination is of still more
importance in the life of the plants when they grow in cold regions and
are deprived of warmth: sun lichens are, therefore, to be looked for in
northern or Alpine regions rather than in the tropics.
_b._ COLOUR-CHANGES DUE TO LIGHT. Lichens growing in full sunlight
frequently take on a darker hue. _Cetraria islandica_ for instance in an
open situation is darker than when growing in woods; _C. aculeata_ on
bare sand-dunes is a deeper shade of brown than when growing entangled
among heath plants. _Parmelia saxatilis_ when growing on exposed rocks is
frequently a deep brown colour, while on shaded trees it is normally a
light bluish-grey.
An example of colour-change due directly to light influences is given by
Bitter[872]. He noted that the thallus of _Parmelia obscurata_ on pine
trees, and therefore subject only to diffuse light, grew to a large size
and was of a light greyish-green colour marked by lighter-coloured lines,
the more exposed lobes being always the most deeply tinted. In a less
shaded habitat or in full sunlight the lichen was distinguished by a much
darker colour, and the lobes were seamed and marked by blackish lines and
spots. Bruce Fink[873] noted a similar development of dark lines on the
thallus of certain rock lichens growing in the desert, more especially on
_Parmelia conspersa_, _Acarospora xanthophana_ and _Lecanora muralis_. He
attributes a protective function to the dark colour and observes that it
seemingly spreads from centres of continued exposure, and is thus more
abundant in older parts of the thallus. He contrasts this colouration
with the browning of the tips of the fronds of fruticose lichens by which
the delicate growing hyphae are protected from intense light.
Galløe[874] finds that protection against too strong illumination is
afforded both by white and dark colourations, the latter because the
pigments catch the light rays, the former because it throws them back.
The white colour is also often due to interspaces filled with air which
prevent the penetration of the heat rays.
A deepening of colour due to light effect often visible on exposed
rock lichens such as _Parmelia saxatilis_ is more pronounced still in
Alpine and tropical species: the cortex becomes thicker and more opaque
through the cuticularizing and browning of the hyphal membranes, and the
massing of crystals on the lighted areas. The gonidial layer becomes,
in consequence, more reduced, and may disappear altogether. Zukal[875]
found instances of this in species of _Cladonia_, _Parmelia_, _Roccella_,
etc. The thickened cortex acts also as a check to transpiration and
is characteristic of desert species exposed to strong light and a dry
atmosphere.
Bitter[876] remarked the same difference of development in plants of
_Parmelia physodes_: he found that the better lighted had a thicker
cortex, about 20-30 µ in depth, as compared with 15-22 µ or even only 12
µ in the greener shade-plants, and also that there was a greater deposit
of acids in the more highly illuminated cortices, thus giving rise to the
deeper shades of colour.
Many lichens owe their bright tints to the presence of coloured
lichen-acids, the production of which is strongly influenced by light and
by clear air. _Xanthoria parietina_ becomes a brilliant yellow in the
sunlight: in the shade it assumes a grey-green hue and yields only small
quantities of parietin. _Placodium elegans_, normally a brightly coloured
yellow lichen, becomes, in the strong light of the high Alps, a deep
orange-red. _Rhizocarpon geographicum_ is a vivid citrine-yellow on high
mountains, but is almost green at lesser elevations.
_c._ SHADE LICHENS. Many species grow where the light is abundant though
diffuse. Those on tree-trunks rarely receive direct illumination and may
be generally included among shade-plants. Wiesner found that corticolous
forms of _Parmelia saxatilis_ grew best with an illumination between 1/8
and 1/17 of full light, and _Pertusaria amara_ from 1/12 to 1/21; both of
them could thrive from 1/3 to 1/56, but were never observed on trees in
direct light. _Physcia ciliaris_, which inhabits the trunks of old trees,
is also a plant that prefers diffuse light. In warm tropical regions,
lichens are mostly shade-plants: Wiesner records an instance of a species
found on the aerial roots of a tree with an illumination of only 1/250.
In a study of subterranean plants, Maheu[877] takes note of the lichens
that he found growing in limestone caves, in hollows and clefts of the
rocks, etc. A fair number grew well just within the opening of the
caves; but species such as _Cl. cervicornis_, _Placodium murorum_ and
_Xanthoria parietina_ ceased abruptly where the solar rays failed. Only
a few individuals of one or two species were found to remain normal in
semi-darkness: _Opegrapha hapalea_ and _Verrucaria muralis_ were found at
the bottom of a cave with the thallus only slightly reduced. The nature
of the substratum in these cases must however also be taken into account,
as well as the light influences: limestone for instance is a more
favourable habitat than gypsum; the latter, being more readily soluble,
provides a less permanent support.
Maheu has recorded observations on growth in its relation to light in the
case of a number of lichens growing in caves.
_Physcia obscura_ grew in almost total darkness; _Placodium murorum_
within the cave had lost nearly all colour; _Placodium variabile_
var. deep within the cave, sterile; _Opegrapha endoleuca_ in partial
obscurity; _Verrucaria rupestris_ f. in total obscurity, the thallus much
reduced and sterile; _Verrucaria rupestris_ in partial obscurity, the
asci empty; _Homodium_ (_Collema_) _granuliferum_ in the inmost recess of
the cave, sterile, and the hyphae more spongy than in the open.
Siliceous rocks in darkness were still more barren, but a few odd lichens
were collected from sandstone in various caves: _Cladonia squamosa_,
_Parmelia perlata_ var. _ciliata_, _Diploschistes scruposus_, _Lecidea
grisella_, _Collema nigrescens_ and _Leptogium lacerum_.
_d._ VARYING SHADE CONDITIONS. It has been frequently observed that on
the trees of open park lands lichens are more abundant on the side of
the trunk that faces the prevailing winds. Wiesner[878] remarks that
spores and soredia would more naturally be conveyed to that side; but
there are other factors that would come into play: the tree and the
branches frequently lean away from the wind, giving more light and also
an inclined surface that would retain water for a longer period on the
windward side[879]. Spores and soredia would also develop more readily in
those favourable conditions.
In forests there are other and different conditions: on the outskirts,
whether northern or southern, the plants requiring more light are to be
found on the side of the trunk towards the outside; in the depths of the
forest, light may be reduced from 1/200 to 1/300, and any lichens present
tend to become mere leprose crusts. Krempelhuber[880] has recorded among
his Bavarian lichens those species that he found constantly growing in
the shade: they are in general species of Collemaceae and Caliciaceae,
several species of _Peltigera_ (_P. venosa_, _P. horizontalis_ and _P.
polydactyla_); _Solorina saccata_; _Gyalecta Flotovii_, _G. cupularis_;
_Pannaria microphylla_, _P. triptophylla_, _P. brunnea_; _Icmadophila
aeruginosa_, etc.
B. EFFECT ON REPRODUCTIVE ORGANS
In the higher plants, it is recognized that a certain light-intensity is
necessary for the production of flowers and fruit. In the lower plants,
such as lichens, light is also necessary for reproduction; it is a
common observation that well-lighted individuals are the most abundantly
fruited. In the higher fungi also, the fruiting body is more or less
formed in the light.
_a._ POSITION AND ORIENTATION OF FRUITS WITH REGARD TO LIGHT. There is
an optimum of light for the fruits as well as for the thallus in each
species of lichen: in most cases it is the fullest light that can be
secured.
Zukal[881] finds an exception to that rule in species of _Peltigera_:
when exposed to strong sunlight, the lobes, fertile at the tips, curve
over so that to some extent the back of the apothecium is turned to
the light; with diffuse light, the horizontal position is retained and
the apothecia face upwards. In the closely allied genera _Nephroma_,
_Nephromium_ and _Nephromopsis_, the apothecia are produced on the back
of the lobe at the extreme tip, but as they approach maturity the fertile
lobes turn right back and they become exposed to direct illumination.
In a well-developed specimen the full-grown fruits may thus become so
prominent all over the thallus, that it is difficult to realize they are
on reversed lobes. In one species of _Cetraria_ (_C. cucullata_) the
rarely formed apothecia are adnate to the back of the lobe; but in that
case the margins of the strap-shaped fronds are incurved and connivent,
and the back is more exposed than the front.
In _Ramalina_ the frond frequently turns at a sharp angle at the point
of insertion of the apothecium which is thus well exposed and prominent;
but Zukal[882] sees in this formation an adaptation to enable the frond
to avoid the shade cast by the apothecium which may exceed it in width.
In most lichens, however, and especially in shade or semi-shade species,
the reproductive organs are to be found in the best-lighted positions.
_b._ INFLUENCE OF LIGHT ON COLOUR OF FRUITS. Lichen-acids are secreted
freely in the apothecium from the tips of the paraphyses which give the
colour to the disc, and as acid-formation is furthered by the sun’s rays,
the well-lighted fruits are always deeper in hue. The most familiar
examples are the bright-yellow species that are rich in chrysophanic acid
(parietin). Hedlund[883] has recorded several instances of varying colour
in species of _Micarea_ (_Biatorina_, etc.) in which very dark apothecia
became paler in the shade. He also cites the case of two crustaceous
species, _Lecidea helvola_ and _L. sulphurella_, which have white
apothecia in the shade, but are darker in colour when strongly lighted.
V. COLOUR OF LICHENS
The thalli of many lichens, more especially of those associated with
blue-green gonidia, are hygroscopic, and it frequently happens that
any addition of moisture affects the colour by causing the gelatinous
cell-walls to swell, thus rendering the tissues more transparent and the
green colour of the gonidia more evident. As a general rule it is the dry
state of the plant that is referred to in any discussion of colour.
In the large majority of species the colouring is of a subdued
tone—soft bluish-grey or ash-grey predominating. There are, however,
striking exceptions, and brilliant yellow and white thalli frequently
form a conspicuous feature of vegetation. Black lichens are rare,
but occasionally the very dark brown of foliaceous species such as
_Gyrophora_ or of crustaceous species such as _Verrucaria maura_ or
_Buellia atrata_ deepens to the more sombre hue.
A. ORIGIN OF LICHEN-COLOURING
The colours of lichens may be traced to several different causes.
_a._ COLOUR GIVEN BY THE ALGAL CONSTITUENT. As examples may be cited most
of the gelatinous lichens, Ephebaceae, Collemaceae, etc. which owe, as in
_Collema_, their dark olivaceous-green appearance, when somewhat moist,
to the enclosed dark-green gonidia, and their black colour, when dry, to
the loss of transparency. When the thallus is of a thin texture as in
_Collema nigrescens_, the olivaceous hue may remain constant. _Leptogium
Burgessii_, another thin plant of the same family, is frequently of a
purplish hue owing to the purple colour of the gonidial _Nostoc_ cells.
The dull-grey crustaceous thallus of the Pannariaceae becomes more or
less blue-green when moistened, and the same change has been observed in
the Hymenolichens, _Cora_, etc.
In _Coenogonium_, the alga is some species of _Trentepohlia_, a
filamentous genus mostly yellow, which often gives its colour to the
slender lichen filaments, the covering hyphae being very scanty. Other
filamentous species, such as _Usnea barbata_, etc., are persistently
greenish from the bright-green Protococcaceous cells lying near the
surface of the thalline strands. Many of the furfuraceous lichens are
greenish from the same cause, especially when moist, as are also the
larger lichens, _Physcia ciliaris_, Stereocaulons, Cladonias and others.
_b._ COLOUR DUE TO LICHEN-ACIDS. These substances, so characteristic of
lichens, are excreted from the hyphae, and lie in crystals on the outer
walls; they are generally most plentiful on exposed tissues such as
the cortex of the upper surface or the discs of the apothecia. Many of
these crystals are colourless and are without visible effect, except in
sometimes whitening the surface, strikingly exemplified in _Thamnolia
vermicularis_[884]; but others are very brightly coloured. These
latter belong to two chemical groups and are found in widely separated
lichens[885]:
1. Derivatives of pulvinic acid which are usually of a bright-yellow
colour. They are the colouring substance of _Letharia vulpina_, a
northern species, not found in our islands, of _Cetraria pinastri_ and
_C. juniperina_[886] which inhabit mountainous or hilly regions. The
crustaceous species, _Lecidea lucida_ and _Rhizocarpon geographicum_, owe
their colour to rhizocarpic acid.
The brilliant yellow of the crusts of some species of Caliciaceae is due
to the presence of the substance calycin, while coniocybic acid gives
the greenish sulphur-yellow hue to _Coniocybe furfuracea_. Epanorin
colours the hyphae and soredia of _Lecanora epanora_ a citrine-yellow and
stictaurin is the deep-yellow substance found in the medulla and under
surface of _Sticta aurata_ and _S. crocata_.
2. The second series of yellow acids are derivatives of anthracene. They
include parietin, formerly described as chrysophanic acid, which gives
the conspicuous colour to _Xanthoriae_ and to various wall lichens;
solorinic acid, the crystals of which cover the medullary hyphae and
give a reddish-grey tone to the upper cortex of _Solorina crocea_, and
nephromin which similarly colours the medulla of _Nephromium lusitanicum_
a deep yellow, the colour of the general thallus being, however, scarcely
affected. In this group must also be included the acids that cause
the yellow colouring of the medulla in _Parmelia subaurifera_ and the
yellowish thallus of some _Pertusariae_.
In many cases, changes in the normal colouring[887] are caused by the
breaking up of the acids on contact with atmospheric or soil ammonia.
Alkaline salts are thus formed which may be oxidized by the oxygen in
the air to yellow, red, brown, violet-brown or even to entirely black
humus-like products which are insoluble in water. These latter substances
are frequently to be found at the base of shrubby lichens or on the under
surface of leafy forms that are closely appressed to the substratum.
_c._ COLOUR DUE TO AMORPHOUS SUBSTANCES. These are the various pigments
which are deposited in the cell-walls of the hyphae. The only instance,
so far as is known, of colours within the cell occurs in _Baeomyces
roseus_, in which species the apothecia owe their rose-colour to
oil-drops in the cells of the paraphyses, and in _Lecidea coarctata_
where the spores are rose-coloured when young. In a few instances the
colouring matter is excreted (_Arthonia gregaria_ and _Diploschistes
ocellatus_); but Bachmann[888], who has made an extended study of this
subject and has examined 120 widely diversified lichens, found that with
few exceptions the pigment was in the membranes.
Bachmann was unable to determine whether the pigments were laid down
by the protoplasm or were due to changes in the cell-wall. The middle
layer, he found, was generally more deeply coloured than the inner one,
though that was not universal. In other cases the outer sheath was the
darkest, especially in cortices one to two cells thick such as those
of _Parmelia olivacea_, _P. fuliginosa_ and _P. revoluta_, and in the
brown thick-walled spores of _Physcia stellaris_ and of _Rhizocarpon
geographicum_. Still another variation occurs in _Parmelia tristis_ in
which the dark cortical cells show an outer colourless membrane over the
inner dark wall.
The coloured pigments are mainly to be found in the superficial tissues,
but if the thallus is split by areolation, as in crustaceous lichens, the
internal hyphae may be coloured like those of the outer cortex wherever
they are exposed. The hyphae of the gonidial layer are persistently
colourless, but the lower surface and the rhizoids of many foliose
lichens are frequently very deeply stained, as are the hypothalli of
crustaceous species.
The fruiting bodies in many different families of lichens have dark
coloured discs owing to the abundance of dark-brown pigment in the
paraphyses. In these the walls, as determined by Bachmann, are composed
generally of an inner wall, a second outer wall, and the outermost sheath
which forms the middle lamella between adjacent cells. In some species
the second wall is pigmented, in others the middle lamella is the one
deeply coloured. The hymenium of many apothecia and the hyphae forming
the amphithecium are often deeply impregnated with colour. The wall
hyphae of the pycnidia are also coloured in some forms; more frequently
the cells round the opening pore are more or less brown.
The presence of these coloured substances enables the cell-wall to resist
chemical reactions induced by the harmful influences of the atmosphere
or of the substratum. The darker the cell-wall and the more abundant
the pigment, the less easily is the plant injured either by acids or
alkalies. The coloured tips of the paraphyses thus give much needed
protection to the long lived sporiferous asci, and the dark thalline
tissues prevent premature rotting and decay.
_d._ ENUMERATION OF AMORPHOUS PIGMENTS:
=1. Green.= Bachmann found several different green pigments:
“Lecidea-green,” colouring red with nitric acid, is the dark blue-green
or olive-green (smaragdine) of the paraphyses of many apothecia in
the Lecideaceae, and may vary to a lighter blue; it appears almost
black in thalline cells[889]. “Aspicilia-green” occurs in the thalline
margin and sometimes in the epithecium of the fruits of species of
_Aspicilia_; it becomes a brighter green on the application of nitric
acid. “Bacidia-green,” also a rare pigment, becomes violet with the
same acid; it is found in the epithecium of _Bacidia muscorum_ and
_Bacidia acclinis_ (Lecideaceae). “Thalloidima-green” in the apothecia
of some species of _Biatorina_ is changed to a dirty-red by nitric acid
and to violet by potash. Still another termed “rhizoid-green” gives
the dark greenish colour to the rhizoids of _Physcia pulverulenta_
and _P. aipolia_ and to the spores of some species of _Physcia_ and
_Rhizocarpon_. It becomes more olive-green with potash.
=2. Blue.= A very rare colour in lichens, so far found in only a few
species, _Biatora_ (_Lecidea_) _atrofusca_, _Lecidea sanguinaria_ and
_Aspicilia flavida_ f. _coerulescens_. It forms a layer of amorphous
granules embedded in the outer wall of the paraphyses, becoming more
dense towards the epithecium. A few granules are also present in the
hymenium.
=3. Violet.= “Arthonia-violet” as it is called by Bachmann is a
constituent of the tissues of _Arthonia gregaria_, occurring in minute
masses always near the cortical cells; it is distinct from the bright
cinnabarine granules present in every part of the thallus.
=4. Red.= Several different kinds of red have been distinguished:
“Urceolaria-red,” visible as an interrupted layer on the upper side of
the medulla in the thallus of _Diploschistes ocellatus_, a continental
species with a massive, crustaceous, whitish thallus that shows a faint
rose tinge when wetted. “Phialopsis-red” is confined to the epithecium of
the brightly coloured apothecia of _Phialopsis rubra_. “Lecanora-red,”
by which Bachmann designates the purplish colour of the hymenium, is
an unfailing character of Lecanora atra; the colouring substance is
lodged in the middle lamella of the paraphysis cells; it occurs also in
_Rhizocarpon geographicum_ and in _Rh. viridiatrum_; it becomes more
deeply violet with potash. M. C. Knowles[890] noted the blue colouring of
_Rh. geographicum_ growing in W. Ireland near the sea and she ascribed
it to an alkaline reaction. Two more rare pigments, “Sagedia-red” and
“Verrucaria-red,” are found in species of Verrucariaceae. These tinge the
calcareous rocks in which the lichens are embedded a beautiful rose-pink.
They are scarcely represented in our country.
=5. Brown.= A frequent colouring substance, but also presenting several
different kinds of pigment which may be arranged in two groups:
=(1) Substances with some characteristic chemical reaction.= These are of
somewhat rare occurrence: “Bacidia-brown” in the middle lamella of the
paraphyses of _Bacidia fuscorubella_ stains a clear yellow with acids or
a violet colour with potash; “Sphaeromphale-brown,” which occurs in the
perithecia and in the cortex of _Staurothele clopismoides_, becomes deep
olive-green with potash, changing to yellow-brown on the application of
sulphuric acid; “Segestria-brown” in _Porina lectissima_ changes to a
beautiful violet colour with sulphuric acid, while “Glomellifera-brown,”
which is confined to the outer cortical cells of the upper surface of
_Parmelia glomellifera_, becomes blue with nitric and sulphuric acids,
but gives no reaction with potash. Rosendahl[891] confirmed Bachmann’s
discovery of this colour and further located it in corresponding cells of
_Parmelia prolixa_ and _P. locarensis_.
=(2) Substances with little or no chemical reaction.= There is only one
such to be noted: “Parmelia-brown,” usually a very dark pigment, which is
lodged in the outer membranes of the cells. It becomes a clearer colour
with nitric acid, and if the reagent be sufficiently concentrated, some
of the pigment is dissolved out. Some tissues, such as the lower cortex
of some _Parmeliae_, may be so impregnated and hardened, that nothing
short of boiling acid has any effect on the cells; membranes less deeply
coloured and changed, such as the cortex of the _Gyrophorae_, become
disintegrated with such drastic treatment. With potash the colour becomes
darker, changing from a clear brown to olivaceous-brown or-green, or in
some cases, as in a more faintly coloured epithecium, to a dirty-yellow,
but the lighter colour produced there is largely due to the swelling up
of the underlying tissues to which the potash penetrates readily between
the paraphyses.
“Parmelia-brown” is a colouring substance present in the dark epithecium
and hypothecium of the fruits of many widely diverse lichens, and in the
cortical cells and rhizoids of many thalli. In some plants the thallus is
brown both above and below, in others, as in _Parmelia revoluta_, etc.
only the under surface is dark-coloured.
_e._ COLOUR DUE TO INFILTRATION. There are several crustaceous lichens
that are rusty-red, the colour being due to the presence of iron. These
lichens occur on siliceous rocks of gneiss, granite, etc., and more
especially on rocks rich in iron. Iron as a constituent of lichens
was first demonstrated by John[892] in _Ramalina fraxinea_ and _R.
calicaris_. Grimbel[893] proved that the colour of rust lichens was due
to an iron salt, and Molisch[894] by microscopic examination located
minute granules of ferrous oxide as incrustations on the hyphae of
the upper surface of the thallus. Molisch held that the rhizoids or
penetrating hyphae dissolved the iron from the rocks by acid secretions.
Rust lichens however grow on rocks that are frequently under water in
which the iron is already present.
Among “rusty” lichens are the British forms, _Lecanora lacustris_, the
thallus of which is normally white, though generally more or less tinged
with iron; it inhabits rocks liable to inundation. _L. Dicksonii_ owes
its ferruginous colour to the same influences. _Lecidea contigua_ var.
_flavicunda_ and _L. confluens_ f. _oxydata_ are rusty conditions of
whitish-grey lichens.
Nilson[895] found rusty lichens occurring frequently in the
Sarak-Gebirge, more especially on glacier moraines where they were
liable, even when uncovered by snow, to be flooded by water from the
higher reaches. It is the thallus that is affected by the iron, rarely if
ever are apothecia altered in colour.
BACHMANN’S PIGMENT REACTIONS
----------------+---------------+-------------+-------------+----------+
Name of Pigment | Colour | KOH | NH₃ | Ba(OH)₂ |
or Lichen | | | | |
----------------+---------------+-------------+-------------+----------+
Lecidea-green | green | | | |
| | | | |
Aspicilia-green | green | | | |
| | | | |
Bacidia-green | green | | | |
| | | | |
Thalloidima- | green | violet | | |
green | | | | |
| | | | |
Rhizoid-green | bluish-green | olive-green | | |
| | to brown | | |
| | | | |
_Biatora | blue | dissolves | | |
atrofusca_ | | with | | |
| |greenish-blue| | |
| | colour | | |
| | | | |
_Phialopsis | brick-red | | dirty | |
rubra_ | | | purple-red | |
| | | | |
Lecanora-red | purple-red | | deep violet | |
| | | | |
_Sagedia | bluish-red | blue |greenish-blue| blue |
declivum_ | | (green) | then | |
| | | grey-black | |
| | | | |
_Verrucaria | rose-red | dark-green | |dark-green|
Hoffmanni_ f. | | | | |
_purpurascens_ | | | | |
| | | | |
_Bacidia |yellowish-brown| violet | violet | violet |
fuscorubella_ | | | | |
| | | | |
_Sphaeromphale | leather-brown | deep | | |
clopismoides_ | | olive-green | | |
| | | | |
_Segestria | yellow-brown | rose-red | | |
lectissima_— | | | | |
perithecia | | | | |
| | | | |
_Segestria | brown and | | | |
lectissima_— | colourless | | | |
entire tissue | | | | |
| | | | |
_Parmelia | leather-brown | | | |
glomellifera_ | | | | |
| | | | |
Parmelia-brown | yellow to |dirty- to | | |
|blackish-brown |olive-brown | | |
----------------+---------------+-------------+-------------+----------+
----------------+---------------+-------------+-------------------------
Name of Pigment | HNO₃ | H₂SO₄ | Special
or Lichen | | | Reactions
----------------+---------------+-------------+-------------------------
Lecidea-green | copper or | | KOH then HCl: blue
| brick-red | |
| | |
Aspicilia-green | | | HNO₃: brighter green
| | |
Bacidia-green | violet | violet | HCl: violet
| | |
Thalloidima- | indistinctly | | HCl: indistinctly
green | purple-red | | purple-red
| | |
| | |
Rhizoid-green | olive-green | |
| | |
_Biatora | violet, then | dissolves | H₂O insoluble
atrofusca_ | yellow, then | |
| decolourized | |
| | |
_Phialopsis | violet | |
rubra_ | | |
| | |
Lecanora-red | | |
| | |
_Sagedia | | |
declivum_ | | |
| | |
_Verrucaria | | | KOH then HNO₃ then
Hoffmanni_ f. | | | H₂SO₄: violet
_purpurascens_ | | | crystals
| | |
_Bacidia | | |
fuscorubella_ | | |
| | |
_Sphaeromphale | | | KOH, then H₂SO₄,
clopismoides_ | | | then HNO₃: blackish
| | |
_Segestria | bright yellow | | dilute H₂SO₄:
lectissima_— | | | bright yellow
perithecia | | |
| | |
_Segestria | | | Strong H₂SO₄: deep
lectissima_— | | | violet, then grey
entire tissue | | |
| | |
_Parmelia | blue, | | CaCl₂O₂: blue, then
glomellifera_ | then violet, | | grey; finally
| at last grey | | decolourized
| | |
Parmelia-brown | bright | |
| red-brown | |
----------------+---------------+-------------+-------------------------