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.