Commensalism

Commensalism, also known as Consortism or Mutualism, is that form of symbiosis in which two organisms (either both plants, both animals, or a plant and an animal) live together on terms of mutual advantage. One of the most striking instances is that of the lichens (q.v.) formerly thought to be autonomous organisms, but now recognised as consisting of one of the lower algae enclosed in the thallus of a fungus, generally ascomycetous. The alga (gonidium) is kept moist, whilst the fungus is nourished on the cast-off cell-walls of the alga. Another similar but even more startling case is that of the mycelia of Elaphomyces, a truffle, and the roots of many ordinary forest-trees, the fungus being supposed to aid in the assimilation-processes of the roots on the sloughing of which it lives. The close association of some hermit-crabs with sea-anemones is only one among multitudes of instances in the animal world; but perhaps the most remarkable are those involved in the correlation of animal and vegetable species. Thus we have flowers contrived in every detail to compel visiting insects to carry away their pollen, whilst the insect's proboscis and legs are modified for the same end and it is rewarded with honey or pollen. Species of Acacia have stipular spines hollowed for the abode of ants that protect the plant from other species of ants which are leaf-eaters, the protectors having also small glandular outgrowths on the points of the leaflets provided for them as partial board in addition to their lodging. Every gradation occurs between mutualist symbiosis and that which is one-sided in its advantages, i.e. parasitism (q.v.).