THE GEOLOGIC FUNCTIONS OF LIFE, 667 



Plant societies.^ — The control of physical conditions has been suffi- 

 cient to develop special associations or societies of plants by fostering 

 those adapted to these conditions and eliminating those that are not. 

 Among these are (1) the hydrophytes ('^ water plants ''), embracing 

 those that grow in water or in very wet situations; (2) xerophytes 

 (^^ drought plants''); embracing the opposite class, which are adapted to 

 very dry situations; (3) mesophytes, including those suited to con- 

 ditions lying between these extremes, the great middle class to which 

 the prevaihng upland vegetation belongs; and (4) the halophytes (^^salt 

 plants"), which are dependent on the presence of certain salts, and 

 embrace such plants as are found on the seacoast, around salt springs, 

 on alkaline flats, etc. The characters which distinguish the xerophytes 

 from the hydrophytes and mesophytes have special geological interest, 

 as they aid in determining the climatic conditions, a feature whose 

 interest increases as the variability of the ancient climates is more fully 

 recognized. 



Within these greater groups there are special minor associations 

 determined by soil, temperature, topography, subjacent strata, and by 

 the relations of the plants to one another. 2 These natural groups are 

 valuable indications of the agricultural capabilities of the districts 

 occupied by them. They may be regarded as the outcome of Nature's 

 experiments in crop-raising, running consecutively through thousands 

 of years. They are natural correlations of compatible members into 

 communities of plants. Some members of the society are obviously 

 dependent on others, as certain forms of undergrowth on the shadow- 

 ing of the upper growth, as of vines upon supporting-trees, etc. There 

 is probably a more occult relation in some cases, the effects of certain 

 plants on the soil being sometimes advantageous to other plants, and 

 sometimes harmful, as illustrated in the conditions that require a rota- 

 tion of crops. 



The chief point of geologic interest lies in the fact that floras are 

 not mere miscellaneous mixtures of plants that happen to live in a given 

 area at a given period, but are organized communities, in a more or less 

 definite sense. They therefore imply more or less definitely the physical 



^ Reference works : Plant Relations, Coulter, 1900, — a convenient elementary 

 work; Schimper, Pflanzengeographie, 1898; Warming, Lehrbuch der oekologischen 

 Pflanzengeographie,.189(i; Co wles, , Botanical Gazette, Vol. XXVII, 1S98. 



' One of the earliest attempts to map these and develop their significance and value 

 is found in Vol. II, Geol. of Wis., 1873-77, Native Vegetation, pp. 176-87. 



