An old wall—Beautiful colours—Insects—Nature's chief aim—Hard times of lichens—Age of lichens—Crusts—Mosses—Lava flows of great eruptions—Colonizing plants—Krakatoa—Vesuvius—Greenland volcanoes—Sumatra—Shale-heaps—Foreigners on railway lines—Plants keep to their own grounds—Precipices and rocks—Plants which change the scenery—CaÑons in America.

AT first sight, and when one is striding along at some four miles an hour, there seems to be nothing at all interesting in an old wall. But if one stops and carefully examines the stones, there is a great deal that is interesting.

Rocks and walls possess a fascination of their own. Probably at least 2000 British plants are only found upon them, and yet of these, the vast majority are so small and inconspicuous that an ordinary person never perceives a single one of them.

It is perhaps on rocks or old walls near the sea that this stone flora is most richly developed. The nearly circular orange-yellow patches of the Lichen Physcia parietina are quite distinct and conspicuous. But any old wall, provided it is well out in the country, is pretty sure to be interesting.

At first it seems to have only a dull grey or neutral tint. But if one goes to four or five feet distance, one discovers that many shades of brown, red, white, and black go to make this grey. But the extraordinary beauty of such a wall is only visible when one peers and scrutinizes the surface very slowly and carefully with the eyes six or seven inches away from it.

In doing this, one is often troubled by rude and ribald boys. A botanical friend indeed complained that he had been for months avoided and shunned as a dangerous wandering lunatic on account of his botanical enthusiasm. But true botanists get accustomed to disagreeable incidents like that, and pay no attention to the vulgar crowd.

The change in an old wall when one looks at it from a few inches distance is most remarkable. The entire surface is spotted or dusted, sprinkled or entirely covered by thick lichen stains and crusts.

The original colour of the stone is nowhere visible. The lichens show the most delicate shades and contrasts in colour; all pleasing and all blending together in harmonious general tones. The fruit of these lichens is like a minute saucer or platter generally with a thin rim or border, but it is exceedingly small, probably only one-sixteenth of an inch in diameter, or even less. The smallest of these crust lichens form continuous, very thin, coatings, covering the stone; and against this background the little saucer-like fruits show up quite distinctly.

The coating itself varies from "bright yellow, pale ochre, citron, chestnut colour, to mouse colour, different shades of grey and green, cream colour, lead colour, blue-black or pure black, tawny, brown, rusty red or pure white." The cups of one kind (Lecidea[77]) are black, whilst those of Lecanora are generally reddish-brown. But they may be a ghostly pale hue which stands out plainly against the grey-green background of the frond. Sometimes they are of the richest deep crimson or lake, set against a pure snow-white crust. Those of Lecanora vitellina are, though tiny, a brilliant yellow, and quite startling when first one notices them. Many of these contrasts and shades are never used by artists, and even from the mere artistic point of view they have great interest.

But if, after spending a few minutes in carefully looking over the rocks at a distance of six or seven inches, one stands up and goes back to four or five feet away, the whole of this colour scheme fades away and there is only the monotonous indeterminate grey or neutral tint of the wall.

Now why is this? Why should these delicate and exquisite shades be wasted on such minute and scarcely distinguishable forms?

There are always two sides from which one can look at any subject, namely the inside and the outside.

From the inside (that is from the point of view of the little lichen itself) these colours are decidedly useful. Small insects crawl about on such walls or hover a few inches in front of them, and to those insects these cups will be as conspicuous and attractive as a scarlet geranium is to ourselves.

Just as we habitually go to look at a geranium, so those insects fly towards the cups and crawl about on them. Then when the spores and dust of the lichen begin to stick in their hairs and feet, they go to a bare place and clean or brush them off. Thus the spores and dust are carried to a new part of the rock, where they will grow if they can find an unoccupied place. The taste in colour of these insects, moreover, is apparently not very different from that of man.

But perhaps a still more interesting point of view is that from the outside. Why are those lichens there? What are they doing, and are they of any use? The general scheme of Nature is to cover the whole world with green, so that every ray of sunlight may find a working leaf or green frond ready to welcome it and use it. Nature abhors bare rock, barren sand, and empty water, and never ceases to try to bring it under that beautiful covering of green plants and active vegetable life which supports both man and animals.

We all know that there is a romance in the story of man's colonies. First the explorer searches out the country; then the pioneer frontiersman settles and builds his log-hut or rough shanty. Next comes the frontier village, which may perhaps in many years' time become a crowded city where active, valuable work is carried on.

The story of the colonizing of rocks and stones by plants is just as vividly interesting. These tiny lichens are almost the first pioneers, and prepare the ground for those that follow. Upon that bare rock, life is terribly severe. The frost shatters it, sunshine heats it until it almost burns the hand in summer. Floods of rain or of sleet beat against it, and it may be frozen over for weeks.

What plant can stand such conditions? Only these minute, tiny, scarce visible lichen films!

Gradually new lichen crusts develop upon it. They cover over the first pioneers; first they suffocate them and afterwards devour their remains. Nature is very businesslike and severe in her working. The lichen crust may be now about one-sixteenth of an inch thick. It is a very slow process. There is a story of a boy who noticed a patch of lichen near his father's door. He went away to Kamschatka or somewhere and came back a very old man of eighty-five years; but he found that the lichen patch was just the same size as when he went away. That, however, is just a story! At any rate, one of these little crust-lichens called Variolaria has been known to increase half a millimetre in size (about a sixtieth part of an inch) between the end of February and that of September.

Now if one tries to realize what the life of such a lichen crust or crottle must be, it is obvious that the stone below it must be a little corroded or weathered, and remains of the first choked pioneers, bacteria, and possibly tiny insects or animalcula will be under the crust, which may now be one-sixteenth of an inch thick.

It is the turn now of other lichens to colonize it. These may be the little trumpet or horn and cup lichens, Cladonias, or perhaps the larger grey kinds, Parmelias and Physcias, which have leaf-like fronds and form circles of perhaps eight to ten inches in diameter. The crust-lichen is overgrown, broken up, disorganized, and devoured by the Parmelias and Cladonias, who are helped by bacteria, insects, and animalcula which shelter below them. These leafy lichens grow much more rapidly.

They may increase two-thirds of an inch in one year.

But very soon after this, one notices a few inconspicuous green mosses; at first in crevices between the stones or in hollows, and not remarkable, they soon increase and form trailing sprays or branches which grow very quickly. Branches of moss four or five inches long extend over the leafy lichens in a season. The Parmelias and Cladonias struggle on, but they cannot keep pace with the rapid life of the moss, and soon our wall is covered by beautiful moss turfs.

Underneath such a turf there may be an inch or so of good soil (dead moss and dust with lichen and insect bodies). Worms, insects, etc., shelter and flourish and multiply in this soil. But the turn of the moss is coming. Here a few grass-blades, there a tiny plant of Sandwort, possibly a Rock Bedstraw, begin to root themselves in the moss.

If people would only let the wall alone, it would soon be festooned with hanging plants, and producing quantities of grass, but somebody is sure to find that it looks very untidy, and everything is torn off the wall, which again looks new and raw and clean. Then of course the pioneer lichens begin again!

Some very interesting and remarkable facts have been discovered about the way in which lavas and basalts have been occupied by the plant world.

In the great volcanic eruption of 1883, the whole island of Krakatoa was covered by hot lava and glowing ashes. In 1884 and 1885 the sunsets were remarkable for a curious fiery red or orange glow, which was popularly supposed to be due to the volcanic dust of that explosion. It is said that the dust travelled three times round the earth, though I do not know on what authority.

However, on Krakatoa island there was left a clean "slate." There were neither bacteria, nor leaf-mould, nor living plants of any kind; no spores or seeds could have endured the fiery furnace of the eruption.

Three years afterwards the botanist Treub visited the island. He found that the rocks had been first covered by thin layers of minute freshwater AlgÆ, but that ferns were then occupying and inhabiting the lavas. Eleven kinds of ferns, and but very few other plants, were discovered.

People were interested in this, and Dr. A. F. W. Schimper then visited another volcano which had been pouring out huge streams of lava in 1843. He found that there were still plenty of ferns, but also numbers of shrubs and other plants. Yet even then there were no trees, and there was no continuous mantle of green plants such as we are accustomed to in this country. He also found many plants growing on the lava which are generally found on the branches of trees, that is, which can do without a thick layer of soil. He also found quantities of a pitcher plant, Nepenthes (which lives mainly on insects caught in its pitchers).

This does not at first sight seem to agree at all with what has been given for the walls. It is true that sometimes in the Highlands, or Lowland and Lakeland Hills, one comes across quantities of the Bladderfern and others growing on the "screes." (These last may be described as streams of broken, angular stones, filling small gullies, and spreading out at the base over a considerable space.) Often these ferns seem to be all that can thrive in amongst the stones. But in a mild and temperate country like our own, one would expect things to proceed differently.

And in fact they do so. Every one must have noticed a green stain which covers wet walls, stones, stucco, even marble statues, and especially tree bark in wet or damp situations. This is a minute green seaweed rejoicing in the name of Pleurococcus. It is a pretty object for the microscope.

This, of course, is the first stage of colonization. It is followed by mosses of sorts.

But there is a more interesting series still in a climate resembling our own. The lava-flows from Mount Vesuvius have been investigated by several observers.

There it was found that the first inhabitants were lichens and small green seaweeds; then "different mosses occupied the lava over which a certain quantity of vegetable dust had been scattered." After this, scattered ferns and even small shrubs could be seen even on flows which were red-hot only twenty years before, whilst on old lava-fields herbs, shrubs, bushes, trees, and even true woods had developed.[78]

Yet in Greenland lava-flows dating from 1724-29 are still only covered by crust-lichens and a very few of the stone-mosses! In Sumatra, on the other hand, the volcano of Tamboro, which in 1815 had entirely destroyed its vegetation, was covered with a fine young wood in 1874![79] The strong heat and abundant moisture of Sumatra favours, whilst the horrible climate of Greenland prevents, the rapid growth of good soil. Just as cities of 20,000 inhabitants can spring up in a few months in the Western United States, whilst the Esquimaux of Greenland have not managed as yet even to live in villages!

The full beauty of this gradual colonization and occupation of bare rock and stones only impresses one properly if one tries to trace the stages, but it is an interesting history.

Near Glasgow one sees great heaps of shale or blaes (generally blackband), which are often mistaken for natural hills. This is or was virgin soil, never occupied by plants, and entirely destitute of leaf-mould or any sort of organic plant-food.

If one scrambles to the top of one of these heaps, it is easy to see all the details of the occupation. Long underground runners of coltsfoot and of horsetail are climbing up the sides, fringes of creeping buttercup, couchgrass, and other hardy weeds occupy, every year, a little more of the flanks, but, on the top, one very soon finds that the dust of the atmosphere, aided by weathering, has afforded a chance to mosses, to hawkweeds, and other rock plants. These in time cover the top, and soon hardy grasses and weeds form a regular turf on the top of the shale.

It is interesting to scramble to the top of one of these heaps, especially in summer. One then begins to realize how every plant attends strictly to its own business.

All over the sides of the heap there will be hundreds of a rare groundsel (Senecio viscosus), which is not really a native, and never occurs except on such places. In a grass field close by hundreds of thousands of Ragwort (Senecio jacobÆa) make a glorious golden carpet; in the marshy part of the meadow the Water Ragwort (Senecio aquaticus) may be found. In the cottage gardens and here and there along the roadside the groundsel (Senecio vulgaris) is flourishing abundantly.

These plants never interfere with or encroach upon one another's grounds. Every year thousands of ragweed and groundsel seeds must be blown on to the shale-heap, but they never manage to grow there.

It is only the foreigner (S. viscosus), accustomed to a very hot and dry climate, and with sticky leaves which catch atmospheric dust and probably insects, that can exist on the bare shaly sides. These slopes of shale are easily heated by the sun, and at the same time radiate the heat rapidly away, so that the Viscid Groundsel must have a very hard time of it. When its roots have worked up the shale a little, and its dead leaves have covered the surface with mould and organic matter, then possibly others (true British plants) can get a footing and suppress it.

Along railway tracks, also, the ballast forms a very hot, a very dry, and a very barren soil. Many of the regular railway-track plants are foreigners from the far south, even from the sunny shores of the Mediterranean. They are mostly annuals, such as the little Toadflax (Linaria minor), which can just manage to exist under those conditions.

Of course, the sides of the banks and of cuttings on railways are generally formed of good earth or soil, and support a rich and flourishing flora of true Britons.

Besides these slow, laborious lichens, mosses, and others which attack rock, there are other plants which are generally called rock plants, though they behave quite differently.

These are those fine hardy Hawkweeds, Roseroots, Sempervivums, Mew, and others which establish their roots in cracks or crevices of the rocks.

Such cracks are soon full of good soil, for the wind blows decayed leaves and dust into them, and the roots are always burrowing into, eating into, and shattering the rocks. Most of them have a circle of leaves which are pressed flat to the ground. Thus they escape the violent winds and storms always common on such crags and precipices. The flowers, however, supported on tough, strong, and flexible stalks, sway freely to and fro in the wind, and can be seen by insects a long way off.

These rock plants are of some importance as stonebreakers and pioneers in a very interesting process.

Wherever a cliff or precipice of stone is exposed, it is "weathered." Water gets into the cracks and freezes in winter. But when water is frozen it expands or widens, and as this happens to the water in the crevices and cracks of rocks, pieces of rock are shivered and broken off. Besides frost and wind and rain, these rock plants help to attack the cliff. Their roots get into the crevices, and there widen and expand, tearing off great slabs and splinters of rock which fall down to the foot of the cliff. Down below plants are every year growing over and covering up or "happing up" with green these bare fragments and splinters. A considerable amount falls down every year, so that the ground is always being raised up below the precipice. At the brow or edge above the precipice, there is also always a loss of rock and stone every year.

So that every year the bare rock exposed becomes smaller and smaller, until eventually a steep, green, grass-covered slope covers over the entire site of that precipice.

Moreover that is not by any means all that plants do in the way of changing the scenery of the country. Look at the outlines of the hills in any part of Great Britain except in the broken, jagged, rocky mountain ranges of Scotland and Wales (also Cumberland, Westmorland, parts of Derbyshire and Dartmoor tors). Everywhere there are smooth, flowing, gently undulating rises and falls. No sharp, abrupt descents break these graceful sweeping curves. If you compare the scenery of a caÑon in the rainless deserts of Western America, the contrast is very striking. There the sides of the valleys are steep cliffs; it is all harsh, precipitous, horrible country, which is obviously very unpleasant and very unattractive to civilized people.

It is this green covering of plants which makes the difference. The rain that falls is not allowed to cut out ragged ravines; it is intercepted and soaks into the grasses, which so keep a smooth, gentle outline over hill and valley.

If you notice the effect of a heavy shower of rain on a road or bare earth, you will see how soon tiny valleys and caÑons and beds of streamlets are cut out. But on the green fields beside the road, there is no change in the surface at all! It seems to be quite unaffected by the heaviest storm of rain.