THE LIFE-HISTORY OF LICHENS AND MOSSES
[Illustration]
The autumn has passed away and we are in the midst of winter. In the
long winter evenings the stars shine bright and clear, and tempt us to
work with the telescope and its helpmates the spectroscope and
photographic plates. But at first sight it would seem as though our
microscopes would have to stand idle so far at least as plants are
concerned, or be used only to examine dried specimens and mounted
sections. Yet this is not the fact, as I remembered last week when
walking through the bare and leafless wood. A startled pheasant rising
with a whirr at the sound of my footsteps among the dead leaves roused
me from my thoughts, and as a young rabbit scudded across the path and I
watched it disappear among the bushes, I was suddenly struck with the
great mass of plant life flourishing underfoot and overhead.
Can you guess what plants these were? I do not mean the evergreen pines
and firs, nor the few hardy ferns, nor the lovely ivy clothing the
trunks of the trees. Such plants as these live and remain green in the
winter, but they do not grow. If you wish to find plant life revelling
in the cold damp days of winter, fearing neither frost nor snow and
welcoming mist and rain, you must go to the mosses, which as autumn
passes away begin to cover the wood-paths, to creep over the roots of
the trees, to suck up the water in the bogs, and even to clothe dead
walls and stones with a soft green carpet. And with the mosses come the
lichens, those curious grey and greenish oddities which no one but a
botanist would think of classing among plants.
The wood is full of them now: the hairy lichens hang from the branches
of many of the trees, making them look like old greybearded men; the
leafy lichens encircle the branches, their pale gray, green, and yellow
patches looking as if they were made of crumpled paper cut into wavy
plates; and the crusty lichens, scarcely distinguishable from the bark
of the trees, cover every available space which the mosses have left
free.
As I looked at these lichens and thought of their curious history I
determined that we would study them to-day, and gathered a basketful of
specimens (see Fig. 28). But when I had collected these I found I had
not the heart to leave the mosses behind. I could not even break off a
piece of bark with lichen upon it without some little moss coming too,
especially the small thread-mosses (_Bryum_) which make a home for
themselves in every nook and corner of the branches; while the
feather-mosses, hair-mosses, cord-mosses, and many others made such a
lovely carpet under my feet that each seemed too beautiful to pass by,
and they found their way into my basket, crowned at the top with a large
mass of the pale-green Sphagnum, or bog-moss, into which I sank more
than ankle-deep as I crossed the bog in the centre of the wood on my way
home.
[Illustration: Fig. 28.
Examples of Lichens. (From life.)
1, A hairy lichen. 2, A leafy lichen. 3, A crustaceous lichen. _f_, _f_,
the fruit.]
So here they all are, and I hope by the help of our magic glass to let
you into some of the secrets of their lives. It is true we must study
the structure of lichens chiefly by diagrams, for it is too minute for
beginners to follow under the microscope, so we must trust to drawings
made by men more skilful in microscopic botany, at any rate for the
present. But the mosses we can examine for ourselves and admire their
delicate leaves and wonderful tiny spore-cases.
Now the first question which I hope you want to ask is, how it is that
these lowly plants flourish so well in the depth of winter when their
larger and stronger companions die down to the ground. We will answer
this first as to the lichens, which are such strange uncanny-looking
plants that it is almost difficult to imagine they are alive at all; and
indeed they have been a great puzzle to botanists.
[Illustration: Fig. 29.
Single-celled green plants growing and dividing (_Pleurococcus_). (After
Thuret and Bornet.)]
Before we examine them, however, look for a minute at a small drop of
this greenish film which I have taken from the rain-water taken outside.
I have put some under each microscope, and those who can look into them
will see the slide almost covered with small round green cells very much
like the yeast cells we saw when studying the Fungi, only that instead
of being colourless they are a bright green. Some of these cells will I
suspect be longer than others, and these long cells will be moving over
the slide very rapidly, swimming hither and thither, and you will see,
perhaps for the first time, that very low plants can swim about in
water. These green cells are, indeed, the simplest of all plants, and
are merely bags of living matter which, by the help of the green
granules in them, are able to work up water and gases into nourishing
food, and so to live, grow, and multiply.
There are many kinds of these single-celled plants in the world. You may
find them on damp paths, in almost any rain-water butt, in ponds and
ditches, in sparkling waterfalls, along the banks of flowing rivers, and
in the cold clear springs on the bleak mountains. Some of them take the
form of tangled threads[1] composed of long strings of cells, and these
sometimes form long streamers in flowing water, and at other times are
gathered together in a shapeless film only to be disentangled under a
microscope. Other kinds[2] wave to and fro on the water, forming dense
patches of violet, orange-brown, or glossy green scum shining in the
bright sunlight, and these flourish equally in the ponds of our gardens
and in pools in the Himalaya mountains, 18,000 feet above the sea.
Others again[3] seize on every damp patch on tree trunks, rocks, or
moist walls, covering them with a green powder formed of single plant
cells. Other species of this family turn a bright red colour when the
cells are still; and one, under the name of Gory Dew,[4] has often
frightened the peasants of Italy, by growing very rapidly over damp
walls and then turning the colour of blood. Another[5] forms the "red
snow" of the Arctic regions, where it covers wide surfaces of snow with
a deep red colour. Others[6] form a shiny jelly over rocks and stones,
and these may be found almost everywhere, from the garden path to the
warm springs of India, from the marshes of New Zealand up to the shores
of the Arctic ocean, and even on the surface of floating icebergs.
[1] _Confervae._
[2] _Oscillariae._
[3] _Protococcus._
[4] _Palmella cruenta._
[5] _Protococcus nivalis._
[6] _Nostoc._
The reason why these plants can live in such very different regions is
that they do not take their food through roots out of the ground, but
suck in water and gases through the thin membrane which covers their
cell, and each cell does its own work. So it matters very little to them
where they lie, so long as they have moisture and sunlight to help them
in their work. Wherever they are, if they have these, they can take in
carbonic acid from the air and work up the carbon with other gases which
they imbibe with the water, and so make living material. In this way
they grow, and as a cell grows larger the covering is stretched and part
of the digested food goes to build up more covering membrane, and by and
by the cell divides into two and each membrane closes up, so that there
are two single-celled plants where there was only one before. This will
sometimes go on so fast that a small pond may be covered in a few hours
with a green film formed of new cells.
Now we have seen, when studying mushrooms, that the one difference
between these green plants and the single-celled Fungi is that while the
green cells make their own food, colourless cells can only take it in
ready-made, and therefore prey upon all kinds of living matter. This is
just what happens in the lichens; and botanists have discovered that
these curious growths are really the result of a _partnership_ between
single-celled green plants and single-celled fungi. The grey part is a
fungus; but when it is examined under the microscope we find it is not a
fungus only; a number of green cells can be seen scattered through it,
which, when carefully studied, prove to be some species of the green
single-celled plants.
Here are two drawings of sections cut through two different lichens, and
enormously magnified so that the cells are clearly seen. 1, Fig. 30 is
part of a hairy lichen (1, Fig. 28), and 2 is part of a leafy lichen (2,
Fig. 28). The hairy lichen as you see has a row of green cells all round
the tiny branch, with fungus cells on all sides of them. The leafy
lichen, which only presents one surface to the sun and air while the
other side is against the tree, has only one layer of green cells near
the surface, but protected by the fungus above.
[Illustration: Fig. 30.
Sections of Lichens. (Sachs.)
1, Section of a hairy lichen, _Usnea barbata_. 2, Section of a leafy
lichen, _Sticta fuliginosa_. 3, Early growth of a lichen. _gc_, Green
cells. _f_, Fungus.]
The way the lichen has grown is this. A green cell (_gc_ 3, Fig. 30)
falling on some damp spot has begun to grow and spread, working up food
in the sunlight. To it comes the spore of the fungus _f_, first
thrusting its tubes into the tree-bark, or wall, and then spreading
round the green cells, which remain always in such a position that
sunlight, air, and moisture can reach them. From this time the two
classes of plants live as friends, the fungus using part of the food
made by the green cells, and giving them in return the advantage of
being spread out to the sunlight, while they are also protected in
frosty or sultry weather when they would dry up on a bare surface. On
the whole, however, the fungus probably gains the most, for it has been
found, as we should expect, that the green cells can live and grow if
separated out of the lichen, but the fungus cells die when their
industrious companions are taken from them.
At any rate the partnership succeeds, as you will see if you go into the
wood, or into an orchard where the apple-trees are neglected, for every
inch of the branches is covered by lichens if not already taken up by
mosses or toadstools.
There is hardly any part of the world except the tropics where lichens
do not abound. In the Alps of Scandinavia close to the limits of
perpetual snow, in the sandy wastes of Arctic America, and over the
dreary Tundras of Arctic Siberia, where the ground is frozen hard during
the greater part of the year, they flourish where nothing else can live.
The little green cells multiply by dividing, as we saw them doing in the
green film from the water-butt. The fungus, however, has many different
modes of seeding itself. One of these is by forming little pockets in
the lichen, out of which, when they burst, small round bodies are
thrown, which cover the lichen with a minute green powder. There is
plenty of this powder on the leafy lichen which you have by you. You can
see it with the magnifying-glass, without putting it under the
microscope. As long as the lichen is dry these round bodies do not grow,
but as soon as moisture reaches them they start away and become new
plants.
[Illustration: Fig. 31.
Fructification of a lichen. (From Sachs and Oliver.)
Apothecium or spore-chamber of a lichen. 1, Closed. 2, Open. 3, The
spore-cases and filaments enlarged, showing the spores. _f_, Filaments.
_sc_, Spore-cases. _s_, Spores.]
A more complicated and beautiful process is shown in this diagram (Fig.
31). If you look carefully at the leafy lichen (2, Fig. 28) you will
find here and there some little cups _f_, while others grow upon the
tips of the hairy lichen. These cups, or fruits, were once closed,
flask-shaped chambers (1, Fig. 31) inside which are formed a number of
oval cells _sc_, which are spore-cases, with from four to eight spores
or seed-like bodies _s3_ inside them. When these chambers, which are
called _apothecia_, are ripe, moist or rainy weather causes them to
swell at the top, and they burst open and the spore-cases throw out the
spores to grow into new fungi.
In some lichens the chambers remain closed and the spores escape through
a hole in the top, and they are then called _perithecia_, while in
others, as these which we have here, they open out into a cup-shape.
This, then, is the curious history of lichens; the green cells and fungi
flourishing together in the damp winter and bearing the hardest frost
far better than the summer drought, so that they have their good time
when most other plants are dead or asleep. Yet though some of them, such
as the hairy lichens, almost disappear in the summer, they are by no
means dead, for, like all these very low plants, they can bear being
dried up for a long time, and then, when moisture visits them again,
each green cell sets to work, and they revive. There is much more to be
learnt about them, but this will be sufficient to make you feel an
interest in their simple lives, and when you look for them in the wood
you will be surprised to find how many different kinds there are, for it
is most wonderful that such lowly plants should build up such an immense
variety of curious and grotesque forms.
* * * * *
And yet, when we turn to the mosses, I am half afraid they will soon
attract you away from the dull grey lichens, for of all plant histories
it appears to me that the history of the moss-plant is most fascinating.
As this history is complicated by the moss having, as it were, two
lives, you must give me your whole attention, and I will explain it
first from diagrams, though you can see all the steps under the
microscope.
Take in your hands, in the first place, a piece of this green moss which
I have brought. How thick it is, like a rich felted carpet! and yet, if
you pull it apart carefully, you will find that each leafy stem is
separate, and can be taken away from the others without breaking
anything. In this dense moss each stem is single and clothed with leaves
wrapped closely round it (see Fig. 33); in some mosses the stem is
branched, and in others the leaves grow on side stalks, as in this
feathery moss (Fig. 32). But in each case every stem is like a separate
plant, with its own tuft of tender roots _r_.
[Illustration: Fig. 32.
A stem of feathery moss. (From life.)
_l_, Leaves. _s_, Stem. _r_, Roots.]
What a delicate growth it is! The stem is scarcely more than a fine
thread, the leaves minute, transparent, and tender. In this pale
sphagnum or bog-moss (Fig. 36, p. 93), which is much larger and stouter,
you can see better how each one of these leaves, though they are so
thickly packed, is placed so that it can get the utmost light, air, and
moisture. Yet so closely are the leaves of each stem entangled in those
of the next that the whole forms a thick springy green carpet under our
feet.
How is it, then, that these moss stems, though each independent, grow in
such a dense mass? Partly because moss multiplies so rapidly that new
stems are always thrusting themselves up to the light, but chiefly
because the stems were not always separate, but in very early life
sprang from a common source.
If, instead of bringing the moss home and tearing it apart, you went to
a spot in the wood where fresh moss was growing, and looked very
carefully on the surface of the ground or among the water of a marsh,
you would find a spongy green mass below the growing moss, very much
like the green scum on a pond. This film, some of which I have brought
home, is seen under the microscope to be a mass of tangled green threads
(_t_, Fig. 34, p. 88) like those of the _Confervae_ (see p. 79), composed
of rows of cells, while here and there upon these threads you would find
a bud (_mb_, Fig. 34) rising up into the air.
This tangled mass of green threads, called the _protonema_, is the first
growth, from which the moss stems spring. It has itself originated from
a moss-spore; as we shall see by and by. As soon as it has started it
grows and spreads very rapidly, drinking in water and air through all
its cells and sending up the moss buds which swell and grow, giving out
roots below and fine stems above, which soon become crowded with leaves,
forming the velvety carpet we call moss. Meanwhile the soft threads
below die away, giving up all their nourishment to the moss-stems, and
this is why, when you take up the moss, you find each stem separate. But
now comes the question, How does each stem live after the nourishing
threads below have died? It is true each stem has a few hairy roots,
but these are very feeble, and not at all like the roots of higher
plants. The fact is, the moss is built up entirely of tender cells, like
the green cells in the lichen, or in the film upon the pond. These cells
are not shut in behind a thick skin as in the leaves of higher plants,
but have every one of them the power to take in water and gases through
their tender membrane.
I made last night a rough drawing of the leaf of the feathery moss put
under the microscope, but you will see it far better by putting a leaf
with a little water on a glass slide under the covering glass and
examining it for yourself. You will see that it is composed of a number
of oval-shaped cells packed closely together (_c_ Fig. 33), with a few
long narrow ones _mr_ in the middle of the leaf forming the midrib.
Every cell is as clear and distinct as if it were floating in the water,
and the tiny green grains which help it to work up its food are clearly
visible.
[Illustration: Fig. 33.
Moss-leaf magnified. (From life.)
Showing the cells _c_, each of which can take in and work up its own
food. _mr_, Long cells of the mid-rib.]
Each of these cells can act as a separate plant, drinking in the water
and air it needs, and feeding and growing quite independently of the
roots below. Yet at the same time the moss stem has a great advantage
over single-celled plants in having root-hairs, and being able to grow
upright and expose its leaves to the sun and air.
Now you will no longer wonder that moss grows so fast and so thick, and
another curious fact follows from the independence of each cell, namely,
that new growths can start from almost any part of the plant. For
example, pieces will often break off from the tangled mass or protonema
below, and, starting on their own account, form other thread masses.
Then, after the moss stems have grown, a new mass of threads may grow
from one of the tiny root-hairs of a stem and make a fresh tangle; nay,
a thread will sometimes even spring out of a damp moss leaf and make a
new beginning, while the moss stems themselves often put forth buds and
branches, which grow root-hairs and settle down on their own account.
[Illustration: Fig. 34.
Polytrichum commune. A large hair-moss.
_t_, _t_, Threads of green cells forming the _protonema_ out of which
moss-buds spring. _mb_, Buds of moss-stems. _a_, Minute green flower in
which the antherozoids are formed (enlarged in Fig. 35). _p_, _p1_,
_p2_, _p3_, Minute green flower in which the ovules are formed, and
urn-plant springing out of it (enlarged in Fig. 35). _us_, Urn stems.
_c_, Cap. _u_, Urn after cap has fallen off, still protected by its
lid.]
All this comes from the simple nature of the plants, each cell doing its
own work. Nor are the mosses in any difficulty as to soil, for as the
matted threads decay they form a rich manure, and the dying moss-stems
themselves, being so fragile, turn back very readily into food. This is
why mosses can spread over the poorest soil where even tough grasses
cannot live, and clothe walls and roofs with a rich green.
[Illustration: Fig. 35.
Fructification of a moss.
A, Male moss-flower stripped of its outer leaves, showing jointed
filaments and oval sacs os and antherozoid cells _zc_ swarming out of a
sac. _zc'_, Antherozoid cell enlarged. _z_, Free antherozoid. P, Female
flower with bottle-shaped sacs _bs_. _bs-c_, Bottle-shaped sac, with cap
being pushed up. _u_, Urn of _Funaria hygrometrica_, with small cap.
_u'_, Urn, from which the cap has fallen, showing the teeth _t_ which
keep in the spores.]
So far, then, we now understand the growth of the mossy-leaf stems, but
this is only half the life of the plant. After the moss has gone on
through the damp winter spreading and growing, there appear in the
spring or summer tiny moss flowers at the tip of some of the stems.
These flowers (_a_, _p_, Fig. 34) are formed merely of a few green
leaves shorter and stouter than the rest, enclosing some oval sacs
surrounded by jointed hairs or filaments (see A and P, Fig. 35). These
sacs are of two different kinds, one set being short and stout _os_, the
others having long necks like bottles _bs_. Sometimes these two kinds of
sac are in one flower, but more often they are in separate flowers, as
in the hair-moss, _Polytrichum commune_ (_a_ and _p_, Fig. 34). Now when
the flowers are ripe the short sacs in the flower A open and fling out
myriads of cells _zc_, and these cells burst, and forth come tiny
wriggling bodies _z_, called by botanists _antherozoids_, one out of
each cell. These find their way along the damp moss to the flower P, and
entering the neck of one of the bottle-shaped sacs _bs_, find out each
another cell or _ovule_ inside. The two cells together then form a
_plant-egg_, which answers to the germ in the seeds of higher plants.
Now let us be sure we understand where we are in the life of the plant.
We have had its green-growing time, its flowering, and the formation of
what we may roughly call its seed, which last in ordinary higher plants
would fall down and grow into a new green plant. But with the moss there
is more to come. The egg does not shake out of the bottle-necked sac,
but begins to grow inside it, sending down a little tube into the moss
stem, and using it as other plants use the ground to grow in.
As soon as it is rooted it begins to form a delicate stem, and as this
grows it pushes up the sac _bs_, stretching the neck tighter and tighter
till at last it tears away below, and the sac is carried up and hangs
like an extinguisher or cap (_c_ Figs. 34, 35) over the top of the stem.
Meanwhile, under this cap the top of the stalk swells into a knob
which, by degrees, becomes a lovely little covered urn _u_, something
like a poppy head, which has within it a number of spores. The growth of
this tiny urn-plant often occupies several months, for you must remember
that it is not merely a fruit, though it is often called so, but a real
plant, taking in food through its tubes below and working for its
living.
When it is finished it is a most lovely little object (_us_, Fig. 34),
the fine hairlike stalk being covered with a green, yellow, or brilliant
red fool's cap on the top, yet the whole in most mosses is not bigger
than an ordinary pin. You may easily see them in the spring or summer,
or even sometimes in the winter. I have only been able to bring you one
very little one to-day, the _Funaria hygrometrica_, which fruits early
in the year. This moss has only a short cap, but in many mosses they are
very conspicuous. I have often pulled them off as you would pull a cap
from a boy's head. In nature they fall off after a time, leaving the
urn, which, though so small, is a most complicated structure. First it
has an outer skin, with holes or mouths in it which open and close to
let moisture in and out. Then come two layers of cells, then an open
space full of air, in which are the green chlorophyll grains which are
working up food for the tiny plant as the moisture comes in to them.
Lastly, within this again is a mass of tissue, round which grow the
spores which are soon to be sown, and which in _Polytrichum commune_ are
protected by a lid. Even after the extinguisher and the lid have both
fallen off, the spores cannot fall out, for a thick row of teeth (_t_,
Fig. 35) is closed over them like the tentacles of an anemone. So long
as the air is damp these teeth remain closed; it is only in fine dry
weather that they open and the spores are scattered on the ground.
_Funaria hygrometrica_ has no lid under its cap, and after the cap falls
the spores are only protected by the teeth.
When the spores are gone, the life of the tiny urn-plant is over. It
shrivels and dies, leaving ten, fifteen, or even more spores, which,
after lying for some time on the ground, sprout and grow into a fresh
mass of soft threads.
So now we have completed the life-history of the moss and come back to
the point at which we started. I am afraid it has been rather a
difficult history to follow step by step, and yet it is perfectly clear
when once we master the succession of growths. Starting from a spore,
the thread-mass or protonema gives rise to the moss-stems forming the
dense green carpet, then the green flowers on some of the leaf-stems
give rise to a plant-egg, which roots itself in the stem, and grows into
a perfect plant without leaves, bearing merely the urn in which fresh
spores are formed, and so the round goes on from year to year.
There are a great number of different varieties of moss, and they differ
in the shape and arrangement of their stems and leaves, and very much in
the formation of their urns, yet this sketch will enable you to study
them with understanding, and when you find in the wood the nodding caps
of the fruiting plants, some red, some green, some yellow, and some a
brilliant orange, you will feel that they are acquaintances, and by the
help of the microscope may soon become friends.
Among them one of the most interesting is the sphagnum or bog-moss (Fig.
36), which spreads its thick carpet over all the bogs in the woods. You
cannot miss its little orange-coloured spore-cases if you look closely,
for they contrast strongly with its pale green leaves, out of which they
stand on very short stalks. I wish we could examine it, for it differs
much from other mosses, both in leaves and fruit, but it would take us
too long. At least, however, you must put one of its lovely transparent
leaves under the microscope, that you may see the large air-cells which
lie between the growing cells, and admire the lovely glistening bands
which run across and across their covering membrane, for the sphagnum
leaf is so extremely beautiful that you will never forget it when once
seen. It is through these large cells in the edge of the stem and leaf
that the water rises up from the swamp, so that the whole moss is like a
wet sponge.
[Illustration: Fig. 36.
Sphagnum moss from a Devonshire bog. (From life.)]
And now, before we part, we had better sum up the history of lichens and
mosses. With the lichens we have seen that the secret of success seems
to be mutual help. The green cells provide the food, the fungus cells
form a surface over which the green cells can spread to find sunlight
and moisture, and protection from extremes of heat or cold. With the
mosses the secret lies in their standing on the borderland between two
classes of plant life. On the one hand, they are still tender-celled
plants, each cell being able to live its own life and make its own food;
on the other hand, they have risen into shapely plants with the
beginnings of feeble roots, and having stems along which their leaves
are arranged so that they are spread to the light and air. Both lichens
and mosses keep one great advantage common to all tender-celled plants;
they can be dried up so that you would think them dead, and yet, because
they can work all over their surface whenever heat and moisture reach
them, each cell drinks in food again and the plant revives. So when a
scorching sun, or a dry season, or a biting frost kills other plants,
the mosses and lichens bide their time till moisture comes again.
In our own country they grow almost everywhere--on walls, on broken
ground, on sand-heaps, on roofs and walls, on trees living and dead, and
over all pastures which are allowed to grow poor and worn out. They
grow, too, in all damp, marshy spots; especially the bog-mosses forming
the peat-bogs which cover a large part of Ireland and many regions in
Scotland; and these same bog-mosses occur in America, New Zealand, and
Australia.
In the tropics mosses are less abundant, probably because other plants
flourish so luxuriantly; but in Arctic Siberia and Arctic America both
lichens and mosses live on the vast Tundras. There, during the three
short months of summer, when the surface of the ground is soft, the
lichens spread far and wide where all else is lifeless, while in moister
parts the Polytrichums or hair-mosses cover the ground, and in swampy
regions stunted Sphagnums form peat-bogs only a few inches in depth over
the frozen soil beneath. If, then, the lichens and mosses can flourish
even in such dreary latitudes as these, we can understand how they defy
even our coldest winters, appearing fresh and green when the snow melts
away from over them, and leave their cells bathed in water, so that
these lowly plants clothe the wood with their beauty when otherwise all
would be bare and lifeless.