CHAPTER IV
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.
FRIENDLY ALLIES BY THE WATER’S EDGE
