LICHENS.
The Lichens are found growing upon the bark of trees, old palings, &c.
Those most easily seen with the naked eye form grey or coloured dryish
patches or pendulous tufts; while the smaller ones are singly easily
overlooked, from their minute size and close adhesion to the _mátrix_ or
body upon which they grow, forming, by their aggregation, the grey or
otherwise-coloured dry and brittle coatings of almost every tree or
decaying branch.
The Lichens derive their nourishment from the air, and not from the
matrix--in this respect differing from the Fungi, with some of which, as
we shall presently see, they agree in the structure of the fruit.
The structure of the Lichens is simple, no distinction of root, stem,
and leaves existing in them, although certain dry root-like fibres exist
in many of them, by which the plant is fixed to the matrix. The whole
consists mainly of a frond or _thal´lus_ (θαλλὀς, a leaf). This is
either raised above the surface of the matrix in a shrubby form, or
spread upon the surface as a flexible lobed layer (Pl. II. fig. 2), or
it is dry and brittle (crustaceous) and closely adherent (Pl. II. fig.
26).
The fructification consists of little saucers, disks, or streak-like
furrows, often of a different colour from the thallus, the structure of
which will be best illustrated by reference to a few common species.
PARMELIA´CEÆ.--_Parmélia parietína_ (Pl. II. fig. 2) is a very common
Lichen, found on the bark of trees, on old palings, &c. It is of an
orange-yellow colour, the thallus being flat, lobed, and scalloped
(crenate) at the margins. The structure of the thallus may serve to
represent that of most of the Lichens. It consists of three layers,--an
upper cortical or epidermic layer, which is continued over the margins
to the under surface of the thallus, to form the under layer; and
between these is the middle or medullary layer. The medullary layer
consists of interwoven fibres, which are more closely packed towards the
upper and under surfaces, so as to give them a cellular appearance. Near
the upper part of the medullary layer, a number of minute rounded green
cells exist, lying loosely in its meshes (Pl. II. fig. 4 _a_). These
green cells (gonidia) appear to correspond to the buds of the higher
plants, and, when detached from the plants, they are capable of growing
into new individuals.
On the upper surface of the thallus of _Parmelia_ (fig. 2) the
fructification may be observed. This consists of saucers or shields
(_apothécia_, ἁποθἡκη, a repository), formed of a raised and expanded
portion of the thallus (fig. 3), and containing the spores. The spores
are enclosed in closely set upright cells, or spore-sacs (figs. 4 _b_
and 5 _b_), called _as´ci_ (ἁσκὀς, a bottle) or thecæ; and intermingled
with the asci are filaments, enlarged and coloured at the ends
(_paraphyses_), which are probably abortive asci.
In systematic works upon the Lichens, the saucers and their contents are
included in the term apothecia, the saucer alone being called the
_excip´ulum_ (_excipulum_, a receiver); the mass of asci and paraphyses
forming the _nucleus_ or _thalámium_ (θἁλαμος, a bed).
The yellow spores are very minute, each ascus containing eight of them,
and they are divided by a transverse partition or septum.
Near the margins of the lobes of the thallus are small dark points.
These are the pouting mouths of little capsules (_spermogonia_) sunk in
the substance of the thallus, and containing numerous filaments,
terminated by very minute stick-shaped bodies (_spermatia_), which break
off and escape through the orifices of the capsules. These are probably
the representatives of the anthers of flowering plants and of the
antheridia of the ferns. They are, however, so difficult to find and
examine, that I must refer to the Dictionary for a further description
and figures of them.
LECIDIN´EÆ.--This family contains the genus _Cladónia_, three or four
species of which are common on boggy heaths, banks, &c., viz. _Cladonia
coccif´era_, the Scarlet Cup-moss (Pl. II. fig. 21); _C. pyxidáta_, the
Common Cup-moss (Pl. II. fig. 23); _C. vermiculáris_ (Pl. II. fig. 22);
and _C. rangiferína_, the Reindeer Moss (Pl. II. fig. 24). The thallus
of these Lichens forms little rounded irregularly overlapping scales,
with scalloped edges, overgrowing the surface upon which the Lichens are
found. The fruit-stalks, or _podétia_ (ποῦς, a foot), are hollow
(fistulose), and either simple and dilated into cups (Pl. II. fig. 23),
or branched with the corners or angles between the adjacent branches
perforated. The apothecia in the young state resemble those of
_Parmelia_ on a small scale; but as they approach maturity, the centre
becomes pushed up, so that the spore-layer is extended over the ends of
the stalks. In _C. coccifera_ and _pyxidata_ the cups are proliferous at
the margin; _i. e._, branches upon which the apothecia are placed arise
from it. The asci and paraphyses are very minute, but do not differ
essentially in structure from those of _Parmelia_. In _C. vermicularis_
the podetia are pointed and more solid than in the other species, the
apothecia forming very minute spots at their apices.
GRAPHID´EÆ.--To this family belong _Gráphis scripta_ (Pl. II. fig. 26)
and _Opeg´rapha betulína_ (Pl. II. fig. 30). These little Lichens are
easily overlooked, from the thin and but slightly raised thallus being
only visible to the naked eye as a discoloration of the bark of the
trees upon which they grow; while the fructification is very minute,
forming little black streaks or _lirel´læ_ (_lira_, a furrow),
irregularly arranged, and resembling somewhat the letters of some of the
Oriental alphabets.
In _Graphis scripta_ (fig. 26) the thallus is thin, somewhat membranous,
smoothish, shining, greyish white, and faintly bordered with black. The
lirellæ (fig. 27) are partly sunk in the bark, winding and narrow, some
being simple, others branched; and they are surrounded by a raised
border, formed by the thallus. The lirellæ are lined at the sides with a
black (carbonaceous) layer or _excip´ulum_, within which are situated
the asci and paraphyses. The spores (Pl. II. fig. 29) are 8-cleft, the
segments being again divided longitudinally into little spores or
sporidia.
In _Opegrapha betulina_ (Pl. II. fig. 30), which is found on the bark of
the birch-tree, the thallus is thin, dirty yellowish white, bordered
with black. The lirellæ (figs. 31, 32) are mostly simple, without a
raised border of the thallus, and the excipulum forms a complete lining
to them. The spores (fig. 33 _a_) are 3-cleft, and taper at the ends.
CALICI´EÆ.--_Calic´ium clavel´lum_ (Pl. II. fig. 6) is a pretty little
Lichen, growing upon old boards and farm-buildings. The thallus is
granular and greyish white. The apothecia (fig. 7) are stalked and
black, but of a lighter colour than the mass of spores forming the
nucleus. The spores are very minute, black, oblong, and divided by a
transverse septum.
The Lichens are divided into two Orders, according to whether the
apothecia are open before the spores are ripe, as in the species noticed
above, or whether the apothecia only open to discharge the ripe spores.
The first Order forms the _Gymnocar´pi_ (γυμνὀς, naked, καρπὀς, fruit);
the second forms the _Angiocar´pi_ (ἁγγεῖον, vessel, capsule).
_Preservation._--The Lichens are readily preserved, on account of their
dry nature; they need simply be kept in a dry place, and glued to pieces
of card. If room is an object, they may be dried under pressure, as in
the case of the flowering plants. When re-moistened, the minute
structures may be easily made out by sections. The smaller ones may be
mounted dry, in cells made of the wax cement (p. 16). The minute
structures keep well in chloride of calcium or glycerine.