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.