THE LIFE-HISTORY OF LICHENS AND MOSSES
[Ill.u.s.tration]
The autumn has pa.s.sed 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 ma.s.s 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 pa.s.ses 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 cla.s.sing 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 s.p.a.ce 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 pa.s.s by, and they found their way into my basket, crowned at the top with a large ma.s.s 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.
[Ill.u.s.tration: 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 gla.s.s 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.
[Ill.u.s.tration: 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 b.u.t.t, 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 sc.u.m 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 sh.o.r.es 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.
[Ill.u.s.tration: 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 cla.s.ses 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-b.u.t.t. 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-gla.s.s, 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.
[Ill.u.s.tration: 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_.
[Ill.u.s.tration: 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 ma.s.s? 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 ma.s.s below the growing moss, very much like the green sc.u.m on a pond. This film, some of which I have brought home, is seen under the microscope to be a ma.s.s 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 ma.s.s 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.