=913. Edaphic[51] formations.=—Edaphic formations may occur in any of the climatic-formation areas. They are controlled by the condition of soil or ground. The condition of the soil is unfavorable for the growth of the general vegetation type of that region, or is more favorable for another vegetation type, so that soil conditions overcome the climatic conditions. These areas include swamps, moors, the strand or beach, rocky areas, etc., as well as oases in the desert, warm oases in the arctic zone, river bottoms in the prairie and plains region, alkaline areas, etc. The edaphic formations may be close or open according to the nature of the soil. The edaphic formations then are infiltrated in the climatic formations, the different vegetation types fitting together like pieces of mosaic, which can be seen in some places from a mountain top, or if one could take a bird’s-eye view of the landscape or from a balloon. =914. Aquatic formations.=—These are made up of water plants and are of two general kinds: fresh-water plant formations in ponds, lakes, streams; and salt-water plant formations in the ocean and inland salt seas. =915. Culture formations.=—Culture formations are largely controlled by man, who destroys the climatic or edaphic formation and by cultivation protects cultivated types, or by allowing land to go to “waste” permits the growth of weeds, though weeds are often abundant in the culture areas. In general the culture formations may be grouped into two subdivisions: 1st, the vegetation of cultivated places; and 2d, the vegetation of waste places, as abandoned fields, roadsides, etc. IV. Plant Societies. =916. Plant societies= are somewhat definite associations of the vegetation of an area marked by physiographic conditions. A single plant society is nearly if not altogether identical with a “_principal formation_,” but is a more popular expression, and besides includes all the plants growing on the area, while in the use of the term “principal formation” we have reference mainly to the dominant plants and the most conspicuous subordinate species.