The term "community" implies a diversity but at the same time a certain organized uniformity in the units. The units are the many individual plants that occur in every community, whether this be a beech forest, a meadow, or a heath. Uniformity is established when certain atmospheric, terrestrial, and other factors are co-operative, and appears either because a certain defined economy makes its impress on the community as a whole, or because a number of different growth-forms are combined to form a single aggregate which has a definite and constant guise. The analysis of a plant-community usually reveals one or more of the kinds of symbiosis as illustrated by parasites, saprophytes, epiphytes, and the like. There is scarce a forest or a bushland where examples of these forms of symbiosis are lacking; if, for instance, we investigate the tropical rain-forest we are certain to find in it all conceivable kinds of symbiosis. But the majority of individuals of a plant-community are linked by bonds other than those mentioned--bonds that are best described as _commensal_. The term _commensalism_ is due to Van Beneden, who wrote, "Le commensal est simplement un compagnon de table"; but we employ it in a somewhat different sense to denote the relationship subsisting between species which share with one another the supply of food-material contained in soil and air, and thus feed at the same table. More detailed analysis of the plant-community reveals very considerable distinctions among commensals. Some relationships are considered in the succeeding paragraphs.