COMRADES OF THE PLANT WORLD
“... _which links by a fraternal tie
The meanest of His creatures with the high._”
--_Lamartine_
The first and greatest problem for every terrestrial creature is to
live. The chief means of doing so is to eat. Therefore, the relation of
being to being and species to species is dominated by the necessity for
food. Among man this fact is somewhat masked and obscured, but in the
rest of the world it is entirely plain and obvious. Again and again on
every hand, we see that plant, animal, and man all maintain their life
impulses by consuming the tissue of their fellows.
In view of this fundamental fact, we can afford to look with some
degree of charity upon that class of plants which are termed parasites.
These interesting creatures are merely carrying out in a very direct
and apparent way a principle which permeates all domains of life. A
Tiger kills its prey; an Ox devours unoffending Grass; the parasitic
Dodder robs some healthy neighbour of part of its juices.
The word “parasite” originally referred to a member of a college of
priests who had their meals in common. Later, it came to mean living
at another’s expense, as large numbers of people did in classical
times. When one realizes that there are twenty-five hundred species
of parasitical seed plants, he hesitates to brand them all as thieves
and degenerates. Taking into consideration plants which depend upon
the soil fungi for part of their sustenance, we should have to call
half the seed plants in the world “parasites.” On a basis of strict
accountability, it would also be necessary to classify all fruits as
“parasites” as they draw nourishment from the parent boughs and give no
return.
The fact is there are very few plants which are not more or less
dependent upon some living fellow creature for their food supply.
Sometimes the relation is strictly reciprocal; sometimes the advantage
appears to greatly favour one or the other of the participants. In
other cases the occurrence arises accidently through chance proximity,
without a conscious pact or deliberate contract.
Edward Step in his illuminating book _Messmates_ sums up the matter
admirably: “Two friends in good health, each able to earn his own
living, agree for the sake of companionship to live together, but each
defraying the cost of his own necessities and luxuries. This is a case
of mutualism. Two other friends also agree to share quarters and have
a common table; but one may be infirm and wealthy whilst the other is
strong and comparatively poor. The infirm one offers to pay two-thirds
of their common expenses if the other will contribute one third,
plus his protection, cheerful companionship or other valuable help.
This is a commensalism. The pair are messmates, each contributing to
hotch-potch according to his ability or endowment, each affording what
the other lacks, and both, therefore, benefitting from the partnership.”
It must be admitted that there are cases of plant companionship in
which, to all human perception, the material benefits seem directly
one-sided, but who can conclusively deny that the nourishment-giving
partner may not receive some psychic or spiritual benefit from the
union? The Orchids and many other tree-parasites bear flowers of
exquisite beauty. Can we be quite sure that the trees do not like to
adorn themselves with gorgeous ornaments of this kind? Such a desire
would be quite natural.
Plants which are low and weak in the scale of evolution are very
prone to enter into symbiotic relations. The Lichens are compound
organisms in which green Algal cells live between fungous threads.
The Fungus sucks up the water and mineral salts from the soil and the
Alga combines them with carbon dioxide from the air to form palatable
food for both. Such plant-partners have been observed to live together
amiably for twenty-five years or more.
The Fungi and all plants which are “pale, fleshy, as if the decaying
dead with a spirit of life had been animated” have no chlorophyll, the
mysterious green substance which is necessary for the production of
starch. They must either make alliances with plants which possess this
vital elixir or live on decaying matter which contains elaborated
food material. Many choose the latter course, but a goodly number,
especially those of primitive structure, have entered into profitable
partnerships.
The minute one-celled plants called Zoochlorella or Zooxanthella
have chosen the fresh water sponge _Ephydatia Fluviatilis_ for their
messmates. Sometimes they live with the Hydra called _Viridis_ and
impart to it a bright green colour.
There are whole regiments of microscopic parasites which thrive on
living plant tissue and cause spots and rust to appear on Apples,
Peaches, Pears and other fruits and number among their cohorts
Rose-blight, Wheat-rust, and various Mildews. The larger messmate does
not receive very much benefit from the relation, in this instance,
except when the minute guests serve to cover a cut or an abrasion with
a protective mantle, just as Mildew shields cheese or jelly from decay.
Cases where Fungi render very valuable services to larger plants are
exemplified by the Monotropa or Indian Pipe. This pallid scavenger
grows on the decaying vegetable matter of the woods. It toils not,
neither does it make plant starch, but it is able to produce pretty,
ghostly flowers and white scale-like leaves. On its roots thrive
species of Fungi which perform the part of root hairs and in return
receive nourishment from their host. Certain authorities claim that the
Fungi get the better of the bargain, as the Monotropa has been known to
maintain its health without them in laboratories. But the fact is the
relation _does_ exist with undisputed benefit to both parties.
Beech Drops germinate in contact with roots of the Beech tree, attach
themselves there and raise yellow, seared stems covered with scales
instead of leaves but bearing perfect flowers. The Broom-Rapes get
their nourishment from the roots of Tobacco and Hemp in the same way.
Prominent among the larger parasitic plants is the Dodder or Devil’s
Thread. This vine derives all its sustenance from other plants and,
as far as can be determined, gives no material return. From this
standpoint, the Dodder is a robber pure and simple, a degenerate
outcast from the community of decent plants. From the viewpoint of
this chapter, it is possible to believe that the host of the Dodder
derives some spiritual or hidden material benefit from the union which
makes it distinctly worth while. If such were not the case, it would
seem that, through ages of evolutionary development, such plants as
Flax would have devised means to escape the Dodder’s clutches.
The Dodder inhabits low ground and pokes an inquiring head above the
surface each spring much like any self-sustaining plant. However, it is
not long before it attaches itself to some lusty neighbour by root-like
suckers, which pierce the stem and extract the nourishing juices. If
the supply seems adequate, the Dodder winds its yellow, yarn-like
tendrils about the host and allows the roots which connect it to the
earth to wither. Its absorbing tubercles look like caterpillar feet;
their cells form a perfect graft with the host and gradually disperse
through its body. If other plants are near enough, the Devil’s Thread
will reach out and tap their food supplies also. A single Dodder
has been known to draw nourishment from five or six other plants of
different families at the same time, thus indicating that it must have
digestive machinery enough to appropriate these varying saps to its own
uses. The Dodder has no chlorophyll and therefore no leaves but bears
pretty little bell-like flowers which later produce seed.
In the tropical jungles are many parasites of brilliant aspect, which,
having no leaves or root hairs, germinate directly on supporting plants
and apply suckers to the tissues of their hosts. When seen from the
ground, their short stems make them seem all flower, and often very
handsome ones. The _Rafflesia Arnoldi_ of Sumatra is a notable example.
Man cannot help condemning such plant practices. Yet all Nature is a
struggle for existence. Does it not require some courage and hardihood
to come out and do in a bold and open way what the rest of the universe
is doing by indirect or underhand methods?
The beautiful Orchids belong to a botanic group of Epiphytes which
may be classified as guests or lodgers. Being green, they are able to
gather their own living from dust, rain and carbon dioxide in the air.
All they ask from their tree-hosts is a branch on which to perch.
There are probably few trees which are not delighted to have such
delicate, fairy-like creatures add to their own beauty and charm. They
wear them much as a woman wears a rose in her hair.
In America there are well-mannered parasites such as the decorative
Spanish Moss so common throughout the South. This plant is normal in
all respects; except that, perched on a kindly tree, it draws all its
nourishment from the air instead of through soil-piercing roots.
The Mistletoe is a perfect example of a mutualist. Early in its aerial
life, it sends a root through the bark of its tree companion and during
the spring and summer, absorbs much food. When winter days come, and
the tree has lost its leaves, the grateful messmate reverses the
process and sends into the heart of its friend the larger part of the
nourishment which it has been able to store up during the prosperous
weeks of summer. The seeds of the Mistletoe are interesting because
they are covered with a sticky fluid which enables them to travel from
tree to tree on the feet of birds.
That some plants are parasites from necessity or laziness rather than
choice is indicated by a Brazilian variety of the Cuckoo-Pint which
sits far up on some tree branch and, like an immense spider, sends down
to the earth long delicate tubes through which it sometimes sucks food
and water.
One of the most interesting facts in plantdom is the alliance
maintained by Clovers, Beans, Vetches and other leguminous plants, with
Bacteria belonging to the class _Pseudomonas_. No soil can be fertile
unless it contains organic compounds of nitrogen. The earth Bacteria
have discovered methods of producing these important substances,
possibly extracting nitrogen distributed through the ground. These
minute parasites attach themselves to the roots of the larger plants,
which promptly enclose them in cysts or nodules where they can lead a
sheltered life and manufacture assimilable food compounds for their
hosts. When they die, the owners of the roots feed upon their bodies.
What is the art of grafting but a form of artificial parasitism? Very
often a branch or cutting is made to form a bodily union with some
plant of an entirely dissimilar species. In some cases, the intruder
sends roots into the tissue of its host like a true dependent. Grafts
of Prickly Pears, Mexican Grapevines and Agaves put forth food-suckers
in the soft flesh of the Giant Cactus or the Barrel Cactus much as they
would do if planted in the earth. There is here no true diffusive union
of partners but mere absorption on the part of the invader.
Even grafting of allied species of Grapes sometimes results in the
young plants sending roots through the tissues of the scion, eventually
reaching the earth by way of the body of the host. In such cases, the
parasite also draws nutriment from its messmate by means of a superior
osmotic pressure.
Almost everything lies in the point of view. No man, no animal, no
plant is so debased and degraded that it does not radiate some little
measure of helpfulness. If “all things work together for good,” even
that member of a plant union which seems to act upon that inverted
principle of “all coming in and nothing going out” has its legitimate
place in the world. As for those numerous examples of share-alike
partnerships, they illustrate the principle of the divine law of love
which lies back of and above the very real hardships and cruelties of
this work-a-day world.
[Illustration: FRIENDLY ALLIES BY THE WATER’S EDGE]