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.
[74] Warming, _Lehrbuch der OEcol Pfl. Geog._
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.
[Illustration: _Stereo Copyright, Underwood & Underwood_
_London and New York_
A RICEFIELD IN CHINA
The proprietor and two of his coolies are in front of the paddy
field. The young rice plants can be seen growing in the water.]
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]
[75] Drude, _l.c._; Schimper, _l.c._; Warming, _l.c._; _Colonial
Reports_, No. 3, Miscellaneous. Schimper, _Indo-Malayische
Strandflora_.
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 carried[75-b] 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.[76] By this time of course the
sand has accumulated farther out to sea and the same process is going
on there.
[76] Flahault, after Schimper, _l.c._
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.