ECOL'OGY, or ŒCOL'OGY of Plants (from Gk. οἶκος, oikos, house + -λογία, -logia, discourse, from λέγειν, legein, to say). That division of botany which has to do with the mutual relations between plant organisms and their environment. Until recently the subject-matter of ecology has not been systematically treated, although certain phases of the subject, such as pollination, seed-dispersal, protection, symbiosis, have been for some time grouped together and denominated 'biology,' especially by German authors, as Kerner (1887). Wiesner (1880), and Ludwig (1895). The first to treat ecology systematically was Warming, of Copenhagen, who. in 1895, gave a résumé of the ecological factors and their influence, and discussed the ecological plant geography of the world. dividing plants into four groups — viz. hydrophytes, mesophytes, xerophytes, and halophytes (qq.v.). In 1898 Schimper gave a much more extended treatment of the world's vegetation, but from a slightly different point of view, basing his main subdivisions on temperature, and discussing in each subdivision the ecological relations of the three types of climatic formations (forest, grassland, desert), and also, but very briefly, of the edaphic formations or plant societies.
Ecology is as yet but incompletely organized, and the subdivisions here proposed are necessarily tentative. There are three more or less distinct lines of ecology, as the subject is now studied: (1) The ecology of the various plants, their tissues and organs — i.e. the study of the relation of plants, tissues and organs to their environment — may be called physiological ecology, since this division of the subject is related to physiology, or may even be classed as a part of it. Physiological ecology has two further subdivisions: Anatomical ecology, which concerns itself with the origin, development, and meaning of plant tissues, and organographic ecology (or simply organography), which concerns itself with the origin, development, and meaning of plant forms and organs. (2) The ecology of edaphic plant societies — i.e. the study of their origin, development, and life relations — may be called physiographic ecology, since this subject is related to physiography, or may even be classed as a part of it. (3) The ecology of climatic plant formations — i.e. the study of the great forest, grass, and desert formations of the globe in relation to climatic factors — may be called geographic ecology, or ecological phytogeography, since this subject is essentially a part of geography. It is more convenient to speak of this third subdivision under the head of Distribution of Plants (q.v.).
Physiological ecology, whether anatomical or organographic, is concerned chiefly with the origin and meaning of the tissues and organs of plants. In the interpretation of this field there are two prominent theories, which may be briefly outlined. The older and commoner theory is based on teleological assumptions, and may be called the theory of purpose or design. The theory of design has also had two aspects: (l)That in which rigid special creations were postulated in order to account for the relations between structures and functions; (2) that in which natural selection, acting for many generations, was supposed to be the cause of specialized structures obviously adapted for certain ends. The newer and less common theory is based on dysteleological assumptions, and may be called the accident or gall theory. Plant structures are regarded as the result of various purely physical and chemical forces; if a structure is of value, it is but a matter of accident. Just as a gall is regarded by every one as the result of forces set in operation by the insect's sting, so all organs and tissues are the inevitable result of mechanical agents. Possibly the correct standpoint is an intermediate one. Structures cannot be built, nor functions carried on, contrary to chemical and physical laws; but within certain limits there is perhaps room for successful variation, and hence for natural selection and the gradual improvement of structures in relation to plant functions. Some of the arguments for natural selection and the teleological view are found in the structure and mechanics of stomata, which are the opposite of what one would theoretically expect; the storage of water or reserve foods is a phenomenon hard to account for without natural selection. Experiments show that some plants are rigid and some plastic; rigid plants have commonly lived for ages in rigid habitats, as is particularly true of desert plants. Plastic plants, on the other hand, often live in variable habitats, the best examples of all plastic plants being the amphibious species. It would seem as if permanence of habitat causes the development of rigid structures, as is well shown in the Begonia, which has remained xerophytic in structure through generations of mesophytic culture — i.e. culture in moist surroundings. And it would seem as if the marvelous plasticity of such plants as Ranunculus, Nasturtium, Polygonum, Proserpinaca, Sium, etc., is a result of long life in variable habitats. The desert plant shows the natural selection of adapted but rigid forms, while the amphibious plant shows the selection of the capacity to vary, a far subtler type of selection. While these arguments favor the natural selection and teleological theories, current dysteleological views have certainly clarified the atmosphere. The extreme views of some teleologists. who see a purpose in everything, must surely be discarded; but it remains for the future to strike a proper balance between the design and accident theories in ecology. For the present, therefore, the ecologist should strive to ascertain the influence of external agents on plant form, and should also seek to discover the advantages possessed by certain forms in the performance of the plant functions, but should leave the determination of the exact relation between form and function to future investigation.
The subject-matter of anatomical ecology is not presented as such in this work. The purely morphological aspects are treated under Anatomy of Plants. Anatomical matter is also presented in connection with the topics Hydrophyte; Xerophyte; {{NIE Article Link|Leaf}; Root; Symbiosis. Organographic ecology will be treated in connection with the various organs of plants (see Leaf; Stem; Root; Seed; Pollination; also Color; Duration; Vegetative Propagation), and with various plant forms. (See Epiphytes; Lianas; Saprophytes; Symbiosis.) The topics treated in physiographic ecology, and to some extent in phytogeography, involve organographic material. Physiographic ecology will be treated from the standpoint of topographic habitats (see Beach Plants; Benthos; Dune Vegetation; Heath; Meadow; Plankton; Rock Plants; Ruderal Plants), as well as from the standpoint of the water relations (see Hydrophyte, Mesophyte, Xerophyte) and chemical relations of the soil. (See Halophyte.) The ecological aspects of geographic botany are presented under Distribution of Plants; Alpine Plant; Desert Vegetation; Forest; Grassland; Thicket.
Bibliography . Costantin, La nature tropicale (Paris, 1899); Flahault, Essai d’une carte botanique et forestière de la France (Paris, 1897); Goebel, Organography of Plants (Oxford, Eng., 1900); Haberlandt, Physiologische Pflanzenanatomie (Leipzig, 1896); Eine botanische Tropenreise (Leipzig, 1893); Lubbock, Buds and Stipules (London, 1899); Ludwig, Lehrbuch der Biologie der Pflanzen (Stuttgart, 1895 ); Wiesner, Biologie der Pflanzen (2d ed., Vienna, 1901); Kerner, Natural History of Plants, trans. by Oliver (London, 1895); Coville, "Botany of the Death Valley Expedition," in United States Department of Agriculture (Washington, 1893); Gray, Scientific Papers (Boston, 1889); Kearnay, Botanical Survey of the Dismal Swamp Region (Washington, 1901); MacMillan, Metaspermæ of the Minnesota Valley (Minneapolis, 1892); Mayr, Waldungen von Nordamerika (Munich, 1890); Merriam, Life Zones and Crop Zones of the United States (Washington, 1898); Pound and Clements, Phytogeography of Nebraska (Lincoln, 1900). Consult also authorities referred to under Botany; Distribution of Plants.