ALLIES OF THE PLANT WORLD
“_I wish I were a willow tree--
Young wind in the green hair of me
And old brown water round my feet,
And a familiar bird to greet._”
--_Elizabeth Fahnestock._
Every division of terrestrial life constitutes a struggle. The plants
grow and carry on their business and social activities so unobtrusively
that we seldom think of them as appealing to arms--yet their whole
existence is a battle royal. They must fight with aspiring neighbours
for every inch of their upward growth, and at the same time wage
incessant warfare against a hundred insects and animal foes.
Under such strenuous conditions, it is only to be expected that the
plants should seek profitable alliances with birds, insects and animals
having interests similiar to their own. Such pacts are described by
botanists as examples of symbiosis; they most frequently occur between
plants and insects, but the plants also have their working agreements
with members of the other two great kingdoms of life. In fact, all
Nature is a vast system of checks and balances, with every creature
preying more or less upon every other creature, except when they can
gain more by joining their efforts. Certain Humming-Birds lie in wait
near plants which by their nectar-sweets attract swarms of insects, and
hard by, Snakes lie in wait for the Birds. The Birds rid the plants
of destroying pests; the part of the Snakes in a beneficent scheme of
existence is not so apparent, but merely because we cannot see good in
a thing is no argument that it does not exist.
Many of the most important alliances of plants are made in response
to the law that “Nature abhors perpetual self-fertilization”. This
principle is one of the greatest in plantdom; there is a constant
necessity for the intercrossing of independent life-streams. The plants
go to great lengths to see that the multiplication and evolution of the
species is properly carried on.
We always associate Bees and flowers, yet it is probable, that, as a
whole, the plants, especially in the tropics, depend more upon Ants
than upon any other insects. Many vegetable folk deliberately employ
them to keep their leaves and stalks free of obnoxious visitors. The
Cow-Horn Orchid, like most plants which perch on trunks and branches,
produces pseudo-bulbs into which its vitality can recede in dry
seasons. There is always a small opening at the bottom of each of these
little tubes, through which Ants enter. They honeycomb the interior
with cells and galleries where they can be perfectly dry in the wettest
weather. On the approach of Caterpillars, Cockroaches and other Orchid
enemies, the residents issue in great swarms to protect their combined
host and home.
The species _Coryanthes_, instead of pseudo-bulbs, grows great masses
of fibrous aerial roots among which the Ants dwell. They are ever ready
to repel invasions of Cockroaches and other crawlers who seek to eat
the tender growing root-tips.
An Epiphyte which is particularly solicitous for the welfare of its
insect allies is the Ant-nest Plant, _Rubiaceae Myrme_. This ingenious
creature not only builds nests but builds them made-to-order. Certain
enlargements on its stem are hollowed out into chambers with connecting
galleries quite ready for their intended tenants. All the Ants have to
do is to move in. The kind that usually enter the plant’s service are
fierce warriors, _Iridiomyrmex Myrmecodiae_, with very powerful stings.
They form a formidable bodyguard.
Sometimes the Ant warriors of such compacts are quite satisfied to
accept the free rental of their snug quarters as sufficient pay and
seek their food elsewhere. More frequently, the alliance includes
“board and lodging” with the plant issuing wages in the form of nectar,
sweet pulp and other food.
The Cherry and Vetch are among plants which secrete a candy-like
substance on their stalks which serves as an allurement for Ants to
climb and establish their homes there. In many cases, these excretions
are also barriers which prevent the Ants from hunting among the
plant’s blossoms for honey, as they would thus destroy the precious
grains of pollen.
The South American Imba-uba Tree, Cecropia, has a hollow trunk in which
Bees and Ants dwell together amicably. The Polygonums Tree of the same
continent has so many Ant allies that it is often entirely hollowed
out by them. The process often operates so far that men break off the
smaller twigs and use them as ready-made pipe stems. The Melastroma
Plant of South America provides pouches on each leaf-stalk for the
benefit of its black guardian Ants. The Tococas and Mermidones also
have Ant-sacs.
In China it is a common practice of the Orange-growers to encourage
the visitation of non-vegetarian Ants by placing selected species on
trees and connecting the trees by bamboo poles over which the faithful
insects can rush their forces to particularly threatened points.
Everyone knows of the large part the industrious Bee plays in the
economy of the plant world. Few plants, there are, which are not aided
in their love-making by this tiny brown buzzer; some flowers depend
upon him entirely in their efforts to propagate the species.
The Bees and their relatives are particularly welcome to the flowers
because they do the work of fertilization so well. Wingless insects
are undesirable because they offer little guarantee that they will
successfully carry pollen to some other flower of the same species.
Even if it is not brushed off in the course of their laborious travels,
they are not at all particular what kind of flowers they visit and so
offer small hope of carrying pollen to its correct destination. Flying
insects of the Bee family seem to have the work of cross-fertilization
directly assigned to them. On each of their separate, pollen-gathering
journeys, they are partial to one particular kind of flower. As they
flit from blossom to blossom of the same species, going in and out of
flower and flower, rubbing against a group of stamens here and brushing
against a pistil there, they fertilize plant after plant in grateful
acknowledgment of the store of sweets they are collecting.
Many and ingenious are the methods which flowers adopt to make sure
that only invited and useful guests come to their nectar-feasts. The
very Ants which guard the lower portions of a plant so well, might
become mere greedy plunderers, if allowed to crawl within the flowers.
It is not often that they do. Sometimes, the stalks and even the petals
of flowers like the Rock-Lichens and the Butter-Wort are coated with
some plant chemical exceedingly disagreeable for an insect to crawl
over. Various alkaloids, resins and oils in the cell juices also make
the flower and its leaves obnoxious to grazing animals. Many plants,
like the Mullein and Stinging-Nettle, use bristles and prickles to
repel Slugs and Caterpillars.
A common protective device is for a flower to place its nectar at the
bottom of a long, narrow tube only accessible to a flying insect having
a proboscis. In the _Antirrhinum_ the entrance to the flower is closed
to small crawlers by a very heavy corolla. Bees, because of their size
and strength, can force their way through. It is said that as soon as
the stigma of this flower has been fertilized, the corolla relaxes and
Ants and their kind are free to enter and partake of such dainties as
are left.
Nettles, Passion-flowers, and Lilies frequently line their interiors
with stiff, in-pointing hairs which oppose a most effective palisade
against anything that crawls, whereas a flyer provided with a proboscis
can stand on the edge and, inserting his straw, drink up the best soda
water in plantdom. This existence of proboscides in insects which help
to cross-fertilize flowers is the very finest example we have of true
mutualism. Here is a case where members of two supposedly different
worlds of life have developed highly specialized organs in order that
they might help each other.
It is said that Charles Darwin, after noting the extraordinary length
of the spur of the Orchid _Angraecum Sesquipedale_ of Madagascar
predicted that some day there would be found in that country a moth
with a proboscis ten to eleven inches long. Not many years after, Dr.
Fritz Müller verified the sagacity of the famous scientist by finding
an insect exactly answering this description.
The Birth-Wort (_Aristolochia Clematitis_) takes no chances with
its insect visitors. In entering it, a Bee brushes easily by the
down-pointing hairs only to find that, when he attempts to go out
again, the bristles present stiff, unyielding obstacles against his
egress. In his excitement at this discovery, he buzzes around quite
angrily and, without noticing it, thoroughly showers the stigma with
pollen and incidentally covers his own body with a good supply to be
carried on to the next stop. When this process is quite complete, the
flower graciously relents, relaxes its hairs and allows the exasperated
insect to escape.
The _Pedicularis_ family uses similiar coercive methods, and by sharp
teeth, forces insect-visitors to take a course through the flowers
which brings them in contact with both stamens and pistils.
The purple Loosestrife, pretty dweller by banks and meadows, sets a
rich table and so always has plenty of insect visitors. It produces six
different kinds of yellow and green pollen, and is therefore sure to
suit every taste. Incidentally it has two different sets of stamens and
stigmas of three different lengths.
Night-blooming flowers only entertain after the sun goes down. All day
long they look withered and dead, but with the coming of the stars,
they open up to show conspicuous white or light-tinted interiors. A
flower like the Silene also exhales a rich, sensuous odor, which, with
its light colour, serves to attract such insects as are abroad at night.
Sycamore and Lime trees have humble allies in the tiny mites which live
in the retreats built of hairs to be found at the places where the
veins of the leaves fork. During the day they hide away from sight, but
at night they come out and scour the leaves clean of noxious bacteria
and fungus spores.
Pollen of different plants, when examined under the microscope, reveals
wonderful facts about the reciprocal relations which exist between
plants and insects. Wind-fertilized plants are nearly always without
any special beauty of form, colour or scent, while plants which are
fertilized by insects are most always conspicuous, brightly coloured
and highly scented. In the same way, pollen of the Hazel, Birch,
and Balsam Poplar, which is carried by the wind, is small, light,
practically spherical and devoid of protuberances. Pollen of the
Primrose, Cowslip and Polyanthus, often carried by insects, is deeply
furrowed, covered with spines and knobs, strung together by sticky
threads and, in other ways, provided with apparatus which enables it to
adhere to any object which it touches.
The pollen of the Hollyhock and the Dandelion consists of large,
beautiful, spherical grains covered with spikes. The Rhododendrons,
Azalias, and Fuchsias produce great masses of grains bound together by
viscid threads. Many of these bits of life-principle are geometric
masterpieces. A pollen grain of the _Cobaea Scandens_ is one of the
most fascinating objects of the microscopic world. It is perfectly
spherical and cut into small hexagonal facets like the eyes of a fly.
Grains of pollen of all kinds vary between one two-thousandth and one
two-hundredth of an inch in diameter.
Alliances between plants and birds are more important than we imagine.
The tropical Humming-birds and the eastern Sun-birds are in habits
exactly like the pollen-carrying insects. To watch one of these
brilliantly coloured creatures hovering over a flower or flying
directly into a blossom after nectar, is to almost always mistake it
for a Butterfly.
Many birds are invaluable allies of the plant world. They devour
thousands of leaf-eating insects per day and so keep down the army of
enemies which would otherwise destroy whole forests. Birds like the
Woodpeckers rid tree bark of wood-boring crawlers.
In the human world every partner does not always live up to his
agreements. And there are evidences that both plants and their allies
sometimes engage in questionable practices, bordering on deception and
chicanery.
The insects are often enough the offenders, and their crime is most
frequently one of robbery. If they can get the sweets they are
after without carrying out their share of the bargain, they will do
so. Bumble Bees have been observed to cut through the flower-walls
of a Nasturtium and so extract its nectar without coming near the
pollen-producing stamens. Sweet Peas frequently ignore the insects and
fertilize themselves. The Hawkweed (_Hieracium_) has so little faith
in insect allies that it produces seeds parthenogenetically, that is,
without the union of sex elements.
Alliances which start out advantageously for both parties sometimes
degenerate into mere sinecures for one or the other. The naturalists
Ihering, Ule and Fiebrig, working in South America, a few years ago
concluded that the association of the plant Cecropia and the Aztecan
Ants, long regarded as a classic example of mutualism, is by far of
greater benefit to the Ants. The openings which the Ants make into
the hollow interiors of this plant also allow the entrance of certain
destructive insects, and the Ants themselves attract Woodpeckers
which damage the plants. It is also alleged that these same Ants,
and the ones which inhabit the _Humboldtia Laurifolia_, are often so
busy feasting on nectar that they do not stop to repel invasions of
foliage-destroying insects.
While man is the greatest enemy of the plant world, he is also at times
its greatest friend. When it is to his advantage or when he is prompted
by a sincere love of Nature, he becomes a strong and helpful ally. He
aids his fellow creatures of the vegetable world when they are sick or
injured and, by improving their environment and protecting them from
attack and danger, enables them to develop to best advantage. A wizard
like Luther Burbank helps them in their efforts at race improvement and
development.
In Egypt and Arabia, man has acted as carrier of pollen for centuries,
and has thus insured an abundant Date crop. The same thing is often
done in other parts of the world with Apples, Pistachios, Melons,
Cucumbers and other plants having unisexual flowers.