Yet there are in every plant-community numerous species which _differ widely_ in the demands they make for light, heat, nutriment, and so on. Between such species there is less competition, the greater the disparity in their wants; the case is quite conceivable in which the _one species should require exactly what the other would avoid_; the two species would then be complementary to one another in their occupation and utilization of the same soil.
There are also obvious cases in which different species are of service to each other. The carpet of moss in a pine forest, for example, protects the soil from desiccation and is thus useful to the pine; yet, on the other hand, it profits from the shade cast by the latter.
As a rule, limited numbers of definite species are the most potent, and, like absolute monarchs, can hold sway over the whole area; while other species, though possibly present in far greater numbers than these, are subordinate or even dependent on them. This is the case where subordinate species only flourish in the shade or among the fallen fragments of dominant species. Such is obviously the relationship between trees and many plants growing on the ground of high forest, such as mosses, fungi, and other saprophytes, ferns, Oxalis Acetosella, and their associates. In this case, then, there is a commensalism in which individuals feed at the same table but on different fare. An additional factor steps in when species do not absorb their nutriment at the same season of the year. Many spring plants--for instance, Galanthus nivalis, Corydalis solida, and C. cava--have withered before the summer plants commence properly to develop. Certain species of animals are likewise confined to certain plant-communities. But one and the same tall plant may, in different places or soils, have different species of lowly plants as companions; the companion plants of high beech forests depend, for instance, upon climate and upon the nature of the forest soil; Pinus nigra, according to von Beck, can maintain under it in the different parts of Europe a Pontic, a central European, or a Baltic vegetation.
There are certain points of resemblance between communities of plants and those of human beings or animals; one of these is the competition for food which takes place between similar individuals and causes the weaker to be more or less suppressed. But far greater are the distinctions. The plant-community is the lowest form; it is merely a congregation of units, among which there is no co-operation for the common weal, but rather a ceaseless struggle of all against all. Only in a loose sense can we speak of certain individuals protecting others, as for example, when the outermost and most exposed individuals of scrub serve to shelter from the wind others, which consequently become taller and finer; for they do not afford protection from any special motive, such as is met with in some animal communities, nor are they in any way specially adapted to act as guardians against a common foe. In the plant-community egoism reigns supreme. The plant-community has no higher units or personages in the sense employed in connection with human communities, which have their own organizations and their members co-operating, as prescribed by law, for the common good. In plant-communities there is, it is true, often (or always) a certain natural dependence or reciprocal influence of many species upon one another; they give rise to definite organized units of a higher order; but there is no thorough or organized division of labor such as is met with in human and animal communities, where certain individuals or groups of individuals work as organs, in the wide sense of the term, for the benefit of the whole community.