ECOLOGY OF See also:

• PLANTS

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, See also:

• PHYSICAL

The difficulty of sharply delimiting edaphic and climatic factors is seen in the See also:

• CASE
• CASE, JOHN (d. 1600)

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 See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

The criticisms were directed chiefly to the inclusion of See also:

• SAND
• SAND, GEORGE (1804-1876)

Recently, Warming' (1909 136), assisted by Vahl, has modified his earlier classification, and adopted the following: A. The soil (in the widest sense) is very wet, and the abundant water is available to the plant (at least in hydrophytes). 1. Hydrophytes.—These include plants of the See also:

• PLANKTON

Oxylophytes.—These plants, sometimes spoken of as " bog xerophytes," grow in soils which contain an abundance of free humous compounds, and include plants which grow on fens and moors. 4. Psychrophytes.—These include the plants which grow on the See also:

• COLD (in O. Eng. cald and ceald, a word coming ultimately from a root cognate with the Lat. gelu, gelidus, and common in the Teutonic languages, which usually have two distinct forms for the substantive and the adjective, cf. Ger. Kolte, kalt, Dutch koude

Psammophytes.—These are plants which grow on sand and ggravel. Plants of sand-See also:

• DUNES

Sclerophyllous formations, e.g., garigues, maquis, and forests of See also:

• EVERGREEN

This. new See also:

• SYSTEM

Hydro-xerophytes (" bog xerophytes ").—Plants which live in wet, peaty soils, and which possess aeration channels and xenophilous leaves; e.g., Cladium Mariscus, Eriophorum angustifolium, Rubus Chamaemorus, and Vaccinium Vitis-Idaea. The term " oxylophyte " is open to the objection that some peaty See also:

• WATERS, TERRITORIAL

The plant association is sometimes referred to in technical See also:

• LANGUAGE (adapted from the Fr. langage, from langue, tongue, Lat. lingua)

" Remote regions which are floristically distinct . may possess areas physically almost identical and yet be covered by different formations" (Clements,' 1905 : 203). For example, the sand dunes of See also:

• NORTH
• NORTH, BARONS
• NORTH, MARIANNE (1830—1890)
• NORTH, ROGER (1653-1734)
• NORTH, SIR THOMAS (1535?-16o1?)

There is much difference of See also:

• OPINION (Lat. opinio, from opinari, to think)

See also:

• SOC

A tropical rain forest exhibits great variety both of species of plant and of plant forms. There is great diversity in the trees and masses of tangled lianes, and a See also:

• WEALTH

Still greater drought induces desert vegetation; but, as deserts are more characteristic of subtropical districts, they are discussed later on. Mangrove swamps, or tropical tidal forests, occur in saline or brackish swamps on See also:

• FLAT (a modification of O. Eng. flet, an obsolete word of Teutonic origin, meaning the ground beneath the feet)

When these forests become degenerate, maquis and garigues respectively are produced. Maquis and garigues are characterized by the abundance of shrubs and under-shrubs, especially by shrubby Leguminous plants, and by species Cistus and Lavandula. Secondly, there are forests of coniferous trees. In the Mediterranean region, even at comparatively See also:

• LOW, SETH (1850- )
• LOW, WILL HICOK (1853- )

In inland localities, where the rainfall is much See also:

• LOWER

At Beni Ounif and See also:

• COLOMB, PHILIP HOWARD (1831-1899)

Cold Temperate and Frigid Districts.—In the coldest portion of the north temperate See also:

• ZONE (Gr. 'WV77, a girdle, from 'wvvLvay to gird)

3. Physically and physiologically dry habitats, with the accompanying plant communities of sand dunes and sandy heaths with little humus in the soil. 4. Habitats of See also:

• MEDIUM

Although many plants typical of fresh water are able to grow also in brackish water, there are only a few species which appear to be quite confined to the latter habitats in this country. Such species perhaps include Ruppia maritima, R. spiralis, Zannichellia maritima, Z. potycarpa, Potamogeton interrupt us (= P. flabellatus), and Naias marina., In See also:

• FRESHWATER

Strengthening tissue of all kinds (and sometimes even the phloem) is more or less rudimentary. The stems are frequently characterized by aeration channels, which connect the aerial parts with the parts which are buried in practically airless mud or silt. Submerged leaves are usually filamentous or narrowly ribbon-shaped, thus exposing a large amount of surface to the water, some of the dissolved gases of which they must absorb, and into which they must also excrete certain gases. Stomata are often absent, absorption and See also:

• EXCRETION (Lat. ex, out of, cernere, cretum, to separate)

Xerophytes.—These plants have devices (a) for procuring water, (b) or for storing water, (c) or for limiting transpiration; and these adaptations are obviously related to the physically or physiologically dry habitats in which the plants live. Plants of physically dry habitats, such as deserts and sand dunes, have frequently See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

The latter, of course, greatly increase transpiration. A " cushion plant " (Anabasis aretioides) of the north-western Sahara, frequently shows dead leaves on the exposed See also:

• SIDE
• SIDE (mod. Eski Adalia)

Some halophytes tend. to lose their succulence when cultivated in a non-saline soil; and some non-halophytes tend to become succulent when cultivated in a salty soil; there is, it need scarcely be stated, little or no See also:

• EVIDENCE (Lat. evidentia, evideri, to appear clearly)

2 Lesage, " Recherches experimentales sur les modifications des feuilles chez les plantes maritimes," in Rev, gen. de See also:

• BAT

It is sometimes maintained, for example, by Schimper, that their xerophytic characters are related to the physiological dryness of the habitat: this, however, is denied by others who maintain (Clements, 1905: 127) that the xerophytism is due to the persistence of ancestral structures. It is possible, of course, that each explanation is correct in particular cases, as the views are by no means mutually exclusive. With regard to the occurrence of plants, such as Juncus effusus, which possess xerophytic characters and yet live in situations which are not ordinarily of marked physiological dryness, it should be remembered that such habitats are liable to occasional physical drought; and a plant must eventually succumb if it is not adapted to the extreme conditions of its habitat. The xerophytic characters being present, it is not surprising that many marsh plants, like Juncus effusus and Iris pseudacorus, are able to survive in dry situations, such as See also:

• BANKS, GEORGE LINNAEUS (1821-1881)
• BANKS, NATHANIEL PRENTISS (1816–1894)
• BANKS, SIR JOSEPH
• BANKS, THOMAS (1735-1805)

Their stomata are frequently not limited to the underside of the leaves, but may occur scattered all over the epidermal surface. The epidermal cells may contain chlorophyll. Strengthening tissue is feebly developed. Many sciophytes are herbaceous tropophytes, and are dormant for more than See also:

• HALF

The following plants, in England, are calcifuge: Lastraea Oreopteris, Holcus mollis, Carex echinata, Spergula arvensis, Polygala serpyllacea, Cytisus 2 T. G. Hill, " Observations on the Osmotic Properties of the Root-Hairs of certain Salt Marsh Plants," in The New Phytologist (1908), vol. vii. scoparius, Potentilla procumbens, Callum hercynicum (=G. saxatile), Gnaphalium sylvaticum, See also:

• DIGITALIS

In England, the number of calcicole species is greater than the number of silicolous species. It would therefore be curious if it were proved that lime acts on plants as a poison. It is said that some plants may be calcicoles in one geographical district and not in another. However, until more is known of the exact chemical composition of natural—as contrasted with agricultural—soils, and until more is known of the physiological effects of lime, it is impossible to decide the vexed question of the relation of lime-loving and lime-shunning plants to the presence or absence of calcium carbonate in the soil. From such points of view as this, it is indeed true, as Warming has recently stated, " that ecology is only in its See also:

• INFANCY