Page:EB1911 - Volume 21.djvu/791

Rh subdivided plant geography into floristic plant geography and ecological plant geography. The former is concerned with the division of the earth's surface into major districts characterized by particular plants or taxonomic groups of plants, with the subdivision of these floristic districts, and with the geographical distribution (both past and present) of the various taxonomic units, such as species, genera, and families. On the other hand, ecological plant geography seeks to ascertain the distribution of plant communities, such as associations and formations, and enquires into the nature of the factors of the habitat which are related to the distribution of plants—plant forms, species, and communities. In a general way, floristic plant geography is concerned with species, ecological plant geography with vegetation. The study of the distribution of species dates back to the time of the early systematists, the study of vegetation to the time of the early botanical travellers. Humboldt, for example, defined his view of the scope of plant geography as follows: “C'est cette science qui considère les vegetaux sous les rapports de leur association locale dans les différents climats” (1807: 14).

The Habitat.—The term habitat, in its widest sense, includes all the factors of the environment which affect a plant or a plant community, though the term is frequently used to signify only some of these factors. The factors of the habitat may be grouped as follows: geographical, physical, and biological.

Geographical Factors.—Geographical position determines the particular species of plants which grow in any particular locality. This matter is bound up with the centres of origin and with the past migrations of species, and such questions are usually treated as a part of floristic plant geography. Here, therefore, floristics and ecology meet. Flahault and Schröter, in defining the term habitat, appear to exclude all geographical factors. They state that “the term habitat is understood to include everything relating to the factors operative in a geographically defined locality, so far as these factors influence plants” (1910: 24); but the exclusion of geographical and historical factors from the concept of the habitat does not appear to be either desirable or logical.

Physical Factors.—These are frequently classified as edaphic or soil factors and climatic factors; but there is no sharp line of demarcation between them. Edaphic factors include all those relating to the soil. The water content of the soil, its mineral content, its humus content, its temperature, and its physical characteristics, such as its depth and the size of its component particles are all edaphic factors. Climatic factors include all those relating to atmospheric temperature, rainfall, atmospheric humidity, and light and shade. Factors connected with altitude, aspect, and exposure to winds are also climatic such are often spoken of as physiographical factors. The difficulty of sharply delimiting edaphic and climatic factors is seen in the case of temperature. Soil temperature is partly dependent on the direct rays of the sun, partly on the colour and constitution of the soil, and partly on the water content of the soil. Again, the temperature of the air is affected by radiation from the soil; and radiation differs in various soils.

Biological Factors.—These include the reactions of plants and animals on the habitat. Here again, no sharp boundary-line can be drawn. In one sense, the accumulation of humus and peat is a biological factor, as it is related to the work of organisms in the soil; but the occurrence or otherwise of these organisms in the soil is probably related to definite edaphic and climatic conditions. Again, the well-known action of earthworms may be said to be a biological work, but the resulting aeration of the soil causes edaphic differences, and earthworms are absent from certain soils, such as peat. The pollination of flowers and the dispersal of seeds by various animals are biological factors; but pollination and dispersal by the wind cannot be so regarded. The influence of man on plants and vegetation is also a biological factor, which is frequently ignored as such, and treated as if it were a thing apart.

When the nature and effect of ecological factors have become more fully understood, it will be possible to dispense with the above artificial classification of factors, and to frame one depending on the action of the various factors; but such a classification is not possible in the present state of knowledge.

Ecology and Physiology.—Whilst our knowledge of the nature and effect of habitat is still in a very rudimentary condition, much progress has been made in recent years in the study of plant communities, but even here the questions involved in relating the facts of the distribution of plant communities to the

factors of the habitat are very imperfectly understood. This is due to a lack of precise knowledge of the various habitat factors and also of the responses made by plants to these factors. Until much more advance has been made by ecologists in the investigation of the nature of habitat factors, and until the effect of the factors on the plants has been more closely investigated by physiologists, it will remain impossible to place ecology on a physiological basis: all that is possible at present is to give a physiological bias to certain aspects of ecological research. Obviously no more than this is possible until physiologists are able to state much more precisely than at present what is the influence of common salt on the plants of salt-marshes, of the action of calcium carbonate on plants of calcareous soils, and of the action of humous compounds on plants of fens and peat moors.

Ecological Classes.—Many attempts have been made to divide plants and plant communities into classes depending on habitat factors. One of the best known classifications on these lines is that by Warming. Warming recognized and defined four ecological classes as follows:—

Hydrophytes.—These live in a watery or wet substratum, with at least 80% of water. Warming included plants of peat-bogs among his hydrophytes.

Xerophytes.—These are plants which live in very dry places, where the substratum has less than 10% of water

Halophytes.—These are plants living in situations where the substratum contains a high proportion of sodium chloride.

Mesophytes.—These are plants which live in localities which are neither specially dry nor specially wet nor specially salty.

Such terms as hydrophytes, xerophytes, and halophytes had been used by plant geographers before Warming's time e.g., by Schouw; and the terms evidently supply a want felt by botanists as they have come into general use. However, the terms are incapable of exact definition, and are only useful when used in a very general way. The above classification by Warming, although it was without doubt the best ecological classification which had, at the time, been put forward, has not escaped criticism. The criticisms were directed chiefly to the inclusion of sand dune plants among halophytes, to the exclusion of halophytes from xerophytes, to the inclusion of “bog xerophytes” among hydrophytes, to the inclusion of all conifers among xerophytes and of all deciduous trees among mesophytes, and to the group of mesophytes in general.

Schimper made a distinct advance when he distinguished between physical and physiological dryness or wetness of the soil. A soil may be physically wet, but if the plants absorb the water only with difficulty, as in a salt marsh, then the soil is, as regards plants, physiologically dry. All soils which are physically dry are also physiologically dry, and hence only the physiological dryness or wetness of soils need be considered in ecology.

Schimper used the term xerophytes to include plants which live in soils which are physiologically dry, and the term hygrophytes those which live in soils which are physiologically wet or damp. Schimper recognized that the two classes are connected by transitional forms, and that it is useless to attempt to give the matter a statistical basis. It is only in a general sense like Schimper's that such ecological terms as xerophytes have any value, and it is not possible, at least at present, to frame ecological classes, which shall have a high scientific value, on a basis of this nature. Whilst Schimper objected to the constitution of a special category, such as mesophytes, to include all plants which are neither pronounced xerophytes nor pronounced hygrophytes, he recognized the necessity of a third class in which to place those