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ECOLOGY] plants which, like deciduous trees and bulbous plants, are hygrophytes during one season of the year and xerophytes during another season of the year. Such plants, which comprise the great majority of the species of the central European flora, Schimper termed tropophytes.

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 plankton, or microphytes that float free on water, of the pleuston, or macrophytes which float on or are suspended in water, and of the benthos, or all aquatic plants which are fixed to the substratum.

2. Helophytes.—These are marsh plants which normally have their roots in soaking soil but whose branches and foliage are more or less aerial. Warming admits there is no sharp limit between marsh plants and land plants; and it seems equally obvious that there is no sharp limit between some of his helophytes and some of his hydrophytes. For example, the difference between aquatic plants with floating leaves, such as the yellow water-lily (Nymphaea lutea) and those with erect leaves, such as Typha angustifolia, is probably more apparent than real. Among helophytes, Warming places plants of the reed swamp, and includes such trees as the alder (Alnus rotundifolia), willows (e.g., Salix alba, S. fragilis, S. cinerea, S. pentandra), birch, and pine, when these grow in marshy places.

B. The soil is physiologically dry.

3. 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 cold soils of subniveal and polar districts.

5. Halophytes.—These are plants which grow on saline soils.

C. The soil is physically dry.

6. Lithophytes.—These are plants which grow on “true rock,” but not “on the loose soil covering rock, even though this may entertain species that are very intimately associated with the rock. Still to this limitation an exception must be made in favour of the vegetation growing in clefts and niches” (Warming, 1909: 240). Many Algae, lichens, and mosses are included among lithophytes and also Saxifraga Aizoon, S. oppositifolia, Silene acaulis, and Gnaphalium luteo-album.

7. Psammophytes.—These are plants which grow on sand and gravel. Plants of sand-dunes, whether in maritime or inland localities, are psammophytes, as well as plants (such as Calluna vulgaris) of dune heaths, dune “bushland” or scrub, and dune forest.

8. Chersophytes.—Here are placed certain “xerophytic perennial herbs” which occur on “particular dry kinds of soil, such as limestone rocks, stiff clay, and so forth” (Warming, 1909: 289).

D. The climate is very dry, and the properties of the soil are decided by climate.

9. Eremophytes.—Under this term, are placed plants of deserts and steppes.

10. Psilophytes.—Here are placed plants found in “savannah-vegetation,” viz. (i.) “thorny savannah-vegetation, including: (a) orchard-scrub, (b) thorn-bushland and thorn-forest; (ii.) true savannah: tropical and sub-tropical savannah; (iii.) savannah-forest, including bush-forest in Africa and ‘campos serrados’ in Brazil” (Warming, 1909: 293 et seq.).

11. Sclerophyllous formations, e.g., garigues, mäquis, and forests of evergreen oaks (Q. Ilex, Q. Ballota, Q. Suber), and of Eucalyptus spp.

E. The soil is physically or physiologically dry.

12. Coniferous forest formations, e.g., of Pinus sylvestris, Picea excelsa, Abies pectinata, Larix sibirica, L. decidua.

F. “Soil and climate favour the development of mesophilous formations.”

13. Mesophytes.—Warming defines mesophytes as “plants that show a preference for soil and air of moderate humidity, and avoid soil with standing water or containing a great abundance of salts” (1909: 317). Under mesophytes, Warming places plants occurring in “Arctic and Alpine mat-grassland and mat-herbage,” in “mat-vegetation of the Alps,” in meadows, in pasture on cultivated soil, in “mesophytic bushland,” in deciduous dicotyledonous forests, and in evergreen dicotyledonous forests.

This new system of Warming's, whilst probably too involved ever to come into general use, must be taken as superseding his older one; and perhaps the best course open to botanists is to select such terms as appear to be helpful, and to use the selected terms in a general kind of way and without demanding any precise definitions of them: it must also be borne in mind that the

various classes are neither mutually exclusive nor of equivalent rank. From this point of view, the following terms will perhaps be found the most serviceable:—

Hydrophytes (submerged aquatic plants).—Plants whose vegetative organs live wholly in water; e.g., most Algae, many mosses, such as Fontinalis spp., and liverworts, such as Jungermannia spp.; a few Pteridophytes, such as Pilularia spp., Isoëtes spp.; several flowering plants, such as Potamogeton pectinatus, Ceratophyllum spp., Hottonia palustris, Utricularia spp., Littorella lacustris.

Hemi-hydrophytes (swamp plants, marsh plants &c.).—Plants whose vegetative organs are partly submerged and partly aerial; Vaucheria terrestris, Philonotis fontana, Scapania undulata, Marsilia spp., Salvinia natans, Azolla spp., Equisetum limosum, Typha angustgolia, Phragmites communis, Scirpus lacustris, Nymphaea lutea, Oenanthe fistulosa, Bidens cernua.

Hygrophytes.—Plants which are sub-evergreen or evergreen but not sclerophyllous, and which live in moist soils; e.g., Lastraea Felix-mas, Poa pratensis, Carex ovalis, Plantago lanceolata, and Achillaea Millefolium.

Xerophytes.—Plants which grow in very dry soils; e.g., most lichens, Ammophila (Psamma) arenaria, Elymus arenarius, Anabasis aretioides, Zilla macroptera, Sedum acre, Bupleurum spinosum, Artemisia herba-alba, Zollikofferia arborescens.

Halophytes.—Plants which grow in very saline soils; e.g., Triglochin maritimum, Salicornia spp., Zygophyllum cornutum, Aster Tripolium, Artemisia maritima. It should be recognized, however, that “a halophyte, in fact, is one form of xerophyte” (Warming, 1909: 219).

Sclerophyllous Plants.—These are plants with evergreen leathery leaves, an typical of tropical, sub-tropical, and warm temperate regions; e.g., Quercus Suber, Ilex Aquifolium, Hedera Helix, Eucalyptus Globulus, Rosmarinus officinalis. Sclerophyllous leaves are usually characterized by entire or sub-entire margins, a thick cuticle, small but rarely sunken stomata, a well-developed and close-set palisade tissue and a feeble system of air-spaces.

Hydro-xerophytes (“bog xerophytes”).—Plants which live in wet, peaty soils, an which possess aeration channels and xerophilous leaves; e.g., Cladium Mariscus, Eriophorum angustifolium, Rubus Chamaemorus, and Vaccinium Vitis-Idaea. The term “oxylophyte” is open to the objection that some peaty waters are alkaline, and not acidic as the term implies. Many plants of peaty soils are sclerophyllous.

Tropophytes.—Plants which are hygrophytes during some favourable part of the year and xerophytes during the rest of the year; e.g., deciduous trees and shrubs, deciduous herbaceous plants with underground perennating organs, and annuals and ephemerals.

Plant Communities.—The study of plant communities (Formationslehre or synecology) has made much progress in recent years. Even here, however, general agreement has not been reached; and the questions involved in relating the facts of the distribution of plant communities to the factors of the habitat are very imperfectly understood. Plant communities may be classified as follows:—

A plant association is a community of definite floristic composition: it may be characterized by a single dominant species; or, on the other hand, it may be characterized by a number of prominent species, one of which is abundant here, another there, whilst elsewhere two or more species may share dominance. The former are pure associations, and are well illustrated by a heather moor, whereCalluna vulgaris is the dominant plant. The latter are mixed associations, such as fens, where different facies are produced by the varying abundance of characteristic plants, such as Cladium Mariscus, Phragmites communis, Molinia coerulea, Calamagrostis lanceolata, and Juncus obtusiflorus. The different facies are possibly related to slight differences in a generally uniform habitat: it is unscientific to regard them as due to chance; still, in the majority of cases, the causes of the different facies have not been demonstrated. A local aggregation of a species other than the dominant one in an association brings about a plant society; for example, societies of Erica Tetralix, of Scirpus caespitosus, of Molinia coerulea, of Carex curta, of Narthecium ossifragum, and others may occur within an association of Calluna vulgaris. The plant societies are also doubtless due to slight variations of the habitat.

The plant association is sometimes referred to in technical language; the termination -etum is added to the stem of the generic name, and the specific name is put in the genitive. Thus