Migrations of Plants
“_Race after race of leaves and men
Bloom, wither and are gone;
As winds and water rise and fall
So life and death roll on._”
We are so in the habit of thinking of plants as fixed and static things
that it rarely occurs to us that they migrate over the earth’s surface
quite as extensively as do men or animals.
While it is probably true that vegetation originated simultaneously
at different points on the globe’s surface, not much observation is
necessary to indicate that it does not always stay where it is put.
Plants are peculiar and native to certain lands in a very definite
way, but their love of adventure often carries them to the far corners
of the earth. They are the most energetic and effective colonizers in
existence. The complete history of plantdom would include the stories
of invasions, conquests and revolutions quite as stirring as anything
in human annals.
If it is absorbing to follow the racial movements of man, ancient and
modern, it is equally fascinating for a lover of plants to investigate
_their_ migratory habits. We have exact records of many of their
travels and can make interesting conjectures about the rest.
To a layman, the present distribution of plants may seem chaotic. He
reads that certain families are natives of Europe and Australia, or
North America and Africa and are absent from all intervening countries.
The Alpine species _Primulas_ and _Saxifrages_ are common to both
the Arctic and the Antarctic. There are fifty-eight European and New
Zealand species which are identical. The British Grass _Poa Annua_ is
also found in the Andes of Brazil. Through what thousands of years of
change and evolution have these things come about! Yet the results are
no more complex than was the filling of America with its mixed and
conglomerate human population.
In a general way, there is a measure of fixity to plant distribution.
Certain plants have elected the tropics as their home; and only under
the greatest stress of circumstance can they be induced to go elsewhere.
Tropical heat and moisture make for luxuriance of vegetation. There
is a much greater variety there than in the North. Woody Vines climb
the tallest trunks, where they intermingle their leaves and blossoms
with those of their host. Gorgeous Air Plants beautify and perfume the
forest. Stately Palms wave magnificent bouquets of pendulous fronds.
As we travel away from the equator, the vegetation takes on a simpler
aspect. There are more annuals and more herbs. The number of Ferns,
Grasses, and catkin-bearing Trees, like the Alder and the Birch,
increase. The limited growing seasons make for a more restricted
accumulation of tissue. Such tropic plants as have braved the rigours
of the colder climates have dwindled much in size. The Castor Oil Tree
becomes a humble annual (_Ricinus Communis_) only three to eight feet
in height. Other tropical trees become so small that temperate zone
folk tread them under foot.
When we get into the polar regions, all the plants take on a stunted
and dwarfed appearance and, in some cases, retire almost entirely
under ground. The number of genera and species is much reduced. The
Oak, Walnut, Chestnut and Elm are replaced by the hardy conifers. At
the point where vegetation becomes almost extinct are dwarf Birches,
Willows and polar Blackberries (_Rubus Arcticus_). The simple Mosses
and Lichens mark the last lingering evidences of life.
A curious feature of plant life in the polar regions is the rapid
growth which it often exhibits. The summer of the Far North is short
but it is one day of intense and blinding light. The sun shines
continually throughout each twenty-four hours. By virtue of its
stimulating power, plants are able to perform in a few weeks processes
of development which take months under ordinary conditions.
It is illuminating to take a single country in a more favoured climate
and, as far as possible, trace its plant history. The British Isles,
because of their limited area, are a convenient field of study. An
investigation of their settlement by plants gives us many hints about
prehistoric climatic and geographical changes.
Geologists generally believe that the British Isles were once joined
to the mainland of Europe. It was at this time that they were settled
by vegetation. Some of this plant life came from Spain and some from
southwest France; there was also a Germanic group. The floating ice of
the glacial period brought over hardy visitors from the Scandinavian
peninsula. A few plant immigrants arrived from North America and landed
on the west coast of Ireland.
St. Helena is an isolated volcanic mass built up seventeen thousand
feet from the bed of the ocean. It therefore has its own peculiar
vegetation, a portion of which is believed to have been evolved on the
spot from the one-celled state. According to Sir Joseph Hooker, forty
out of fifty flowering plants and ten out of twenty-six Ferns “with
scarcely an exception cannot be regarded as very close specific allies
of any other plants at all.” Sixteen of the Ferns are common to Africa,
India or America and were probably carried there by the wind. Ocean
currents also brought other species from Africa.
In 1883, a most interesting thing occurred on the Asiatic island of
Krakatoa. A violent volcanic eruption wiped every vestige of life
off its surface. When the flow of lava ceased and the earth cooled
once more, Krakatoa was to all intents and purposes a volcanic island
newly risen from the sea. It presented the exact analogy of a recently
created bit of land waiting to be settled by the plants. In 1883, it
was as barren as the face of the moon. In 1888, a Mr. Hemsley described
its appearance as follows:--
“The first phase of the new vegetation, was a thin film of microscopic
fresh-water Algae, forming a green, slimy coating, such as may often
be seen on damp rocks, and furnishing a hygroscopic condition, in the
absence of which it is doubtful whether the Ferns by which they were
followed could have established themselves. Both Algae and Ferns are
reproduced from microscopic spores, which are readily conveyed long
distances by winds. Eleven species of Ferns were found, all of very
wide distribution, and some of them had already become common the
fourth year after the eruption. Scattered here and there among the
Ferns were isolated individuals of flowering plants, belonging to such
kinds as have succulent seed-vessels eaten by birds, or such as have a
light, feathery seed-vessel like the Dandelion and a host of others,
and are wafted from place to place by the winds.
“On the seashore there were young plants and seeds (or seed-vessels
containing seeds) of upwards of a dozen other herbs, shrubs and trees,
all of them common on coral islands, and all known to have seeds
capable of bearing long immersion in sea water without injury. Among
the established seedlings were those of several large trees, and a
Convolvulus that grows on almost all tropical coasts, often forming
runners one hundred yards in length. There were Cocoanuts also, though
none had germinated.”
The farther such an island is from the land, the longer will vegetation
take to get established. Darwin found that the isolated islands of
Keeling, after thousands of years of existence, contained only twenty
kinds of flowering plants.
Although plants have no legs they are not devoid of mobility. When man
uses the propulsive power of steam to travel by, he shows no greater
ingenuity than do plants in their use of special devices of locomotion.
Species like the Tumble Weed (_Amarantus Albus_) pull up stakes, and,
consigning themselves to the swift autumn winds, race across country
at great speed, scattering seeds as they go. The Utriculariae or
Bladderworts are true sailors and float about on inland streams like
little ships. The Duckweeds and Wolffias also have aquatic habits.
However, most plants prefer to travel in embryo. In the form of small
and microscopic seeds the force of gravity has little influence on
them, and they can journey for long and incredible distances.
To this end practically every seed in existence is provided with
some apparatus or appendage designed to help it make its way in the
world. The Elm, the Linden, and the Ash bear winged seeds, which
are so efficient in riding the breeze that they are really miniature
aeroplanes. The double wings of the Maple are very much like those of
an insect. The seeds are released from their container in such manner
as to acquire a whirling motion as they fall.
The progeny of the Willow is provided with long projecting hairs which
curl together to form a tiny balloon. Feathery attachments called
pappus enable the children of the Dandelion, the Thistle and the Fire
Weed to go on long jaunts of exploration.
The seed-pods of the Sycamore are great rollers. Even ordinary nuts and
fruits may be blown to considerable distances by the strong winds of
autumn. The many edible seeds and fruits are carried gratis by birds
and animals. The Mistletoe, for instance, is distributed entirely by
them.
Walnuts, Butternuts, and Acorns bear water travel well, as do certain
of the hard seeds. The Arrowhead (_Sagittaria_) has a self-made
water-wing on which its offspring float.
Plant seeds, which like to travel on animals, all provide themselves
with grappling irons in the shape of sharp hooks, spurs and spines with
which they cling to their carriers. Everybody in the northern United
States knows of the avidity with which the Cockle-bur clings to any
passing object. The Touch-me-not (_Impatiens_), the Wistaria, and a
host of others, actually shoot their seeds from their pods as from a
gun.
Every vagrant breeze, every purling brook, every deep river, every
ocean current, is a highway of travel in plantdom. The birds, the
beasts, the insects, and not least, man himself, are involuntary
vehicles on which our vegetable friends tour the world. The spores of
Mosses, Lichens, Fungi and other cryptogams are so light that they
find no difficulty in mounting into the air and traveling across the
Atlantic or Pacific Oceans at will.
The complete record of plant conquests would fill many volumes. Their
operations have extended into every land and have had influence on the
world’s history. It very often happens that plant invaders become so
quickly and thoroughly naturalized in a strange country that they go
a long way toward supplanting the original inhabitants in a very short
time.
It was Darwin who first noticed the extensive conquests of the Cardoon
Artichoke (_Cynara Cardunculus_) in South America. In one section,
these prickly plants covered an area of several hundred square miles,
having entirely superceded the aborigines.
It is well known that the most troublesome of the American weeds are of
British origin. On the other hand, the American water weed _Anacharis_
blocks up small English streams. The grass called _Stipa Tortilis_
has captured the steppes of southern Russia. The love of change seems
to be an inherent tendency in plantdom. The Pigweed and the Morning
Glory have come north from the tropics. The Canada Thistle, originally
a foreigner in North America, has spread all over Canada and New
England. The American _Erigeron Canadense_ has emigrated to all parts
of the world. The flora of Scandinavia, like its people, are aggressive
colonizers. More than one hundred and fifty species have reached New
Zealand alone and nearly as many have established themselves in the
eastern United States.
Some plants seem to be able to adapt themselves to any climate and
therefore are born explorers, but the greater number are too fastidious
regarding conditions of soil, heat, light and moisture to thrive well
everywhere. It is a noticeable fact that the most successful plant
invaders usually come in the wake of human colonizers and stick to the
sphere of man’s influence. For example, the Butter-and-Eggs (_Linaria
Linaria_) has followed the railroad tracks almost entirely over the
tropical and semi-tropical world. Sometimes, however, hardy plants
advance into the primeval jungle, there to give battle to its lusty
inhabitants.
On the whole, annuals have a better chance than perennials to gain a
foothold in a new country. Every spring the weeds, grasses, and common
flowering plants have to start all over again from a seed beginning.
The spores of newcomers, therefore, have almost an equal chance
with the established inhabitants. On the other hand, the bodies of
perennials occupy the land in close-packed ranks all the year, ready
to dispute every inch of ground with an aggressor. It is very hard for
new plants to gain entrance into a well-grown forest.
Man has been of tremendous aid in the distribution of plants over the
earth’s surface. Either consciously or unconsciously he takes his
plants with him wherever he goes.
It was the Emperor Chang-Chien who carried the Bean, Cucumber, Lucerne,
Saffron, Walnut, Pea, Spinach and Watermelon from Asia to China about
200 B. C. The period of Roman conquest was a great epoch in the history
of plant migrations. The Peach and the Apricot first became prominent
as fruits at that time. Roman generals introduced the Pear, Peach,
Cherry, Mulberry, Walnut and many ornamental shrubs into England.
From an obscure native of Bengal, the Sugar Cane has become an
important plant of wide distribution. Coffee, a wild berry of Arabia,
is now the chief crop of whole countries in the West Indies and South
America. The yellow Maize of America has become a citizen of the world.
The weak and humble Wheat is the sole possessor of thousands of square
miles of land in America, Russia and elsewhere.
All this has been wrought by man’s efforts. When it is to his interest,
he fights the battles of plantdom, and because of his superior
knowledge and equipment is of tremendous service. Sometimes, however,
he gives aid to his plant friends through motives that are quite
unselfish. A romantic story is related of a French naval officer named
Declieux who once elected to carry a Coffee Plant to the Colony of
Martinique. The supply of water ran low during the voyage, and, rather
than see the plant die, the man shared his daily glass with it, at
considerate discomfort to himself.
Until man becomes all-wise, he will continue to make mistakes; and
not least of these will be in connection with his investigations into
the mysteries of Nature. It has happened more than once that he has
introduced some new plant into an old land, or vice versa, and lived to
thoroughly regret his action.
Sometime in 1890, a generously inclined individual threw a Water
Hyacinth into the St. Johns River in Florida. In the space of a few
short years, that single plant had multiplied so prodigiously as to
seriously impede navigation, lumbering and fishing.
Jack London tells of a similiar thing that happened in Hawaii: “In
the United States, in greenhouses and old-fashioned gardens, grows a
potted flowering shrub called Lantana; in India dwells a very noisy
and quarrelsome bird known as the Myna. Both were introduced into
Hawaii--the bird to feed upon the cut-worm of a certain moth; the
flower to gladden with old associations the heart of a flower-loving
missionary. But the land loved the Lantana. From a small flower that
grew in a pot, the Lantana took to itself feet and walked out of the
pot into the missionary’s garden. Here it flourished and increased
mightily in size and constitution. From over the garden wall came the
love call of all Hawaii, and the Lantana responded to the call, climbed
over the wall, and went a-roving and a-loving in the wild woods.
“And just as the Lantana had taken to itself feet, by the seduction of
its seed it added to itself the wings of the Myna, which distributed
its seed over every island in the group. From a delicate,
hand-manicured, potted plant of the greenhouse, it shot up into a
tough, and belligerent swashbuckler a fathom tall, that marched in
serried ranks over the landscape, crushing beneath it and choking to
death all the sweet native grasses, shrubs and flowers. In the lower
forests, it became jungle, in the open, it became jungle only more so.
It was practically impenetrable to man. The cattlemen wailed and vainly
fought with it. It grew faster and spread faster than they could grub
it out.”
Then ensued a battle royal between man and plant. The man called to
his aid hosts of insect mercenaries. “Some of these predacious enemies
of the Lantana ate and sucked and sapped. Others made incubators out
of the stems, tunnelled and undermined the flower-clusters, hatched
maggots in the hearts of the seeds, or covered the leaves with
suffocating fungoid growths. Thus simultaneously attacked in front and
rear and flank, above and below, inside and out, the all-conquering
swashbuckler recoiled. Today, the battle is almost over, and what
remains of the Lantana is putting up a sickly and losing fight.
Unfortunately, one of the mercenaries has mutinied. This is the
accidently introduced Mani Blight, which is now waging unholy war upon
garden flowers and ornamental plants, and against which some other army
of mercenaries must be turned.”
Such unfortunate occurrences are sure to become more and more
infrequent as plant emigration and immigration finds itself under
increasingly drastic governmental regulation.
The Foreign Seed and Plant Introduction Service of the United States
Department of Agriculture makes a scientific examination of all plants
brought into the United States for propagation purposes. It rids them
of objectionable Bacteria and insect pests and refuses them admittance
entirely if its experts decide that the newcomers will be harmful or
injurious in any way.
The agents of the Service are constantly scouring the far corners of
the earth for new and rare plants. In the twenty-four years of its
existence it has introduced from abroad some fifty thousand specimens
of seeds and plant cuttings. Some of the successful immigrants have
been Feterita (from Egypt), Sudan Grass, Bamboo and Alfalfa. New
Zealand has yielded new types of Potatoes. Dwarf Almonds and strange
Cherries and Apricots have come from Turkestan. All these have proven
of commercial importance, as has Durum Russian Wheat, credited with
opening up new areas in the Northwest, and the Navel Orange from Brazil
which has created for itself a California industry covering thirty
thousand acres and valued at fifteen million dollars per annum.
Painstaking and scientific methods are best when man attempts to aid
Nature in her evolutionary processes, especially when they are in
connection with the migration and distribution of plants.