=911.= The term plant formation is applied to associations of plants of the same kind, though there is a great difference in the use of the word by different writers which leads to some confusion.[50] It is sometimes applied to an association of individuals of a species, or of several species occupying a rather definite area of ground where the soil conditions are not greatly different (individual formation); by others it is applied to the plants of a definite physiographic area, as a swamp, moor, strand, or beach, bank, rock hill, clay hill, ravine, bluff, etc. (principal formation); and in a broad sense it is applied to the plants of climatic regions, of those in bodies of water, etc. (general formations). Space here is too limited to discuss all these kinds of formations, but the nature of the general formations will be pointed out. The general formations may be grouped into four divisions: 1st. Climatic formations. 2d. Edaphic formations. 3d. Aquatic formations. 4th. Culture formations. =912. Climatic formations.=—Climatic influences extend over wide regions, so that climate controls the general type of vegetation of a region. In the sense of control there are two climatic factors, temperature and moisture, especially soil moisture. Temperature exerts a controlling influence over the vegetation type only where the total heat during the period of growth and reproduction is very low. This occurs in polar lands and at high elevations where the climate is alpine. In the temperate and tropical regions of the globe moisture, not heat, controls the general vegetation type. These vegetation types in general are coincident with rainfall distribution, and Schimper recognizes here three types, which with the arctic-alpine type would make four climatic formations as follows: