LICHENS.
LICHENS are essentially air plants, being nourished, like the Algæ, by
the medium in which they grow. They vary from a pulverulent or dry
papillose crust, to a leathery or horny expansion, and even acquire an
erect stem. They are independent of the matrix to which they are
attached. Hence they spread their coloured frond, or thallus, in
circular or indefinite patches on old walls, the tiles of houses,
stones, and rocks. They appear in large expansions of red, golden
yellow, grey or white, on barren heaths, under plantations, and on the
stems of aged trees; while others of them often hang from the branches
like long shaggy grey hair, and many form forests of miniature bushes on
the northern plains. The lichen is the last trace of vegetation on the
tops of the mountains, and on the arctic deserts. Some lichens are
patient of severe cold, yet in general they prefer heat and moderate
warmth, and they love bright light so much, that they are usually
barren, or else yield little fruit under shade. Though differing greatly
from fungi in slowness of growth, length of life, and the power of
forming chlorophyll, they resemble them in having a mycelium in their
youth, and in their ascigerous fructification. A perfect lichen without
an ascus would be an anomaly, for the asci contain the true fruit,
associated with vertical threads or elongated cells called paraphyses,
which sometimes bear secondary spores on their summit. The asci with
their paraphyses inclosed in vessels of various shapes, called
perithecia, are aggregated in discs or shields, which form projections
on the surface of the plant. Some of these discs are closed, and give
egress to the spores through a fracture or pore on their surface; others
are open cups of various forms, either with stalks or sessile on the
frond, and through these the spores have egress. Hence the whole order
of lichens is naturally divided into two groups, according as their
discs are open (Gymnocarpei), or closed (Angiocarpei), the first being
incomparably the most important. Fig. 37 shows open, and sections of
closed cups, or perithecia.
[Illustration: Fig. 37. Lichens:—_a_, Trypethelium Sprengelii, pustules,
with sporidia; _b_, Verrucaria variolosa, section of perithecium with
sporidium; _c_, Endocarpon lacteum, thallus with section and fruit; _d_,
Stegobolus Berkeleianus, portion of plant with ascus, and sporidia.]
The highest type of horizontal lichens, of which fig. 37 _b_ is a
perpendicular section, has a firm, spreading, superficial crust or
surface, formed of oblong coloured cells, or of coloured filaments
closely aggregated, and which covers two distinct layers of cellular
tissue. In the layer immediately below the surface, the cells are
globular and of a paler colour; the second layer, or marrow, which is
the origin of fructification, consists of lax, detached, branching rows
of elongated cells and gelatinous matter. These branches of cells spring
up from a fourth layer, which is the base of the plant. It is of a
strong, firm, and tough nature, composed of interlaced filaments and is
often ribbed on the under-side. White fibres fix the base of the plant
to the surface on which it is spread; they are the remains of a mycelium
or matted mass of fibres, from whence lichens spring, which vanishes
when the plant is full grown.
[Illustration: Fig. 38. _a_, Sporopodium Leprieurii, ascus; _b_,
Coccocarpia smaragdina, section; _c_, Lecanora affinis, section.]
Globular bodies of vegetable green called gonidia, like those at the
base of fig. 39 _a_, are arranged in regular parallel rows, and placed
between the surface and the base of the lichen, as in fig. 38 _b_ and
_c_; they are said to proceed from the medullary layer, though Mr.
Berkeley has seen them springing from the threads of the mycelium of
Parmelia parietina. The gonidia find their way to the air through rents
in the surface of the plant, and are washed off by the rains, after
losing a little of their green colour. When they germinate they only
produce a _facsimile_ of the mother plant, as buds do in the highest
classes. But according to the microscopic observations of M. Tulasne,
the true fruit differs little if at all from the asci-bearing fungi.
Through the open discs of the higher lichens, sporidia are discharged
from perpendicular septate asci which, with their paraphyses, are
imbedded in the substance below. The asci are formed by the elongation
of some of the cells of that layer into cylindrical septate vessels,
generally containing from four to eight sporidia, the ultimate result of
as many free cells. In whatever part of a lichen the perithecia may be
placed, the asci and paraphyses invariably originate in the medullary
layer.
[Illustration: Fig. 39. _a_, Paulia perforata, gonidia, paraphysis, and
asci; _b_, Calicium tympanellum, perithecium and sporidia; _c_, Graphis
Leprevostei, with excipulum, asci, and sporidium.]
The thallus or frond of many of these lichens is irregularly covered
with thick convex scales, in each of which are concealed white or grey
bodies, which become flask-shaped, with pores in their extremities. When
mature, they exactly resemble the bottle-shaped perithecia in the genus
Sphæria among fungi. They are lined with branching filaments or other
supports, ending in minute ovoid particles, which escape in myriads from
the flask-shaped vessels long before the spores appear. They are motile,
though not provided with cilia, and are supposed to be fertilizing
particles like pollen, since they do not germinate as spores do. In this
vast family of open-shielded lichens, which comprises every form and
habit of the plant, the preceding type, which is the highest, undergoes
many modifications; but the essential character remains the same, and
the group is so natural, that the species run into one another so as to
leave no very striking distinction.
Commencing with the GYMNOCARPEI, the Parmeliacei first claim notice as
containing the highest types which lichens are capable of assuming, and
as abounding in species. The disc, which is the hymenium, is orbicular
or kidney-shaped, and surrounded by the frond, without any proper
excipulum or cortical covering. A large portion of our most widely
diffused lichens, whether growing on the ground or attached to rocks and
trees, belong to this tribe; they form patches two or three feet in
diameter, which are often of marvellous antiquity, and they grow so
slowly that even small patches are of great age. This tribe consists of
three distinct groups: in two of these the plants are horizontal and
sometimes foliaceous; in the third they are vertical, often branched,
and occasionally pendulous.
The latter group of this large division of lichens comprises the genera
allied to Usnea, which are erect, centripetal lichens, that is to say,
their body or thallus is an upright and generally cylindrical stem, in
the centre of which the layer of marrow, the origin of fructification,
is condensed, and the open discs, or hymenia, are in connection with it,
whether they be situated at the ends of the branches, or on the surface
of the upright stems. The Usneæ have three forms, the pendulous, the
inflated and erect, and the branched or shrubby. The Evernia jubata is
an example of the first, and remarkable for its long, pendulous,
cylindrical branches hanging down like bunches of bluish grey hair. It
is often attached to the branches of aged larch trees, and is
exceedingly picturesque. With the exception of Evernia vulpina and
flavicans, which are brightly coloured, all the rest of the genus have
dull tints. The genus Ramalina has the dull tints of the Evernia, and
some species approach very nearly in form to that genus. The Ramalina
polymorphum and R. scopulorum abound in dyes, while the R. calicaris is
rich in gluten.
Lichens have lost much of their importance since the discovery of the
coal tar colours; nevertheless they afford dyes still in use. In many
species the dying principle is colourless like aniline; but it yields
the most brilliant colours by means of alkalies. The Parmelia parietina,
common on walls and the roofs of houses, gives the chrysophonic acid, a
colourless liquid which becomes bright yellow when treated with an
alkali; and in like manner the vulpinic acid, given by Evernia vulpina,
gives a brown dye. Notwithstanding the quiet greyish green tints of the
Rocella fuciformis and R. tinctoria, no lichens are richer in the purple
substance known as orchil, from which, by means of soda or potash, the
valuable blue substance litmus is manufactured, so important as a test
for acidity. Many of the small moss-like lichens yield orchil, but none
in such abundance as the R. fuciformis, which grows in Madeira, Angola,
Madagascar, and South America. Oxalic and usnic acids are produced by
lichens; indeed the usnic acid combined with green and yellow resins
seems to be more or less a constituent of various lichens. It is evident
that the colour of the dyes is altogether independent of the colour of
the lichen from whence they are obtained.
These aërial plants have a marvellous power of decomposing the light,
and adorning themselves in brilliant hues. In our own country, where the
colouring is often cold from the excess of verdure, the lichen affords a
happy relief by giving a little warmth to the landscape. Even at Rome,
where nature is so gorgeously coloured, the ruins owe much of their
picturesque beauty to the red, white, and golden lichens with which they
are clothed.
The Usneæ are perhaps the most beautiful of the lichens, the colours
being sometimes brilliant, the forms elegant, and when the broad discs
are amply ciliated, the appearance is very striking. The same species
are widely diffused, but the colours are brighter in exotic specimens.
Usnea melaxantha and Usnea Taylori are splendid productions.[63] The
Usneæ are cosmopolitan in genera and species. The genus Cetraria, of
which the Iceland moss is a well known species, forms a connection
between the vertical and horizontal lichens; its thallus is neither
cylindrical nor quite erect, though it becomes more so towards maturity.
The Cetraria tristis has only that degree of inclination which arises
from its crowded mode of growth, and springs like a sea-weed from a
little peltate disc.
The most typical species of lichens occur in the second group of
Euparmeliaceæ, or Parmeliacei proper, in which the disc is at first
closed, and surrounded after expansion by a border arising from the
thallus or frond. The thallus is always horizontal, and expands from the
centre towards the circumference. The genus Sticta, belonging to this
group, is often highly foliaceous, and is not excelled by any horizontal
lichen in brightness of colouring or elegance of form. Even in our own
country the Sticta pulmonacea spreads over a wide area, and is
remarkable for its pitted frond. In this genus the under-side of the
plant is covered with a delicate velvety down, amidst which are
scattered round white spots, which penetrate to the medullary strata.
The erratic lichens are among the peculiarities of the genus Parmelia.
The Parmelia saxatilis, common on stones and boulders of the primary and
metamorphic formation, curls up into a ball, only fixed to its matrix by
a slender thread, which soon gives way, and the ball being dry and as
light as a balloon, is driven bounding by the winds, over the sheep
walks and downs of England. The globular Lecanora esculenta sometimes
suddenly covers large tracts in Armenia, Persia, and Tartary, where the
plants are eaten by the cattle and by the nomade tribes. This species,
with Lecanora affinis, is largely used by the inhabitants of the
countries east of the Levant, and in some parts of Africa is mixed with
wheat in grinding. These species are found, scattered over the ground
without any attachment, in the form of rugged truffle-like objects of
the size of walnuts. Sometimes they are piled together in strata a few
inches thick, by the whirlwinds, after traversing the air for many
miles, which gives rise to the histories of the miraculous descent of
food. During a scarcity, a shower of these lichens fell at Erzeroum, and
there are other well-authenticated instances recorded.[64]
The Peltigeri, or the third group of Parmeliacei, are so named from the
target-like discs on their surface, covered by a veil, which afterwards
disappears. The species frequently spread their beautifully foliaceous
fronds upon the ground, and as the fruit is marginal, it gives the
thallus a digitate appearance. They are often spotted by a little red
fungus. The genus Solorina has the fruit scattered over the frond; in
the Solorina saccata it is at first superficial, but a number of fibres
are formed on the under-side of the discs, which penetrate the soil, and
draw them down below the general surface—a very peculiar structure, in
strong contrast with that of the Solorina crocea, which has veins on the
under-surface but no fibres, and as no rootlets are sent out from the
base of the disc, it remains superficial. M. Tulasne discovered
fertilizing particles in several species of this group.
The order of the Lecidinei contains numerous species of the most varied
habits. There is a distinct and regular series upwards in the genus
Lecidea, the species of which are always crustaceous, and often form
merely a thin, close, adherent fibrous stratum on the hardest flint or
quartz. Some of them disintegrate the hard rocks on which they spread,
possibly by the action of some acid which they contain. From this low
type, the genus rises to the exalted forms with erect branching stems
clothed with foliaceous scales, and brightly coloured. The order is
distinguished by an orbicular disc, contained in a distinct excipulum or
cortical envelope, which is open from the earliest age; but it is
frequently obliterated afterwards by the development of that part of the
medullary stratum where the fruit is formed, the disc in consequence
becoming convex and capitate. This transformation takes place in the
highest forms of the genus Lecidea. The genus Bæomyces has rose or
chestnut coloured convex discs, supported on a stem called a podetium.
The genus Cladonia has a foliaceous thallus producing free scale-like
fronds, from the midst of which spring cylindrical or cup-shaped
podetia, which are sprinkled with leaves. The margins of the cups or
tips of the branches bear an abundant crop of convex, irregular brown,
or deep red discs, often as brightly coloured as sealing wax.
The species of Lecidea bear severe cold, and are wonderfully long lived.
Mr. Berkeley mentions that there are patches of the Lecidea geographica
which probably date from almost fabulous periods.
The fronds of the order Collemacei are gelatinous; the medullary stratum
seems to occupy the whole, though in two species of the genus there is a
distinct epidermic cellular coat. There are moniliform strings of minute
gonidia in the gelatinous matter, and others that are single, arising
from the division of each gonidium and its supporting thread into two:
both kinds differ from the gonidia in other lichens. The fruit-bearing
discs on the surface are open. Members of this order occur in Europe,
Africa, and Australia.
The order Coccocarpei is mainly distinguished by having orbicular discs
entirely deprived of the cortical envelope called an excipulum, or, if
it does exist, it is confounded with the thin membranaceous thallus. The
discs spring at once from the medullary stratum, and contain asci and
sporidia, similar to those of minute fungi (Sphæriæ). Some species of
the genus Coccocarpia only differ from Lecidea in the total absence of
an excidium. The order is chiefly parasitic, and in some cases the whole
plant is little more than a mass of fructification, parasitic upon and
continuous with the substance of other lichens, at whose expense they
live, thus forming an exception to the general habit of lichens, which
are fed by the atmosphere alone. M. Tulasne has discovered that the
genera Abrothallus and Scutula, though consisting almost entirely of
fruit, produce secondary spores—they are the only lichens in which they
occur; while Phacopsis and Celidium bear spermatogonia, which, analogous
to antheridia, contain minute fertilizing particles. The parasitic
genera occur in most parts of Europe and North America.
The Pyxinei are horizontal foliaceous lichens, for the most part fixed
by the centre. They have orbicular discs, and form one of the most
singular groups, both with regard to the superficial fruit, and the
curious convolutions of the perithecia. The fruit-cup, or excipulum, is
at first closed, and in the genus Gyrophora the disc produces a number
of partial fructiferous discs on the original fruit-bearing surface. As
usual the cells from whence the asci spring belong to the medullary
stratum; the border on the contrary is cortical. The thallus is always
foliaceous, and more or less peltate, but it becomes dry and brittle
when exposed to drought. Several of the species have tufts of strong
rootlets by which the frond is fixed to rocks, and some have deep pits,
with corresponding swellings on the upper-surface, from whence they have
their popular name of Tripe de Roche. These lichens, together with some
species of Gyrophora, afforded a miserable sustenance to Sir George Back
and his companions during their journey along the frozen regions of
Arctic America in quest of the north-west passage. It is evident that
plants which derive their sustenance solely from the medium in which
they live, whether air or water, can never become a permanent and
wholesome food for man, though erratic lichens and laver may be eaten
from necessity, certainly not from choice. In the Gyrophora the
perithecia are convolute, in the Umbilicaria they are not; but both of
these genera, which have their seat in Arctic Europe and America,
possess fertilizing particles like most of their class.
In the order Graphidei the disc is linear, simple, or branched, with or
without an excipulum, which is carbonaceous. Many of these lichens are
crustaceous, and in most of the genera the perithecia are much
elongated, pointed at both ends. When they are parallel, or placed at
different angles to each other, they form groups like Japanese or
Chinese characters, whence their name. The genera are determined by the
position of these perithecia, which are much varied. M. Tulasne has
found linear male particles immersed in the crust of some of these
lichens. They have their principal seat in tropical America, though a
few species occur in the frigid and temperate zones of both hemispheres.
The order Glyphidei has no true excipulum; the coloured discs are at
first immersed in the medullary stratum of a crustaceous thallus, the
crust then rises into distinct expansions, in the centre of which the
coloured discs are set like gems in a mosaic. ‘There is in fact no true
border to the disc, the perithecium being reduced to a thick conical
base, from which proceed immediately the asci and paraphyses; each
individual hymenium being surrounded by the intervening medullary matter
injected, as it were, into the interstices.’ The whole surface of
Chiodecton monostichum is productive, and in that genus M. Tulasne found
vessels in the form of little scattered perithecia, containing filiform
curved fertilizing particles. The species of this order are almost
wholly tropical, though the Chiodecton myrticola has been found in
Ireland.
The order Caliciei consists of horizontal lichens, with generally an
ill-developed crust; the discs, which are at first covered by a veil,
are contained in a stalked, or more rarely sessile, excipulum, looking
like little flat-headed pins stuck into the crust; the veil at length
vanishes, and exposes a pulverulent mass of spores, which adhere so
loosely in the Calicium inquinans, that they soil the finger if touched;
in other cases they come out of their ascus like little necklaces. The
species of these lichens are almost entirely confined to Europe and
North America.
In the second division of Lichenaceæ, the ANGIOCARPEI, the discs are
enclosed in an excipulum, which projects from the surface of the plant,
and ultimately discharges the spores from a rupture or pore in its
surface. Besides the fructification consisting of perithecia containing
paraphyses and asci with their enclosed sporidia, there is a nucleus in
each disc, either of firm or deliquescent matter.
Some of the lichens of this group are parasitic, others are aquatic. The
order Limboriei, like parasitic fungi, begin their existence under the
thick skin of the leaves of tropical plants, spread their crustaceous
thallus over their surface, and destroy their beauty, by stopping up
their pores, and preventing the admission of light to their tissues. The
excipulum and perithecia are black, and the latter burst in an irregular
fissure, and are in most cases covered by a beautifully sculptured
crust.
The crustaceous fronds of the Verrucariei are often so thin as to be
inseparable from the substance over which they spread. The excipula are
closed, the walls of the perithecia are often black, and in some species
more or less crowded round a columella. In the Verrucaria muralis
fertilizing particles have been discovered. The plants are widely
distributed, and at least one species spreads its crust over the smooth
stones in running streams.
In the group Endocarpei, the perithecia are immersed in the substance of
the plant, which has, for the most part, a foliaceous horizontal crust,
and a gelatinous nucleus. Some species grow on stones, perpetually or
periodically submerged, or, if not under water, continually wet with its
spray. The Lichina, a genus of the group Lichinei, lives on marine
rocks, and is often dripping with salt-water, and often suddenly dried
up.
The Sphærophorei, or sphere-bearing lichens, have upright stems bearing
globular fruit at the extremity of their numerous branches. At first,
the fruit is only indicated by a swelling, but in time the outer bark
bursts, and exposes the contents of the perithecium, which consist of
asci and paraphyses seated on a central columella. The sporidia are
beautiful objects under the microscope on account of their spherical
form, which is rare among lichens, and their more or less deep blue
tint—a colour by no means peculiar to the sporidia of this order of
plants, for in other orders they are bright scarlet, olive, golden
yellow, or brown.