ON UNDERGROUND LIFE
Mother-earth--Quarries and Chalk-pits--Wandering atoms--The
soil or dirt--Populations of Worms, Birds, Germs--Fairy
Rings--Roots miles long--How roots find their way--How they
do the right thing and seek only what is good for them--Root
versus stones--Roots which haul bulbs about--Bishopsweed--Wild
Garlic--Dandelion, Plantain--Solomon's Seal--Roots throwing down
walls--Strength of a seedling root.
The word "Adam" means red earth. Poets and essayists still regularly
write about Mother-Earth and, in so doing, admit one of the most
interesting and wonderful facts in Nature.
If you go to some quarry or cliff where a section has been cut,
laying bare the original rock below; then (with Hugh Miller) you may
reflect on the extraordinary value of those few inches of soil which
support the growth of all our trees and of all our cultivated plants.
It is probable that plant-roots _never_ go deeper than about thirty
feet. All our food, our energy, and activity depend therefore on this
thinnest surface-layer of an earth which is 8000 miles in diameter.
But in most places the depth of true soil is far less than thirty
feet, generally it is not more than thirty inches, and by far the
most valuable part of it is a very thin layer five or six inches
thick.
It is in this true soil that the roots gain their nourishment, and
not only roots, for whole populations of worms, of germs, of
insects, even of birds and the higher animals, live upon it. To
it return the dead leaves, the bodies of dead insects, and waste
products of all kinds. Within it, they are broken to pieces and
worked up again by the roots of other plants in order to form
new leaves, new insects, and food for bird and beast. Just as in
engine-works, you may see old engines, wheels, and scrap-iron being
smashed into pieces; they are melted down and again worked up into
engines of some improved design.
[Illustration: THRASHING CORN IN CHILE
Mares are driven at the gallop round the circle, and so beat the
corn out of the ear with their hoofs. They do this for twenty
minutes at a time, and are then made to go the other way round.]
On a chalk-cliff, which dates from the long-distant Cretaceous
period, the entire thickness formed by the yearly work of plants for
millions and millions of years is often less than a foot in depth,
and probably only four to five inches are true soil.
But this is an exceptionally thin stratum, although it is capable of
producing rich turf, fat snails, and excellent mutton. In peat-mosses
and in those buried forests which form the coalfields, vegetable
matter may accumulate in deposits of thirty feet of coal. Yet these
stores of carbonaceous matter seem to be at first sight miserly and
selfish, at least from a vegetable point of view.
They resemble the gold and silver withdrawn from circulation in the
world by some Hindoo miser and buried deep within the earth. Yet
somebody is pretty certain to find out and make use of such stores
eventually.
In the case of the peat and coalfields, an animal of sufficient
intelligence to utilize them has already been produced, and now they
are used by man as fuel.
It is very important to remember that the soil is a sort of last home
to which the particles of carbon, of nitrate, and minerals always
return after their wanderings in the bodies of plants, of insects, or
of other animals. They probably rest but a short time before they
again set off on new adventures.
One might say the same of the water, and of the carbonic acid gas
and oxygen of the atmosphere, for the water, falling as rain upon
the earth, trickles down to the underground water-level. Then it
immediately begins to rise up between the particles of earth and is
promptly caught and sucked in by the roots, only to be again given
out by their leaves. The carbonic acid gas and oxygen also are always
entering and leaving the foliage. Even the nitrogen of the air is not
left alone in the atmosphere. There are small germs in the soil which
are able to get hold of it and make it into valuable nitrates.
More curious still is the fact that electric charges can be used
to change the comparatively useless air-nitrogen into useful
manures. Probably the farmer will some day make his own nitrates by
electricity.
The structure of the soil or earth is a most interesting and romantic
part of botany. It is true that a "radical" disposition is necessary
if one is to go to the root of the matter, but, unless we do this, it
is impossible to realize the romance of roots.
Down below is the unaltered rock, sand, or clay. Next above it comes
the subsoil, which consists of fragments of the rock below, or of
sand, clay, etc., more or less altered by deep-going roots. Even in
this subsoil, bacteria or germs may be at work, and the burrows of
worms and insects often extend to it. Next above the subsoil comes
the true soil; there is plenty of the stones, soil, sand, or whatever
it may be that constitutes the subsoil, but its richness consists
in its contents of valuable minerals, and especially of broken-up
leaves, corpses of insects, and manure. Above this true soil are
first the leaf-mould of two years ago, then that of the year before
last, and _on the top_ is the leaf-mould and other decayed products
of last winter.
All these upper layers are full of life and activity, which probably
goes on vigorously all the year round.
The population of worms is especially important. The worm is a
voracious and gluttonous creature: it is for ever swallowing bits
of leaves and rich soil. Inside its body there are lime-glands
which act upon the vegetable food and improve its quality as
manure. The worm comes up to the surface at night or early morning
and leaves the worm-casts upon it. The rain then washes the rich,
finely-divided matter of the casts down into the soil again. It is
said that there are about 160,000 worms at work in an acre of good
soil. Yet their life is full of danger. A keen-eyed population of
blackbirds, thrushes, starlings, peewits (plover), and partridges
are always watching for and preying upon the poor worm. Even in his
burrows, which may be six feet deep, he is not safe, for the mole
(_moudiewarp_) is also both very hungry and very active, and delights
in eating him.
In the soil also and even deeper in the subsoil are many insects;
some hibernate in the winter, and at other times actively gnaw the
roots of plants or devour dead leaves and twigs (see Chapter xxiii.).
Thus there are many burrows and holes, so that there is no want of
air in the soil, which is indeed necessary both for these creatures
and also for the roots of the plants.
Rain comes down through the soil, carrying with it carbonic acid,
mineral salts, and also germs or bacteria, which form perhaps the
most important population of all.
No work could be carried on without their help; it is bacteria which,
at every stage of decay, assist in breaking up leaves, twigs,
insects' bodies, worm-casts, and other manures. The way in which they
work is too difficult to explain here, but to get an idea of the
romance of the underground world one must try to picture to oneself
these swarms and myriads of germs and bacteria all incessantly and
busily engaged at their several duties. In the uppermost layers there
are probably in a single cubic inch of good soil from 54,000,000 to
400,000,000 of these microbes. Many are absolutely necessary to the
harvest; a few may be of little importance, but there are sure to be
some of those dangerous sorts which might devastate a continent with
disease in a single summer.
There are also quantities of other fungi. The fairy rings which one
sees year after year in widening circles of bright, fresh green
are the work, not of fairy footsteps, but of an underground fungus
(_Marasmius oreades_ and others). Its threads are thin, white, and
delicate; they attack the roots of grasses, etc., on the outer side
of the ring. It is therefore on this outer side yellow, dry, and more
or less withered. On the inner side, however, the grass is luxuriant
and of a rich bright green. Here the fungus has died off, and its
remains, as well as those of the plants which it destroyed, form a
rich manure for the new grass following on its track. Every year
the ring widens; at a certain time in summer one sees the irregular
line of mushroom-like fungi which are formed by the destructive
underground absorbing threads. This, however, is but one of the
underground fungi. There are many kinds; some are useful, others are
very destructive.
Upon the upper surface of the soil there falls not only rain, but
another sort of rain consisting of seeds, dead leaves, insects'
bodies, fungus spores, bacteria, and dust.
Every year when the ploughman turns the sod there is a revolution in
the whole of these populations.
So far nothing has been said about the roots themselves, which
penetrate, explore, and exploit all these layers of dead leaves,
soil, and subsoil.
The length of roots produced is very much greater than any one would
suppose. A one-year-old Scotch fir seedling when grown in sand
produced in a season a total length (branches, etc.) of no less than
thirty-six feet of root. The total surface of this root system was
estimated to be about twenty-three square inches. This little Scotch
fir after six months' growth was laying under contribution a cone of
earth twenty to thirty inches deep and with a surface of 222 square
inches. In certain kinds of corn the same author estimated the total
length of the roots as from 1500 to 1800 feet. S. Clark estimated the
length of the roots of a large cucumber plant as amounting to 25,000
yards (fifteen miles), and made out that it was occupying a whole
cubic yard of ground.
Clover roots are said to go down to depths of six or nine feet, but
many weeds go deeper still. Coltsfoot, for instance, may be found,
according to a friend of mine, living at a depth of twenty spades. In
Egypt and other places the roots of acacias go down to twenty feet or
even further, so that they can tap the water supplies, which are at a
great depth.
But a still more extraordinary fact is the manner in which the
root-branches arrange to grow in such a way that they search every
part of the soil.
The main root in many plants grows straight down, or as nearly as it
can do so. Its branches are inclined downwards at a quite definite
angle which is often 30°-45° to the surface. Moreover, these branches
come off in quite a regular way. Each keeps growing in its own
special direction to the east, south-east, or west, or whatever it
may be, of its parent root.
Have they some extraordinary sense of the direction of the points
of the compass? It is said that if a side root, which is growing,
say for instance downwards and westwards, is turned in some other
direction, it will after a time resume its original westerly voyage.
This fact is a most extraordinary one, if true, but it can scarcely
be said that it has been proved, and, as will be shown later, there
are other curious facts in the behaviour of roots which might explain
the experiment without assuming that roots know the points of the
compass.
If one cuts a branch of willow and plants it upside down in the
earth, it will very likely take root and grow. Its appearance will
be most extraordinary, for the roots will grow downwards, whilst the
branches, instead of growing in the direction of the old branches,
turn round and grow upwards.[34]
[34] Kerner and Oliver, _l.c._, vol. 1, p. 88.
Why do roots generally grow downwards? The fact is so familiar that
the difficulty of answering does not, at first sight, seem so great
as it really is.
Pfeffer, the great physiologist, has the following interesting
comparison. Suppose a man is trying to find his way in the dark, then
a single lingering ray of light gives him an impulse to walk towards
it.[35] So our root, also in the dark, feels the pull of gravity and
endeavours to grow downwards. Others have compared the direction of
gravity to the sailor's compass, and suppose that the root is guided
in the same sort of way.
[35] _Annals of Botany_, 1904.
But a young, vigorous root making or forcing its way in darkness
through stones and heavy earth is a most interesting and fascinating
study.
There are the most extraordinary coincidences in its behaviour. It
has the property of always doing exactly the right thing in any
emergency.
It is of course intended to keep below the ground and in the dark.
So we find that if roots are uncovered, they will turn away from the
light and burrow into the earth again. They avoid light just as a
worm would do.
Roots are of course intended to absorb or suck in water. If there is
a drain in the soil or a place where water collects, the roots will
grow towards that place. Very often they form a dense spongy mass of
fibres which may almost choke the drain. Along a riverside one can
often find great fibrous masses of tree roots near the water. But how
does the root learn that the water is there and turn away from its
original track to find it? It certainly does so!
Then again, Herr Lilienfeld has recently shown that roots seem able
to turn away from poisonous materials in the soil and to seek out and
grow towards valuable and nutritious substances. He found that peas,
beans, sunflower, and other roots were very sensitive to different
substances in the soil, and were directly attracted by what was good
for them and turned aside from what was unwholesome.
This property and the power of growing towards water probably explain
the mysterious sense of direction alluded to above, for roots will
take a line which has not been exhausted by their neighbours.[36]
[36] Lilienfeld, _Beihefte z. Botan. Centralblatt_, Band XIV.,
abth 1, pp. 131-212. The facts were denied by Newcombe and
Rhodes, _Bot. Gazette_, 36, 1904.
But of all these wonderful properties, the most remarkable is the way
in which roots find their way past stones and other obstacles in the
soil. They insinuate themselves into winding cracks and crawl round
stones with an ingenuity that makes one wonder if they can possibly
be without some sort of intelligence.
It is the very tip or end of the young root that seems to be
responsible; for if, in the course of its journeyings underground, it
should strike a stone or something hard, the root does not grow on
and flatten itself.
But some sort of message is sent back from the tip to the growing
part which is a short distance behind it. After this message has
been received, the growing part begins to curve sideways, so that
the tip is brought clear of the obstacle and can probably proceed
triumphantly upon its way. The inexplicable part is that the growing
part which curves has never been touched at all, but simply answers
to the message from the tip.[37]
[37] If the growing part itself touches a stone it curves round
the stone, not away from it--the reverse of the reaction at the
tip!
This is perhaps the most reasonable and intelligent behaviour found
in the whole vegetable world, and it is not surprising that Darwin
compared the root-tip to a brain.
These extraordinary responses fill one with astonishment, but
there are others still more interesting and remarkable. It will be
remembered that we have already shown how different the soil is at
different levels. The subsoil, soil, and uppermost layers are all
quite different from one another.
This may explain why it is that many plants seem to prefer to develop
their roots at one particular depth below the surface. Not only so,
but they find their own favourite level in the most persevering way.
If, for instance, you sow a barley-corn at too great a depth, the
seed germinates and forms a few roots, but it immediately sends out a
stem which grows upwards towards the light. As soon as this stem has
reached the proper place, which is just below the surface, there is
an enormous development of roots, which begin to search and explore
their favourite stratum of soil.[38]
[38] Pfeffer, _l.c._, p. 139.
In some few cases one can see in a dim sort of way the reason for the
level which certain plants prefer. Thus the underground stems of the
common Thistle, which are very long and fleshy, are found just a few
inches below the level usually reached by plough or spade. This makes
it very difficult to tear them out. Even if grubbers with long spikes
which reach as deep as these buried stems are driven through the
ground, it generally happens that the stems are only cut in pieces
and not dragged up. These hardy weeds are not much injured by little
accidents of this kind, for each separate bit will form upright
thistle stems next year. In fact if one cuts this fleshy subterranean
runner of the Thistle into pieces a quarter of an inch long, each
piece will probably become a Thistle.
Sometimes indeed these weeds are carried from one field to another
by pieces of them sticking in the very machines which are used to
eradicate them.
The Bishopsweed is one of the hardest cases. The writer was once
ambitious enough to try to dig up an entire plant of this horrid
weed. The first foot or so revealed no sign of the end of the
branching runners, and it was not until a hole about four feet deep
and five feet across had been excavated that there was any sign of an
end to the plant.
When it was at last removed, the original deeply buried stem was
found to give off branches which again branched in a most complicated
manner, until almost every green shoot of Bishopsweed[39] within a
space six feet in diameter was seen to be really a branch of this
one original plant! So to eradicate the plant it would have been
necessary to dig over the whole garden to a depth of at least five or
six feet.
[39] This weed is a cure for gout, and seems to have been called
Bishopsweed because it was supposed that gout was a common
ailment of bishops!
How did the stem get down to such a depth below the surface? This is
one of the most curious stories in plant life, and the process which
we shall now try to describe has only been explained within the last
few years.[40]
[40] By the classical researches of Rimbach.
The seed of the Wild Garlic (_Allium ursinum_) lies at first upon the
surface of the ground, but it is soon buried by a growth of the stalk
of the seed-leaf, which pushes the germ down below the earth. As soon
as it is buried, roots are formed and pass obliquely downwards, where
they become fixed by forming root-hairs all round themselves. These
root-hairs round every root hold its tip firmly in the earth; then
these same roots contract or shorten, which of course hauls down the
root a little deeper in the earth. One might compare it to a few men
hauling down a balloon by ropes attached to the car. About September
to November, roots of quite a different character are formed; these
explore the surrounding soil and gather in food and moisture.
Then the roots rest during the winter, when the buds and young leaves
are being formed. In April the buds begin to push out their leaves
and a new ring of roots appear. These April roots are quite different
from the September ones. They again fix themselves firmly and then
contract, becoming fully a third shorter than they were originally.
The bulb is dragged down still deeper below the surface. It flowers
in May and fruits in June and July. Then in September the same
series of operations begins again. The process goes on until the
plant is three to five inches below the ground.
It follows from all this, that every year the roots find new ground
to explore and utilize. Nor is the Wild Garlic at all exceptional in
this respect. A great many plants have roots which contract and drag
the bulb or stem after them deeper into the earth. Something of the
same sort happens, for instance, to Bramble branches. They arch or
droop over, when growing, so that the end touches the earth. On the
underside of the tip, as soon as it begins to rest on the ground,
roots are formed. These roots make their way into the ground, and
then, when fixed, they shorten or contract, so that the end of the
branch is dragged down to a depth of several inches. After this has
happened the old branch generally dies away, and a young, vigorous
Bramble develops from its buried tip.
Raspberry branches also are often buried; their roots become coiled
or rolled in a very curious manner. The end of the root becomes
firmly attached in the soil, and then the rest of it revolves like a
tendril so as to draw the stem deeper into the earth.[41]
[41] Scott Elliot and Fingland, _Trans. Nat. Hist. Soc. Glasgow_,
vol. 5, New Series, part ii., 1897-8.
On any ordinary roadside in the country one is sure to find the
rosettes of the common Dandelion and of the Rats-tail Plantain
(_Plantago major_). These are two of the most interesting plants in
the world, although they are vulgarly common. How is it that their
leaves are always at the level of the ground? The stem is always
growing upwards; every year fresh circles of leaves are formed above
the older ones. Yet the crown of the stem is never so much raised up
above the ground that the toe of a boot would be likely to knock it
off. It is always kept so deep in the earth, that it is by no means
easy to kick or "howk" the crown out of the ground.
The Dandelion root contracts very strongly at the end of the season,
and by this shortening or contraction keeps its leaves just at the
soil level. The Plantain sends out about forty to sixty oblique
downward-growing roots, which fix themselves in the soil by throwing
out branch roots. These forty to sixty roots are at first about ten
inches long, but, as soon as they are firmly attached, they contract,
and pull the stem with its crown of leaves about one-third of an inch
deeper. This is just enough to keep the leaves flat on the ground and
to prevent any possible injury from passers-by.
So that in finding their favourite level in the soil, plants are
often pulled or hauled about by the roots. But they are not always
moved by the roots. Even though buried in darkness, they seem able in
some way to tell when they are in the most favourable position.
Every gardener knows that Autumn Crocus and other bulbs do not remain
in the same position. They wander below ground in a curious and
inexplicable fashion.
The Solomon's Seal has an underground, fleshy stem, which prefers to
grow at a definite depth. If it is planted close to the surface, then
the point of the next year's little fleshy bud turns downwards; next
year it again turns downwards, and so on every year, until the stem
has reached its proper depth. Then it grows horizontally. Similarly,
if it is planted too deep it grows upwards.
Thus if one wishes to realize the underground life of plants, one
must picture to oneself:--
1. The usual descending roots, whose system of branching may be
compared to the ordinary branching above ground. It is often not
unlike the reflection in water of the tree itself, such as one might
see on a fine winter's day along the shore of some still lake.
2. The bold, exploring, horizontal runners of Couchgrass, Thistle,
Bishopsweed, etc., vigorously pushing their way at a depth too great
for the gardener's spade.
3. All sorts of bulbs, runners, and roots being slowly hauled or
dragged about till they get into exactly the right position, but
never remaining for two years in exactly the same place. All have
their favourite depth[42]--
[42] See Rimbach's researches.
Herb Paris 2/3 to 1-3/4 inches deep.
Solomon's Seal 1-1/3 to 2-1/3 " "
Cuckoo Pint (_Arum maculatum_) 2 to 4 " "
Colchicum (Autumn Crocus) 3-1/3 to 5-1/3 " "
Asparagus 6-3/8 to 13-1/8 " "
The water evaporating on the surface of the soil must, as it rises
from the permanent water-level below, pass the gauntlet of all
these thirsty rootlets and their hairs. Tree-roots will be ready to
intercept it at ten feet depth, many herbaceous plants will suck it
in at depths of five to six feet, and in the upper layers of soil it
will have to pass root-system after root-system from Asparagus to
Paris, so that very little will be lost.
Perhaps of more importance are the bacteria-germs, and dissolved
mineral salts in the rainwater as it trickles down from the surface.
The soil particle acts as a filter: at every inch of the descent some
of the bacteria and salts will be left, so that by the time the level
of Asparagus has been reached there will be exceedingly few, and the
water is comparatively speaking pure. The effect of this vigorous
underground life is often visible on the surface. Roots, and
particularly tree-roots, are often extraordinarily strong. Kerner, in
his invaluable _Natural History of Plants_, has a beautiful picture
of a young larch tree which had grown in a fissure of a huge boulder.
In attempting to grow, the root had forced up part of this stone. It
was estimated that it had lifted a weight of 3000 lb., though it was
only some ten inches in diameter.
Along a dry-stone wall, or even near houses, the growth of tree-roots
very often damages the entire wall, which may be entirely overthrown
if the tree is too near. The force of the growth of the roots is so
great that even a six-foot stone wall cannot keep them down.
Quite a young seedling root, in forcing itself through the soil, may
exercise a pressure of two-thirds to four-fifths of a pound!
This is of course necessary, if one remembers that it has to drive
itself through the earth, pushing aside and compressing the earth
particles along its course.