WANDERING FRUITS AND SEEDS
Ships and stowaway seeds--Tidal drift--Sheep, broom, migrating
birds--Crows and acorns--Ice--Squirrels--Long flight of
birds--Seeds in mud--Martynia and lions--The wanderings
of Xanthium--Cocoanut and South Sea Islands--Sedges and
floods--Lichens of Arctic and Antarctic--Manna of Bible--The
Tumble weeds of America--Catapult and sling fruits--Cow
parsnips--Parachutes, shuttlecocks, and kites--Cotton--The use
of hairs and wings--Monkey's Dinner-bell--Sheep-killing grasses.
The ways in which fruits and seeds are scattered abroad over the face
of the earth form one of the most fascinating chapters in the story
of Plant Life.
There is an infinite number of ingenious contrivances, so many indeed
that it is not at all easy to explain them.
However, suppose yourself seated on a grassy cliff near Eastbourne or
Brighton.
Looking lazily out over the blue waters, you see Norwegian timber
ships and steamers of all kinds, from the little coasting "Puffing
Billy" to the huge liner departing for Australia or South Africa.
Plants are probably using every steamer; in the straw of the packing
cases, in the cargoes of corn or grain, in the ore, and in the
ballast, there are sure to be seeds. Such stowaways are mostly weeds,
but of course many valuable garden, farm, orchard, and forest seeds
are being intentionally exported.
Looking down on the seashore, you will notice the high-water mark,
a yellowish brown line of floated rubbish which is quite distinct
even at a distance. If you now go down and examine it closely (not
a particularly pleasant operation, seeing that so much is in a
decomposing condition) you will find many seeds amongst the corks and
bits of straw, seaweed, and objectionable, if lively, animalcula, and
very likely also pieces of plants, such as willow branches, which
might quite easily take root.
On the coast of Norway, and on our own western seaboard, the fruits
of a West Indian bean (_Entada scandens_) are occasionally to be
found, and its seeds are probably able to germinate. We know that in
long-past geological ages they were floating round the estuary of the
Thames, where they occur as fossils. It has been found by experiment
that fruits and seeds are not killed although they have floated for a
year or more in salt water. Thus ocean currents are utilized to carry
fruits and seeds.
But from our comfortable seat on the South Downs, still more can be
learnt of wandering seeds. The wind which blows across the downs
carries with it hundreds of winged or hairy fruits, all of them
exquisitely fashioned as miniature airships, aeroplanes, or other
winged contrivances. The wind is an important distributer of seeds.
One of the South Down sheep is trailing behind it a piece of bramble
which has caught in its wool; others, which have been grazing on the
broken cliff-edge where Agrimony, Forget-me-not, and Burdock are
flourishing, are certain to have spiny or sticky fruits entangled
in their wool. Animals therefore carry seeds in their wool or fur.
If it should happen to be a fine, sunny afternoon, and if there are
any plants of Broom near by, it is quite likely that you may, every
now and then, hear a faint, sudden crack. This will be the Broom at
work scattering its seeds by itself. The little pod, when it dries,
contracts in such a way that it splits with a sudden explosive pop,
and the seeds are sent flying to a distance of three or four feet.
This curious fact was observed in 1546 by the naturalist Boek. The
Whin and many other plants act in the same way, for the dry fruit
becomes elastic and coils up spirally, flinging away the seed.
But here also, on the southern shore of England, we are at a main
station of arrival and departure for migrating birds. A Landrail or
other marsh bird might be flushed in France, and might quite easily
cross the Channel with French mud sticking to its plumage. In this
mud, or in its crop, there may be seeds or fruits which will be left
in an English pond. This method is probably a very important one,
for these plants growing in duck-haunted places are amongst the most
widely distributed of all.
Mr. Reid has a very interesting discussion on this point. The crow or
rook could quite well cross the British Channel now. In the days when
Britain was covered with ice and snow, the gap between the French and
the English shore was only half the present width. There was at that
time much flat land with oak forest bordering the French coast.
Mr. Reid shows that it is probable that rooks regularly carry about
acorns in the cup, for he found seedling oaks associated with empty
acorn husks, stabbed and torn in a peculiar way. "On October 29th of
1895, in the middle of an extensive field, bordered by an oak copse
and scattered trees, I saw a flock of rooks feeding and passing
singly backwards and forwards to the oaks. On driving the birds away,
and walking to the middle of the field I found hundreds of empty
acorn husks and a number of half-eaten, pecked acorns."[113] So that
crows may have brought the acorns that colonized Britain with oak
forest in the earliest historical period.
[113] Reid, _Origin of the British Flora_.
Another means of dispersal is not so obvious on the South Downs. In
the Arctic region a glacier breaks away at its tongue into icebergs,
which float off and are stranded somewhere perhaps hundreds of miles
distant. Upon these icebergs are stones and soil and plants which
may be carried to a great distance from their original place. In the
Glacial period or Great Ice Age, ice may have been an important help
in distributing plants, but at present it is difficult to find a good
example.
From all this it is clear that in order to carry plants to new
countries and new homes, everything that moves on the earth's surface
can be employed. Not only the wind, but ocean currents, river waters,
icebergs, and floating ice are used. Migrating birds, mammals, and
especially the most restless and unsettled animal of all, viz. man,
are at work consciously and deliberately, or unconsciously and
accidentally, carrying the seeds to form new forest, grasslands, or
harvests in other countries.
The subject is in truth so vast that it is difficult to select the
most interesting and important cases.
The way in which squirrels, rats, voles, and lemmings devour nuts and
the like often leads to the distribution of the fruit. A squirrel
may, like a human being, forget where its store was buried, or be
driven from the place. Then some of those forgotten nuts will grow
into trees.
Birds are known to travel enormous distances. It is said that one
little Arctic bird travels from Heligoland to Morocco in a single
flight. It would not, at first sight, seem likely that seeds and
fruits could be carried by birds; yet Darwin saw that this might
possibly be the case. The mud and slime in which so many birds find
the small insects which they require is full of seeds. An Austrian
botanist, Kerner von Marilaun, examined the mud scraped from the
beaks, feathers, and legs of a number of wading and marsh-birds. He
found in it the seeds of no less than thirty-one different water
and marsh plants (Grasses, Sedges, Toad-rush, etc.). This showed,
as is very often the case, that Darwin was the first to discover a
very important point. It is also interesting to find that these ugly
little freshwater mud and marsh plants are at home almost everywhere,
from the Arctic circle to Tierra del Fuego and from Peru to Japan.
The most extraordinary cases known of sticking fruits and spines
are the Martynias and Harpagophytons of South Africa. The fruit is
covered by hooked claws, and becomes a regular pest wherever it
occurs. Deer, antelopes, and other animals get their hoofs entangled
in the fruit, and the wretched creatures have to limp about until the
hard thorny fruit is trodden to pieces. Dr. Livingstone says that the
fruit gets into the nostrils of grazing animals which cannot possibly
remove it themselves, and so have to wait patiently till the herdsman
comes to take it out. According to Lord Avebury, lions may sometimes
be destroyed by these horrible fruits. When a lion is rolling on
the sand, the claws (an inch long) stick in his skin, and when the
lion tries to tear it away with his teeth his mouth gets full of the
fruits and he cannot eat, and perishes miserably of starvation.[114]
[114] Ludwig, _Biologie d. Pflanzen_.
[Illustration:
_Stereo Copyright, Underwood & Underwood_
_London & New York_
A COCOANUT GROVE IN CEYLON]
Some of our common British fruits are most perfectly planned to stick
or entangle themselves in the wool of sheep or in people's clothes.
These, such as the Goosegrass (Robin-run-the-Hedge), Burdock,
Forget-me-not, Sanicle, Avens, etc., have very often been described.
It is only necessary to examine one's clothes after a walk through
rough, broken ground to discover some of them, and the ingenuity
and neatness of their tiny hooks, harpoons, or prongs can then be
realized. We shall give one or two instances of some other spiny
plants. There is, for instance, Xanthium, which is one of the Daisy
flowers or Composites. Unlike most of this order, its little fruits
possess no wind-hairs. The outside of the head of flowers is covered
by strong curved little crooks. These get so entangled in wool or
hair that they become a perfect pest to wool merchants. In 1814
Xanthium was unknown in the Crimea, but by 1856 it had covered the
whole of the peninsula. In 1828 the Russian cavalry horses brought it
on their manes and tails into Wallachia, from whence it travelled to
Servia. Servian pigs carried it into Hungary. In 1830 it was taken
in wool to Vienna. By 1871 it had reached Paris and Edinburgh. In
1860 Frauenfeld saw horses in Chile whose manes and tails were so
felted together with thousands of these fruits that the animals could
scarcely walk. In Australia, where it first appeared in 1850, it has
caused a very serious loss to the wool merchants and squatters. The
loss has been put at 50 per cent. by some authorities.[115]
[115] Ludwig, _l.c._, after Ihne, Frauenfeld, Shaw.
We have already alluded to the transference of fruits and seeds
by ocean currents. In the _Challenger_ expedition, no less than
ninety-seven kinds of marine floating fruits were observed.
Amongst these the most important is the Cocoanut. The nut sold in
this country is not the whole fruit, but only the inside shell.
In the natural state this is enclosed in a dense mass of fibres,
which form the valuable "coir" used for brushmaking and a variety of
purposes.
The entire outside of the fruit is covered by a smooth white skin.
The whole fruit is about the size of a man's head, and is so light
that it floats easily in the water. It has in fact been carried by
the waves to uninhabited islands all over the South Seas. It is a
very great blessing to Polynesia, for a tree yields thirty to fifty
nuts, and four of these nuts will furnish enough food for one day.
Coprah and the oil extracted by boiling the inside are also valuable.
Spirit or toddy can be made from the young buds. The leaves are used
for thatching and the trunk for timber.
There are other very curious palm fruits which are also carried by
water. Sir Joseph Hooker mentions the large, round fruits of Nipa, as
big as a cannon-ball, turned over by the paddles of the steamer in
the muddy waters at the Ganges mouth (_Himalayan Journal_).
In this country a search in the rubbish left by a spate or freshet
along a riverside is sure to furnish many floating fruits or seeds.
Most of these are small and rather difficult to see. Perhaps the most
interesting are those of the Sedges. The real fruit is only about
one-sixteenth of an inch in size, but it is enclosed in a little sack
or bag a quarter of an inch long and with a narrow opening, so that
it floats quite easily. Many willow branches, pondweeds, hornweeds,
and the like, are also found in the rubbish left by floods, and these
can often take root.
It is, however, in the exquisite modifications of those fruits which
are blown by the wind that we find the most beautiful contrivances
of all. They are effective also. Seeds are often so small as to
be like dust particles, and such may be carried in the air to
almost incredible distances. That of _Goodyera repens_ weighs only
1/200,000,000 of a pound, that of _Monotropa_, ·000,000,006 lb. It
is no doubt by the wind that the spores of lichens are carried from
one mountain to another. On a map of the world the distance from the
Arctic to the Antarctic, between the North and South Poles, seems
enormous. Moreover, the amount of water, desert, tropical forest,
and cultivated land in this extent of country is very great. There
are but few rocks on which lichens could manage to grow. And yet of
the Antarctic Lichens in the South Polar regions, and which are also
European species, more than 73 per cent. are found in the Arctic or
North Polar regions.[116]
[116] Darbishire, _Trans. and Proc. of Bot. Soc. Edin._, vol. 23,
part 1.
An Arctic lichen spore probably travelled from Scandinavia to
the German and Swiss Alps, another journey took it to the Atlas
Mountains, thence to Abyssinia, again to Mount Kenia, and from there,
somehow, it wandered to the South Orkneys or King Edward VII Land.
While talking of lichens, one must not forget the Manna of the Bible
(_Lecanora esculenta_) and two other species, which form warted,
wrinkled masses on rocks. It breaks off and may be carried away by
the wind, or in heavy rain it may be washed into depressions of the
soil, where a man can pick up 8 to 12 lb. in a day.
It "is used as a substitute for corn in years of famine--being ground
in the same way and baked into bread.... It is also remarkable that
all the great so-called rains of manna, of which news has come from
the East to Europe, especially those of the years 1824, 1828, 1841,
1846, 1863, and 1864, occurred at the beginning of the year, between
January and March, i.e. at the time of the heaviest rains.... The
inhabitants of the district actually thought that the manna had
fallen from heaven, and quite overlooked the fact that this vegetable
structure grew and developed (although only in isolated patches and
principally as crusts on stones) in the immediate neighbourhood of
the spots where they collected it."[117]
[117] Kerner, _Natural History of Plants_, vol. 2.
Amongst the wind-blown fruits and seeds there are cases in which
entire plants are dragged out of the soil and hurried away by the
wind, which rolls them over and over. They may be blown along for
days together. The seeds drop out by the way. In this country one
rarely sees anything of the sort, but in the Prairies of North
America, when under cultivation, these tumble-weeds are a serious and
expensive pest. Sometimes the farmers dig trenches to catch them, or
they may put up fences against which the tumble-weeds become piled or
heaped up until they blow over the top.
It is not very much use to give the names of these weeds, for they
are mostly rare or not British species. Such tumble-weeds are
generally nearly spherical in general form and have a short, rather
weak, root which is easily torn out of the ground. In some grasses,
such as "Old Witch," a well-known pest of the United States, the
grass-stalk, with many flowers on it, is pulled out of its sheath and
blown away.
But it is more usual for the fruits or seeds themselves to break
off the parent plant, and to be carried away by the wind. To this
end we find the most extraordinary changes. Although the flower may
droop from its stalk, the latter becomes upright and grows quite
a considerable length when the seed or fruit is dispatched on its
wanderings. This will raise the fruit or seed as high as possible
above the surrounding grasses.
Then in some cases the fruit opens to allow the seed to escape.
Small holes appear in it, or the fruit splits. As the dry, elastic,
withered stalk swings to and fro in the wind, the seeds are swung out
of these openings, and starting with a certain momentum the wind will
carry them often to a surprising distance from their parents. In wet
or rainy weather these holes or slits generally close together, and
no seeds are sent forth on their travels. The little holes in the top
of a poppy-head by which the seeds are swung out have little flaps,
which close over and shut them up in wet weather.
Some plants make a sort of catapult to sling or hurl their fruits.
Kerner von Marilaun was the first to describe some of these curious
arrangements. He had brought home some fruits of _Dorycnium
herbaceum_ and laid them on his writing-table. "Next day as I sat
reading near the table, one of the seeds of the Dorycnium was
suddenly jerked with great violence into my face." Some of the
neatest catapult fruits are those of _Teucrium flavum_. (There
is a British species, the Woodsage, but it has not got the same
arrangement.) When the petals have fallen off, the four small fruits
are left inside the cup-like sepals; the flower-stalk when dry is
very elastic, and if an animal touches the sepals it swings violently
and shoots out one of the fruits. But that is by no means the whole
of the process: there are hairs arranged spirally in the throat of
the sepals, and these give a spin or twirling motion like that of a
rifle-bullet to the fruit. The fruit also flies out of the sepals in
a line of flight which is inclined at an angle of about forty-five
degrees to the horizon; at this angle, as is well known, the
trajectory or distance travelled will be the greatest possible.
But by far the best way to understand these questions is to try
with some common weeds in the country towards the end of summer or
beginning of autumn. If either the Cow Parsnip or wild Angelica,
or Myrrhis, be gathered and kept till it is quite dry, then if you
take it by the stalk and swing it to the full extent of the arms
the fruits fly off to fifteen (or more) feet away. Every part is
elastic--not only the main stalk, but the thin separate stalks of
the flowers and also the delicate piece by which each half-fruit is
attached. The half-fruits themselves are also so made that they are
of exactly the right shape to take a long flight.
Ever since the days of Icarus, one of the unsatisfied ambitions of
mankind has been to fly like a bird, to "soar into the empyrean," and
to be no longer chained to the earth's surface.
It is a very curious study, that of the many and diverse inventions,
almost always useless and very often fatal, by which men have
endeavoured to solve this problem. Every one of these can be
paralleled amongst the many neat contrivances of wind-borne fruits
and seeds. The principle of the "parachute," which is more or less
like an umbrella, is found in both fruits and seeds. One of the
most beautiful is the Dandelion fruit, where a series of the most
exquisite branched hairs springs from the top of the slender shaft
which carries the little hard fruit. Most of the Composite or
Dandelion order have, however, more of the "shuttlecock" idea. There
is a row or crown of stiff and spreading or feathery hairs.
[Illustration: _Stereo Copyright, Underwood & _Underwood_
_London & New York_
COTTON-FIELDS IN GEORGIA, U.S.A.
Negroes picking the cotton harvest.]
The classical person above alluded to seems to have copied the
bird's wing, sticking on feathers with wax, which of course melted
in the sun with the usual result to the inventor of flying machines.
Many seeds have regular wings which act like those of the bat or
flying squirrel. One of the most exquisite of all is the seed
of Bignonia. The Dahlia fruit has also a flying wing, and a great
many others might be mentioned. Major Baden-Powell experimented with
kites, which were supposed to raise a man high enough in the air to
take observations of the enemy's movements. But a most exquisite
"kite" is that of the Lime tree. The little fruit is hung from a
broad, flying bract, and as it very slowly sinks to the ground it
solemnly turns round and round. That is because the pressure of the
air acts on the flat bract just as it does on an aeroplane, and
forces it to revolve. So the fruit remains a long time in the air,
and may be carried to nearly a hundred yards away from its parent
tree.
The Traveller's Joy (_Clematis_) and the Cotton have their seeds
covered all over by many entangled hairs, which act like a piece of
fluff, so that the wind blows the seed away.
No one has discovered the original wild Cotton plant. The robes of
the priests in Egyptian temples were made of it. It was introduced
into Spain by the Arabs when they invaded that country. When the
Spaniards attacked the half-civilized Indian people of Central and
South America, they found cotton was regularly cultivated there.
Its history in England is rather interesting. In the days of Queen
Elizabeth the great English industry was the production of woollen
cloth from Yorkshire sheep. A penalty of £20 was imposed, even as
late as 1720, on any person who imported or even wore cotton cloths.
Yet this was unable to stop the growth of the trade which, thanks to
the Flemings and Huguenots who took refuge from religious persecution
in this country, eventually became our gigantic textile industry
employing millions of factory hands.
The advantage of these wings and hairs is at once seen if one
compares the time that a fruit or seed takes to fall through a given
height, first with its wings or hairs, and then after they have been
cut off.
An Artichoke fruit, for instance, will take nearly eight seconds to
reach the ground from a height of a few feet. But if you cut away its
hairs, it will touch the ground in a little more than one second. A
Sycamore fruit of which the wing has been removed falls to the ground
in about a quarter of the time that it takes when it has not been
injured, so that the wing helps it to fly to four times the distance
that it could reach if it had none. The Ash fruit also remains twice
as long in the air as it would do if it had no wing; and so on.
We shall finish this chapter by describing two very extraordinary
cases.
The Sandbox tree is a native of tropical America. The fruit, as
large as an orange, consists of a number of rounded pieces, each
with a single seed inside. When ripe each piece splits off, making a
noise like the report of a pistol. The plant is sometimes called the
Monkey's Dinner Bell. These pieces may be thrown to a distance of
fifty-seven feet from the parent plant.
Even more remarkable are the hygroscopic grasses. There are four
of them, which are widely separated as regards distribution, for
one (_Stipa capillata_) lives in Russia, another (_Stipa spartea_)
in North America, a third (_Aristida hygrometrica_) is found in
Queensland (Australia), and the fourth (_Heteropogon contortus_)
belongs to New Caledonia.
Yet all these four grasses are said to kill sheep, and do so in a
manner that is almost identical. The mechanism is as follows.
The fruit is like that of most grasses, enclosed in a folded leaf,
the bract (or glume), which in these particular cases is produced
into a very long fine tapering hair or awn. This awn is sensitive to
changes in the _moisture_ of the air. It is strongly hygrometric:
in wet weather it straightens itself, and it coils into corkscrew
spirals in dry weather. The widened part of the base, which contains
the grain, tapers into a sharp, very hard point; upon this there are,
on the outside, many stiff hairs, which point backwards away from the
sharp tip.
Now, suppose this fruit to fall on the ground, the awn or tail is
sure to be entangled in neighbouring grasses or herbs, but the hard
point will rest upon the ground. Every coil and twist made by the
entangled awn or tail will push the point a little deeper into the
earth, and the backward-pointing stiff hairs will prevent its being
pulled out of the soil.
Therefore all these modified contrivances ensure that the seed will
bury itself.
But supposing that one of these fruits falls upon a sheep's back.
Then an exactly similar process will go on. The seed will be forced
through the skin into the body of the sheep. In fact, if it should
fall above any soft or vulnerable part of the animal, the sheep will
very likely be killed.
As a matter of fact, sheep are said to be killed by these grasses in
all those four countries, distant though they are from one another.
We have endeavoured in this chapter to give some faint notion of the
hundreds and thousands of ingenious contrivances utilized by plants
in order to ensure the dispersal and future prosperity of their
children.
Every species is always trying to colonize new ground, to seek fresh
fields and new pastures. Plants are not content to keep to the old
habitats, but every species tries to scatter its pioneers over
all the neighbouring country, so that, as often happens, if it is
exterminated or suppressed in one locality, new generations luxuriate
elsewhere.