_Like commensals._--When a plant-community consists solely of individuals belonging to one species--for example, solely of beech, ling, or Aira flexuosa--then we have the purest example of like commensals. These all make the same demands as regards nutriment, soil, light, and other like conditions; as each species requires a certain amount of space and as there is scarcely ever sufficient nutriment for all the offspring, a struggle for food arises among the plants so soon as the space is occupied by the definite numbers of individuals which, according to the species, can develop thereon. The individuals lodged in unfavorable places and the weaklings are vanquished and exterminated. This competitive struggle takes place in all plant-communities, with perhaps the sole exceptions of sub-glacial communities and in deserts. In these _open communities_ the soil is very often or always so open and so irregularly clothed that there is space for many more individuals than are actually present; the cause for this is obviously to be sought in the climatically unfavorable conditions of life, which either prevent plants from producing seed and other propagative bodies in sufficient numbers to clothe the ground or prevent the development of seedlings. On such soil one can scarcely speak of a competitive struggle for existence; in this case a struggle takes place between the plant and inanimate nature, but to little or no extent between plant and plant. That a congregation of individuals belonging to one species into one community may be profitable to the species is evident; it may obviously in several ways aid in maintaining the existence of the species, for instance, by facilitating abundant and certain fertilization (especially in anemophilous plants) and maturation of seeds; in addition, the social mode of existence may confer other less-known advantages. But, on the other hand, it brings with it greater danger of serious damage and devastation wrought by parasites.