Through Magic Glasses and Other Lectures Part 6

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 gla.s.s slide under the covering gla.s.s 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.

[Ill.u.s.tration: 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 ma.s.s or protonema below, and, starting on their own account, form other thread ma.s.ses.

Then, after the moss stems have grown, a new ma.s.s 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.

[Ill.u.s.tration: 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 gra.s.ses cannot live, and clothe walls and roofs with a rich green.

[Ill.u.s.tration: 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 k.n.o.b 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 s.p.a.ce 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 ma.s.s 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 ma.s.s 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-ma.s.s 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.

[Ill.u.s.tration: 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 cla.s.ses 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 lat.i.tudes 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.

CHAPTER V

THE HISTORY OF A LAVA STREAM

[Ill.u.s.tration]

It is now just twenty-two years ago, boys, since I saw a wonderful sight, which is still so fresh in my mind that I have to look round and remember that it was before any of you were born, in order to persuade myself that it can be nearly a quarter of a century since I stood with my feet close to a flowing stream of red-hot lava.

It happened in this way. I was spending the winter with friends in Naples, and we were walking quietly one lovely afternoon in November along the Villa Reale, the public garden on the sea-sh.o.r.e, when one of our party exclaimed, "Look at Vesuvius!" We did so, and saw in the bright sunlight a dense dark cloud rising up out of the cone. The mountain had been sending out puffs of smoke, with a booming noise, for several days, but we thought nothing of that, for it had been common enough for slight eruptions to take place at intervals ever since the great eruption of 1867. This cloud, however, was far larger and wider-spread than usual, and as we were looking at it we saw a thin red line begin some way down the side of the mountain and creep onwards toward the valley which lies behind the Hermitage near where the Observatory is built (see Fig. 37). "A crater has broken out on the slope," said our host; "it will be a grand sight to-night. Shall we go up and see it?" No sooner proposed than settled, and one of the party started off at once to secure horses and men before others engaged them.

[Ill.u.s.tration: Fig. 37.

Somma. Vesuvius.

Vesuvius, as seen in eruption by the author, November 1868.]

It was about eight o'clock in the evening when we started in a carriage for Resina, and alighting there, with buried Herculaneum under our feet, mounted our horses and set forward with the guides. Then followed a long ascent of about two hours and a half through the dark night. Silently and carefully we travelled on over the broad ma.s.ses of slaggy lava of former years, along which a narrow horse-path had been worn; and ever and anon we heard the distant booming in the crater at the summit, and caught sight of fresh gleams of light as we took some turning which brought the glowing peak into view.

Our object was to get as close as possible to the newly-opened crater in the mountain-side, and when we arrived on a small rugged plain not far from the spot, we alighted from our horses, which were growing frightened with the glare, and walked some distance on foot till we came to a ridge running down the slope, and upon this ridge the lava stream was flowing.

Above our heads hung a vast cloud of vapour which reflected the bright light from the red-hot stream, and threw a pink glow all around, so that, where the cloud was broken and we could see the dark sky, the stars looked white as silver in contrast. We could now trace clearly the outline of the summit towering above us, and even watch the showers of ashes and dust which burst forth from time to time, falling back into the crater, or on to the steep slopes of the cone.

If the night had not been calm, and such a breeze as there was blowing away from us, our position would scarcely have been safe; and indeed we were afterwards told we had been rash. But I would have faced even a greater risk to see so grand a spectacle, and when the guide helped me to scramble up on to the ledge, so that I stood with my feet within a few yards of the lava flow, my heart bounded with excitement. I could not stay more than a few seconds, for the gases and vapour choked me; but for that short time it felt like a dream to be standing close to a river of molten rock, which a few hours before had been lying deep in the bowels of the earth. Glancing upwards to where this river issued from the cone in the mountain-side, I saw it first white-hot, then gradually fading to a glowing red as it crept past my feet; and then looking down the slope I saw it turn black and gloomy as it cooled rapidly at the top, while through the cracks which opened here and there as it moved on, puffs of gas and vapour rose into the air, and the red lava beneath gleamed through the c.h.i.n.ks.

We did not stay long, for the air was suffocating, but took our way back to the Hermitage, where another glorious sight awaited us. Some way above and behind the hill on which the Observatory stands there is, or was, a steep cliff, and over this the lava stream, now densely black, fell in its way to the valley below, and as it fell it broke into huge ma.s.ses, which heeling over exposed the red-hot lava under the crust, thus forming a magnificent fiery cascade in which black and red were mingled in wild confusion.

This is how I saw a fresh red-hot lava stream. I had ascended the mountain some years before, when it was comparatively quiet, with only two small cones in its central crater sending out miniature flows of lava (see Fig. 38). But the crater was too hot for me to cross over to these cones, and I could only marvel at the ma.s.s of ashes of which the top of the mountain was composed, and plunge a stick into an old lava stream to see how hot it still remained below. Peaceful and quiet as the mountain seemed then, I could never have imagined such a glorious outburst as that of November 1868 unless I had seen it, and yet this was quite a small eruption compared to those of 1867 and 1872, which in their turn were nothing to some of the older eruptions in earlier centuries.

[Ill.u.s.tration: Fig. 38.

The top of Vesuvius in 1864. (After Nasmyth.)]

Now it is the history of this lava stream which I saw, that we are going to consider to-day, and you will first want to know where it came from, and what caused it to break out on the mountain-side. The truth is, that though we know now a good deal about volcanoes themselves, we know very little about the mighty cauldrons deep down in the earth from which they come. Our deepest mines only reach to a depth of a little more than half a mile, and no borings even have been made beyond three-quarters of a mile, so that after this depth we are left very much to guesswork.