has all happened and is still happening. Each mountain torrent that
leaps from the Tyrolese Alps bring down in its lap a rich mass of mud,
which has gradually spread over a strip of sea some forty or fifty
miles wide, from the base of the mountains to the modern coast-line of
the province. Near the sea--or, in other words, at the temporary
outlet--it forms banks and lagoons, of which those about Venice are the
best known to tourists, though the least characteristic. For miles and
miles between Venice and Trieste the shifting north shore of the
Adriatic consists of nothing but such accumulating mud banks. Year
after year they push farther seaward, and year after year fresh islets
and shoals grow out into the waves beyond the temporary deltas. In
time, therefore, the gathering mud banks of these Alpine torrents must
join the greater mud bank that runs rapidly seaward at the delta of the
Po. As soon as they do so the rivers must rush together, and what was
once an independent stream, emptying itself into the Adriatic, must
become a tributary of the Po, helping to swell the waters of that great
united river. The Adige has now just reached this state: its delta is
continuous with the delta of the Po, and their branches interosculate.
The Mincio and the Adda reached it ages since: the Piave and the
Livenia will not reach it for ages. In Roman days Hatria was still on
the sea: it is now some fifteen miles inland.
From all this you can gather why the existing Po flows far from the
Alps and nearer the base of the Apennines. The Alpine streams in far
distant days brought down relatively large floods of glacial mud;
formed relatively large deltas in the old Lombard bay; filled up with
relative rapidity their larger half of the basin. The Apennines, less
lofty, and free from glaciers, sent down shorter and smaller torrents,
laden with far less mud, and capable therefore of doing but little
alluvial work for the filling in of the future Lombardy. So the river
was pushed southward by the Alpine deposits of the northern streams,
leaving the great plains of Cisalpine Gaul spread away to the north of
it.
And this land-making action is ceaseless and continuous. About Venice,
Chioggia, Maestra, Comacchio, the delta of the Po is still spreading
seaward. In the course of ages--if nothing unforeseen occurs meanwhile
to prevent it--the Alpine mud will have filled in the entire Adriatic;
and the Ionian Isles will spring like isolated mountain ridges from the
Adriatic plain, as the Euganean hills--those 'mountains Euganean' where
Shelley 'stood listening to the pæan with which the legioned rocks did
hail the sun's uprise majestical'--spring in our own time from the dead
level of Lombardy. Once they in turn were the Euganean islands, and
even now to the trained eye of the historical observer they stand up
island-like from the vast green plain that spreads flat around them.
Perhaps it seems to you a rather large order to be asked to believe
that Lombardy and Venetia are nothing more than an outspread sheet of
deep Alpine mud. Well, there is nothing so good for incredulity, don't
you know, as capping the climax. If a man will not swallow an inch of
fact, the best remedy is to make him gulp down an ell of it. And,
indeed, the Lombard plain is but an insignificant mud flat compared
with the vast alluvial plains of Asiatic and American rivers. The
alluvium of the Euphrates, of the Mississippi, of the Hoang Ho, of the
Amazons would take in many Lombardies and half-a-dozen Venetias without
noticing the addition. But I will insist upon only one example--the
rivers of India, which have formed the gigantic deep mud flat of the
Ganges and the Jumna, one of the very biggest on earth, and that
because the Himalayas are the highest and newest mountain chain exposed
to denudation. For, as we saw foreshadowed in the case of the Alps and
Apennines, the bigger the mountains on which we can draw the greater
the resulting mass of alluvium. The Rocky Mountains give rise to the
Missouri (which is the real Mississippi); the Andes give rise to
Amazons and the La Plata; the Himalayas give rise to the Ganges and the
Indus. Great mountain, great river, great resulting mud sheet.
At a very remote period, so long ago that we cannot reduce it to any
common measure with our modern chronology, the southern table-land of
India--the Deccan, as we call it--formed a great island like Australia,
separated from the continent of Asia by a broad arm of the sea which
occupied what is now the great plain of Bengal, the North-West, and the
Punjaub. This ancient sea washed the foot of the Himalayas, and spread
south thence for 600 miles to the base of the Vindhyas. But the
Himalayas are high and clad with gigantic glaciers. Much ice grinds
much mud on those snow-capped summits. The rivers that flowed from the
Roof of the World carried down vast sheets of alluvium, which formed
fans at their mouths, like the cones still deposited on a far smaller
scale in the Lake of Geneva by little lateral torrents. Gradually the
silt thus brought down accumulated on either side, till the rivers ran
together into two great systems--one westward--the Indus, with its four
great tributaries, Jhelum, Chenab, Ravee, Sutlej; one eastward, the
Ganges, reinforced lower down by the sister streams of the Jumna and
the Brahmapootra. The colossal accumulation of silt thus produced
filled up at last all the great arm of the sea between the two mountain
chains, and joined the Deccan by slow degrees to the continent of Asia.
It is still engaged in filling up the Bay of Bengal on one side by the
detritus of the Ganges, and the Arabian Sea on the other by the
sand-banks of the Indus.
In the same way, no doubt, the silt of the Thames, the Humber, the
Rhine, and the Meuse tend slowly (bar accidents) to fill up the North
Sea, and anticipate Sir Edward Watkin by throwing a land bridge across
the English Channel. If ever that should happen, then history will have
repeated itself, for it is just so that the Deccan was joined to the
mainland of Asia.
One question more. Whence comes the mud? The answer is, Mainly from the
detritus of the mountains. There it has two origins. Part of it is
glacial, part of it is leaf-mould. In order to feel we have really got
to the very bottom of the mud problem--and we are nothing if not
thorough--we must examine in brief these two separate origins.
The glacier mud is of a very simple nature. It is disintegrated rock,
worn small by the enormous millstone of ice that rolls slowly over the
bed, and deposited in part as 'terminal moraine' near the summer
melting-point. It is the quantity of mud thus produced, and borne down
by mountain torrents, that makes the alluvial plains collect so quickly
at their base. The mud flats of the world are in large part the wear
and tear of the eternal hills under the planing action of the eternal
glaciers.
But let us be just to our friends. A large part is also due to the
industrious earth-worm, whose place in nature Darwin first taught us to
estimate at its proper worth. For there is much detritus and much
first-rate soil even on hills not covered by glaciers. Some of this
takes its origin, it is true, from disintegration by wind or rain, but
much more is caused by the earth-worm in person. That friend of
humanity, so little recognized in his true light, has a habit of
drawing down leaves into his subterranean nest, and there eating them
up, so as to convert their remains into vegetable mould in the form of
worm-casts. This mould, the most precious of soils, gets dissolved
again by the rain, and carried off in solution by the streams to the
sea or the lowlands, where it helps to form the future cultivable area.
At the same time the earthworms secrete an acid, which acts upon the
bare surface of rock beneath, and helps to disintegrate it in
preparation for plant life in unfavourable places. It is probable that
we owe almost more on the whole to these unknown but conscientious and
industrious annelids than even to those 'mills of God' the glaciers, of
which the American poet justly observes that though they grind slowly,
yet they grind exceedingly small.
In the last resort, then, it is mainly on mud that the life of humanity
in all countries bases itself. Every great plain is the alluvial
deposit of a great river, ultimately derived from a great mountain
chain. The substance consists as a rule of the débris of torrents,
which is often infertile, owing to its stoniness and its purely mineral
character; but wherever it has lain long enough to be covered by
earth-worms with a deep black layer of vegetable mould, there the
resulting soil shows the surprising fruitfulness one gets (for example)
in Lombardy, where twelve crops a year are sometimes taken from the
meadows. Everywhere and always the amount and depth of the mud is the
measure of possible fertility; and even where, as in the Great American
Desert, want of water converts alluvial plains into arid stretches of
sand-waste, the wilderness can be made to blossom like the rose in a
very few years by artificial irrigation. The diversion of the Arkansas
River has spread plenty over a vast sage scrub; the finest crops in the
world are now raised over a tract of country which was once the terror
of the traveller across the wild west of America.
THE GREENWOOD TREE.
It is a common, not to say a vulgar error, to believe that trees and
plants grow out of the ground. And of course, having thus begun by
calling it bad names, I will not for a moment insult the intelligence
of my readers by supposing them to share so foolish a delusion. I beg
to state from the outset that I write this article entirely for the
benefit of Other People. You and I, O proverbially Candid and
Intelligent One, it need hardly be said, are better informed. But Other
People fall into such ridiculous blunders that it is just as well to
put them on their guard beforehand against the insidious advance of
false opinions. I have known otherwise good and estimable men, indeed,
who for lack of sound early teaching on this point went to their graves
with a confirmed belief in the terrestrial origin of all earthly
vegetation. They were probably victims of what the Church in its
succinct way describes and denounces as Invincible Ignorance.
Now, the reason why these deluded creatures supposed trees to grow out
of the ground, instead of out of the air, is probably only because they
saw their roots there.
Of course, when people see a wallflower rooted in the clefts of some
old church tower, they don't jump at once to the inane conclusion that
it is made of rock--that it derives its nourishment direct from the
solid limestone; nor when they observe a barnacle hanging by its sucker
to a ship's hull, do they imagine it to draw up its food incontinently
from the copper bottom. But when they see that familiar pride of our
country, a British oak, with its great underground buttresses spreading
abroad through the soil in every direction, they infer at once that the
buttresses are there, not--as is really the case--to support it and
uphold it, but to drink in nutriment from the earth beneath, which is
just about as capable of producing oak-wood as the copper plate on the
ship's hull is capable of producing the flesh of a barnacle. Sundry
familiar facts about manuring and watering, to which I will return
later on, give a certain colour of reasonableness, it is true, to this
mistaken inference. But how mistaken it really is for all that, a
single and very familiar little experiment will easily show one.
Cut down that British oak with your Gladstonian axe; lop him of his
branches; divide him into logs; pile him up into a pyramid; put a match
to his base; in short, make a bonfire of him; and what becomes of
robust majesty? He is reduced to ashes, you say. Ah, yes, but what
proportion of him? Conduct your experiment carefully on a small scale;
dry your wood well, and weigh it before burning; weigh your ash
afterwards, and what will you find? Why, that the solid matter which
remains after the burning is a mere infinitesimal fraction of the total
weight: the greater part has gone off into the air, from whence it
came, as carbonic acid. Dust to dust, ashes to ashes; but air to air,
too, is the rule of nature.
It may sound startling--to Other People, I mean--but the simple truth
remains, that trees and plants grow out of the atmosphere, not out of
the ground. They are, in fact, solidified air; or to be more strictly
correct, solidified gas--carbonic acid.
Take an ordinary soda-water syphon, with or without a wine-glassful of
brandy, and empty it till only a few drops remain in the bottom. Then
the bottle is full of gas; and that gas, which will rush out with a
spurt when you press the knob, is the stuff that plants eat--the raw
material of life, both animal and vegetable. The tree grows and lives
by taking in the carbonic acid from the air, and solidifying its
carbon; the animal grows and lives by taking the solidified carbon from
the plant, and converting it once more into carbonic acid. That, in its
ideally simple form, is the Iliad in a nutshell, the core and kernel of
biology. The whole cycle of life is one eternal see-saw. First the
plant collects its carbon compounds from the air in the oxidized state;
it deoxidizes and rebuilds them: and then the animal proceeds to burn
them up by slow combustion within his own body, and to turn them loose
upon the air, once more oxidized. After which the plant starts again on
the same round as before, and the animal also recommences _da capo_.
And so on _ad infinitum_.
But the point which I want particularly to emphasize here is just this:
that trees and plants don't grow out of the ground at all, as most
people do vainly talk, but directly out of the air; and that when they
die or get consumed, they return once more to the atmosphere from which
they were taken. Trees undeniably eat carbon.
Of course, therefore, all the ordinary unscientific conceptions of how
plants feed are absolutely erroneous. Vegetable physiology, indeed, got
beyond these conceptions a good hundred years ago. But it usually takes
a hundred years for the world at large to make up its leeway. Trees
don't suck up their nutriment by the roots, they don't derive their
food from the soil, they don't need to be fed, like babies through a
tube, with terrestrial solids. The solitary instance of an orchid hung
up by a string in a conservatory on a piece of bark, ought to be
sufficient at once to dispel for ever this strange illusion--if people
ever thought; but of course they don't think--I mean Other People. The
true mouths and stomachs of plants are not to be found in the roots,
but in the green leaves; their true food is not sucked up from the
soil, but is inhaled through tiny channels from the air; the mass of
their material is carbon, as we can all see visibly to the naked eye
when a log of wood is reduced to charcoal: and that carbon the leaves
themselves drink in, by a thousand small green mouths, from the
atmosphere around them.
But how about the juice, the sap, the qualities of the soil, the manure
required? is the incredulous cry of Other People. What is the use of
the roots, and especially of the rootlets, if they are not the mouths
and supply-tubes of the plants? Well, I plainly perceive I can get 'no
forrarder,' like the farmer with his claret, till I've answered that
question, provisionally at least; so I will say here at once, without
further ado--the plant requires drink as well as food, and the roots
are the mouths that supply it with water. They also suck up a few other
things as well, which are necessary indeed, but far from forming the
bulk of the nutriment. Many plants, however, don't need any roots at
all, while none can get on without leaves as mouths and stomachs. That
is to say, no true plantlike plants, for some parasitic plants are
practically, to all intents and purposes, animals. To put it briefly,
every plant has one set of aerial mouths to suck in carbon, and many
plants have another set of subterranean mouths as well, to suck up
water and mineral constituents.
Have you ever grown mustard and cress in the window on a piece of
flannel? If so, that's a capital practical example of the comparative
unimportance of soil, except as a means of supplying moisture. You put
your flannel in a soup-plate by the dining-room window; you keep it
well wet, and you lay the seeds of the cress on top of it. The young
plants, being supplied with water by their roots, and with carbon by
the air around, have all the little they need below, and grow and
thrive in these conditions wonderfully. But if you were to cover them
up with an air-tight glass case, so as to exclude fresh air, they'd
shrivel up at once for want of carbon, which is their solid food, as
water is their liquid.
The way the plant really eats is little known to gardeners, but very
interesting. All over the lower surface of the green leaf lie scattered
dozens of tiny mouths or apertures, each of them guarded by two small
pursed-up lips which have a ridiculously human appearance when seen
through a simple microscope. When the conditions of air and moisture
are favourable, these lips open visible to admit gases; and then the
tiny mouths suck in carbonic acid in abundance from the air around
then. A series of pipes conveys the gaseous food thus supplied to the
upper surface of the leaf, where the sunlight falls full upon it. Now,
the cells of the leaf contain a peculiar green digestive material,
which I regret to say has no simpler or more cheerful name than
chlorophyll; and where the sunlight plays upon this mysterious
chlorophyll, it severs the oxygen from the carbon in the carbonic acid,
turns the free gas loose upon the atmosphere once more through the tiny
mouths, and retains the severed carbon intact in its own tissues. That
is the whole process of feeding in plants: they eat carbonic acid,
digest it in their leaves, get rid of the oxygen with which it was
formerly combined, and keep the carbon stored up for their own
purposes.
Life as a whole depends entirely upon this property of chlorophyll; for
every atom of organic matter in your body or mine was originally so
manufactured by sunlight in the leaves of some plant from which,
directly or indirectly, we derive it.
To be sure, in order to make up the various substances which compose
their tissues--to build up their wood, their leaves, their fruits,
their blossoms--plants require hydrogen, nitrogen, and even small
quantities of oxygen as well; but these various materials are
sufficiently supplied in the water which is taken up by the roots, and
they really contribute very little indeed to the bulk of the tree,
which consists for the most part of almost pure carbon. If you were to
take a thoroughly dry piece of wood, and then drive off from it by heat
these extraneous matters, you would find that the remainder, the pure
charcoal, formed the bulk of the weight, the rest being for the most
part very light and gaseous. Briefly put, plants are mostly carbon and
water, and the carbon which forms their solid part is extracted direct
from the air around them.
How does it come about then that a careless world in general, and more
especially the happy-go-lucky race of gardeners and farmers in
particular, who have to deal so much with plants in their practical
aspect, always attach so great importance to root, soil, manure,
minerals, and so little to the real gaseous food stuff of which their
crops are, in fact, composed? Why does Hodge, who is so strong on grain
and guano, know absolutely nothing about carbonic acid? That seems at
first sight a difficult question to meet. But I think we can meet it
with a simple analogy.
Oxygen is an absolute necessary of human life. Even food itself is
hardly so important an element in our daily existence; for Succi, Dr.
Tanner, the prophet Elijah, and other adventurous souls too numerous to
mention, have abundantly shown us that a man can do without food
altogether for forty days at a stretch, while he can't do without
oxygen for a single minute. Cut off his supply of that life-supporting
gas, choke him, or suffocate him, or place him in an atmosphere of pure
carbonic acid, or hold his head in a bucket of water, and he dies at
once. Yet, except in mines or submarine tunnels, nobody ever takes into
account practically this most important factor in human and animal
life. We toil for bread, but we ignore the supply of oxygen. And why?
Simply because oxygen is universally diffused everywhere. It costs
nothing. Only in the Black Hole of Calcutta or in a broken tunnel shaft
do men ever begin to find themselves practically short of that
life-sustaining gas, and then they know the want of it far sooner and
far more sharply than they know the want of food on a shipwreck raft,
or the want of water in the thirsty desert. Yet antiquity never even
heard of oxygen. A prime necessary of life passed unnoticed for ages in
human history, only because there was abundance of it to be had
everywhere.
Now it isn't quite the same, I admit, with the carbonaceous food of
plants. Carbonic acid isn't quite so universally distributed as oxygen,
nor can every plant always get as much as it wants of it. I shall show
by-and-by that a real struggle for food takes place between plants,
exactly as it takes place between animals; and that certain plants,
like Oliver Twist in the workhouse, never practically get enough to
eat. Still, carbonic acid is present in very large quantities in the
air in most situations, and is freely brought by the wind to all the
open spaces which alone man uses for his crops and his gardening. The
most important element in the food of plants is thus in effect almost
everywhere available, especially from the point of view of the mere
practical everyday human agriculturist. The wind that bloweth where it
listeth brings fresh supplies of carbon on its wings with every breeze
to the mouths and throats of the greedy and eager plants that long to
absorb it.
It is quite otherwise, however, with the soil and its constituents.
Land, we all know--or if we don't, it isn't the fault of Mr. George and
Mr. A.R. Wallace--land is 'naturally limited in quantity.' Every plant
therefore struggles for a foothold in the soil far more fiercely and
far more tenaciously than it struggles for its share in the free air of
heaven. Your plant is a land-grabber of Rob Roy proclivities; it
believes in a fair fight and no favour. A sufficient supply of food it
almost takes for granted, if only it can once gain a sufficient
ground-space. But other plants are competing with it, tooth and nail
(if plants may be permitted by courtesy those metaphorical adjuncts),
for their share of the soil, like crofters or socialists; every spare
inch of earth is permeated and pervaded with matted fibres; and each is
striving to withdraw from each the small modicum of moisture, mineral
matter, and manure for which all alike are eagerly battling.
Now, what the plant wants from the soil is three things. First and
foremost it wants support; like all the rest of us it must have its
_pou sto_, its _pied-à-terre_, its _locus standi_. It can't hang aloft,
like Mahomet's coffin, miraculously suspended on an aerial perch
between earth and heaven. Secondly, it wants water, and this it can
take in, as a rule, only or mainly by means of the rootlets, though
there are some peculiar plants which grow (not parasitically) on the
branches of trees, and absorb all the moisture they need by pores on
their surface. And thirdly, it wants small quantities of nitrogenous
matter--in the simpler language of everyday life called manure--as well
as of mineral matter--in the simpler language of everyday life called
ashes. It is mainly the first of these three, support, that the farmer
thinks of when he calculates crops and acreage; for the second, he
depends upon rainfall or irrigation; but the third, manure, he can
supply artificially; and as manure makes a great deal of incidental
difference to some of his crops, especially corn--which requires
abundant phosphates--he is apt to over-estimate vastly its importance
from a theoretical point of view.