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CHAPTER XII
ON PLANTS WHICH ADD TO CONTINENTS
Lake Aral and Lake Tschad – Mangrove swamps of West Africa – New mudbanks colonized – Fish, oysters, birds, and mosquitoes – Grasping roots and seedlings – Extent of mangroves – Touradons of the Rhone – Sea-meadows of Britain – Floating pollen – Reeds and sedges of estuarine meadows – Storms – Plants on ships' hulls – Kelps and tangles in storms – Are seaweeds useless? – Fish.
THE way in which the savage, rugged, inhospitable Britain of the Ice Age changed into our familiar peaceful country formed the subject of the last chapter.
But plants do far more than cover the earth and render it fertile, for some of them assist in winning new land from the sea or from freshwater lakes. The Sea of Aral, for instance, or Lake Tschad are rapidly becoming choked up by reeds and other vegetation. Blown sand from the deserts around is caught and intercepted by these reeds, so that fertile pastures are gradually forming in what used to be the open water of a deepish lake.
By far the most extraordinary of all these plants which form new land are the Mangroves.
They are only found in the tropics or sub-tropical regions, and are always along the sea-coast. It is where a river ends in a delta, dividing into intricate and confused irregularly winding creeks, that the mangroves are especially luxuriant.
Such a river will have probably flowed through hundreds of miles of the most exuberant tropical forest, where growth is never checked by the cold grasp of winter.
Its waters are yellowish brown or café au lait coloured, because they are full of mud and of decaying vegetation, with dead leaves and decaying branches floating on the surface. So full are such rivers of decaying material that they have a distinct and unmistakable smell, which has been compared to "crushed marigolds."
So soon as the muddy water reaches the sea, most of its mud is deposited and forms great banks and shoals of shifting odoriferous slime, which confuses and interferes with the discharging mouths of the river.
It is upon these changing, horrible-smelling banks of bottomless slime that the Mangrove is especially intended to develop.
If one takes a canoe in such a delta and paddles inwards on the incoming tide, a dense forest of glossy-green mangroves will be found to cover the whole coast-line, and also to extend far inland by the winding creeks, lagoons, and river channels.
The whole theory of the mangroves becomes clearly revealed as soon as the water begins to sink at low tide. First one notices that the stem of every mangrove ends below, not in a single trunk, but in an enormous number of arched, stilt-like supporting roots. Not only the stem but the branches also give off descending roots, which branch into four or five grasping arched fingers as soon as they get near the water. When they reach the mud, these fingers grow down into it and form a new supporting root to the tree. It is very difficult to give any idea of the extraordinary appearance of these mangrove roots.
Imagine an orchard of very old apple trees in winter, and suppose that one were to cut off every tree and plant it upside down in black mud, and also to crowd them so closely together that the branches were all mixed and confused. This may give an idea of the odd and strange appearance of the root-system in a mangrove forest. Upon these arching roots, even on those which are not yet attached, multitudes of oysters may be seen. There is also a little fish (a sort of perch) which climbs up on to the roots or out of the mud below, and gasps or squatters about in it.
As to the mud itself, it is a horrible, greasy, oozy, black or blue-black slime of bottomless depth. "It is full of organic, putrefying, strongly-smelling material, clearly full of bacteria. The water itself is sometimes covered by a dirty, oily scum, and air-bubbles rising from the bottom, spread out on the surface and let loose their microbes in the atmosphere."74 There are many crocodiles, which may be seen reposing on the mud above high tide. It is difficult to distinguish them from a rough log of wood, but it is still more difficult to kill them, for their scales turn any ordinary bullet. There is scarcely any experience more exasperating than when, after one has taken a long, careful, and accurate aim, one observes the sleeping brute suddenly wake and scurry down into the water with a hideous leer on its face. Sea-cows or manatees are said to live in these creeks. Little ducks of many kinds rise in hundreds and thousands, but the commonest bird is the "curlew" (either a whimbrel or closely allied to it). During the day they sift the mud with their long curved beaks for insects, and at sunset fly down in vast numbers to the mudbanks near the sea. A miserable little white crane called "Poor Joe" is common, and has the same habit. It is not worth shooting, and it is quite aware of the fact. Herons, cormorants, and other birds are often to be seen. Monkeys sometimes visit the mangroves, probably to eat oysters or crabs. There are several kinds of crablike creatures which climb up the roots and may be seen running about all over them. But during the three weeks spent by the writer in the Mahéla creeks of Sierra Leone, it was the insects that made the deepest impression upon him; as soon as the evening falls the mosquitoes appear in myriads and in millions. Such creeks and mangrove swamps are always feverstricken and dangerous, and probably enjoy the very worst climate in the whole world. Of course nowadays, when Sir Patrick Manson and Dr. Ross have discovered that the mosquito carries the malaria germ, it is possible with great care to guard against malaria. One has also the satisfaction of knowing that the mosquito itself cannot be perfectly at ease with all these tiny parasites attacking its digestive organs.
At first sight such swamps appear to be useless, impossible, and dangerous. But that is not the case. No one, of course, would ever willingly reside in mangrove swamps, and the mangroves themselves are of scarcely any use to man, although the bark does sometimes furnish a useful tanning material; but, indirectly, the mangroves are one of the most important of all Nature's geographical agents.
On those horrible, slimy, shifting mudbanks no other plants could manage to exist. If one looks carefully at the seaward side of the last of the mangrove swamps, then it is easy to see that they are colonizing and reclaiming the mud.
Not only do the roots depending from the branches grasp and colonize new mud, but the seedlings are also specially adapted to fulfil the same office.
They remain a long time attached to the parent fruit; they also grow to a considerable length before they fall off. When ready to fall, they have a distinct seedling stem, which swells out towards the base and ends in a pointed root. The seedling is, in fact, like a club hanging upside down and with a pointed end. When it does fall, it goes straight down deep into the mud; then it promptly forms some anchoring roots, and the young mangrove is fixed in new mud and begins to develop. So that the forest continually grows towards the sea.
Such mudbanks soon become pierced by roots in every direction. Then the leaves of the mangroves themselves, as well as silt, soil, and rubbish floating in the water, gradually accumulate about and around these roots. This must raise the level of the ground. Eventually the soil becomes hardened and is above the level of the water. When this happens, the mangrove, which likes salt water about its roots, becomes unhealthy and the ordinary jungle trees kill it and take its place. Thus in course of time, when the jungle is cleared, fertile ricefields may be thriving on what was once a pure, or rather impure, mudbank.
In this way, by the continual development of the mangroves, enormous stretches of land are being added to the continents, and the process continues so long as the character of the coast-line favours it.
The shore-line covered by these mangrove swamps is enormous. In fact, within the tropics one finds them almost everywhere along the seashore, but coral, rock, or an exceedingly dry climate such as that of Arabia or Northern Peru, prevents their growth. Central and South America, West and East Africa, India, Polynesia, Australia, and much of the Asiatic coast-line, is covered by mangroves.
Theophrastus speaks of those in the Persian gulf, and that exceedingly shrewd botanist has some valuable notes about them worth reading even to-day.75
In temperate countries, such as our own, the districts where great rivers enter the sea are for the most part aguish and rheumatic, but, of course, there is nothing so startling and extraordinary as the mangrove swamps.
Yet, even in temperate countries, the work of winning or gaining new land plods steadily onwards, and it is performed by humble, inconspicuous little plants.
Where the Rhone enters the Mediterranean, there are some 40,000 acres of sandy and clayey land called the Camargue. The bare sand near the sea is often flooded and swept by violent storms in winter; anything which tries to grow there is usually carried76 off and destroyed.
[note 75-b: Drude, l. c.; Schimper, l. c.; Warming, l. c.; Colonial Reports, No. 3, Miscellaneous. Schimper, Indo-Malayische Strandflora.]
But, after a time, one finds here and there a solitary plant of a kind of Saltwort (Salicornia macrostachya) which has withstood the strain: its branches gather a little sand and hold it together, and its roots gradually explore and tie down the soil around it. Next winter it can stand the sweep and scour of the stormy water; next summer other plants begin to grow on this tiny sand-heap, and the "touradon," as it is called, is now fairly well established. It goes on growing until it may be, after a few years, six feet in diameter.
Eventually the salt gets washed out of the soil and these little heaps become united by a continuous covering of green plants in which shrubs and then trees begin to grow.77 By this time of course the sand has accumulated farther out to sea and the same process is going on there.
In Britain we have the "sea meadows" of Sea-grass, which covers the submerged sand and mudbanks near the mouths of great rivers.
The waving green grasslike leaves form a rich submarine meadow. They are used for stuffing pillows and cushions, especially in Venice, but their real importance in the world depends upon their being able to tie down and fix permanently those unseen shifting banks which form a real danger to all navigation.
These plants are very remarkable. They lived, no doubt, at one time on the land, like most of the flowering plants. But, like the whale and the seal, they have been driven to take refuge below the ocean. They are not easily seen, and, indeed, one may wander for years along the sea-coast and never suspect that great meadows of Zostera (the Eelwrack, Grasswrack, or Sea-grass) are flourishing under water.
But, one might ask, how is the pollen of its flowers carried? Obviously neither insects nor the wind can be of any service. The pollen of Zostera is, however, of the same weight exactly as the water, so that it neither rises to the surface nor sinks to the bottom, but floats to and fro until it reaches the outspread styles of another plant. This is perhaps the most remarkable arrangement known for the carrying of pollen.
Sometimes along the seashore, or especially on the muddy foreshore of an estuary or tidal river, one can watch those plants which are trying to form new land. One finds generally that there is a broad stretch of marshy meadow interrupted and intersected by small ditches and little winding streams. As one gets towards the shore, Sea-pink, Scurvy-grass, an Aster, and other plants, not to be found elsewhere, become common. Then stretching out into the mud there are rows of curious reeds and sedges.
Try to pull up one of these reeds, and you will find a strong, buried, stringy stem, with hundreds of anchoring roots. These are the pioneers which first fix the sand.
Over the surface of the sand between these upright stems, one often comes upon a most beautiful, glossy, dark-green, velvety cushion. It is composed of a seaweed called Vaucheria, whose twined and interlaced threads form a thick, silky cushion. But it is only beautiful to look at from above. If you pull up a piece of this cushion, you will find that it is growing on black and loathly mud, with many wriggling worms and horrible animalcula. First these pioneer reeds, then this soft, silky carpet of vaucheria, and then the sea-pinks and other estuarine marsh flowers gradually creep forward and extend over the bare muddy sand, so winning it from the sea for the use of cattle.
In the worst winter storms, when the waves are thundering heavily over these sands, it seems as if nothing could resist them. Yet if you go down when the storm is over, no harm has been done: there is the silky green cushion of vaucheria, and there are the lines of pioneer sedges and reeds quite undisturbed!
The reeds bend and sway, yielding to the water; the seaweed is slimy and oily, and the water cannot injure it. But yet the strength of these seaweeds is extraordinary, and, indeed, almost incredible.
More remarkable still, perhaps, are those seaweeds which grow upon rocks, often where the full strength of the waves beats upon them. After a heavy storm, when, perhaps, the great timbers of groins and the heavy concrete blocks of an esplanade have been shattered to pieces and tossed all over the shore, one may go down to the shore and there will be no visible difference in the kelps and tangles of the rocks. Scarcely any seem to have been broken away. Indeed, if one looks in the rubbish left by the last high tide, one finds that when one of these Alarias has been broken away, it is often because the stone itself has been torn out of the rock! One finds broken off stones with the seaweed still attached to them.
The reason is that the outside of the seaweed is oily, slimy, or slippery, so that the water gets no hold of it. The stem and substance is also elastic and surprisingly strong, so that the daily tossing and wrenching when the tides come in and go out has no effect in tearing it away.
But if you go down to a dry dock and look at the hull of a ship which has come in to be cleaned and scraped, you will see that it is entirely covered by seaweeds and shells.
That ship has been driven through the water perhaps at ten miles an hour or more, and yet those delicate-looking seaweeds have held on! It is more surprising still if you can get some of them and examine them with a microscope, for amongst them are tiny, delicate, graceful little fronds and sprays which one would think consisted of nothing but jelly. Yet they have been able to thrive and grow on the ship's hull while it has been hurrying day and night through the sea, in calm or in tempest, and in currents of hot or cold water.
Those seaweeds were called by Horace Algæ inutiles, or useless seaweeds; but are they useless?
Go down to a little pool and watch them waving in the water. Could anything be more beautiful than these little graceful red, yellow, or brown sprays? All sorts of seaslugs, shrimps, and minute animals of weird and wonderful design are clearly living on them. Fishes live upon these animals, and fishes are an extremely useful and excellent food for man.
CHAPTER XIII
ROCKS, STONES, AND SCENERY
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 (Lecidea78) 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.79
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!80 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.
