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.