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480 for about an hour; in the other the soil is trenched in the usual way, but at the bottom of the trench is placed a grid made of iron piping perforated with holes through which steam is blown as soon as the soil has been replaced. The grid is then pulled out and placed in the next trench. The cost before the war was not less than 24 per acre, and in 1921 it varied according to the thoroughness of the steaming from about 90 to 300 per acre. In small nurseries or private glasshouses baking the soil is usually effective and is much cheaper, a coke oven being worked at very little cost. There is, however, a limit below which the cost can- not be brought, and in practice 12 tons or more of coke are needed to steam an acre of soil.

Attempts have therefore been made to find some chemical agent that will prove as effective as heat in dealing with undesir- able organisms.

The method of investigation is to take each organism and find the toxicity of various chemical groupings. An example is as follows:—

Proceeding in this way it is found that chlorcresol and dichlor- cresol are very effective, and they are being studied on a large scale. Some complications arise from the fact that soil bacteria have re- markable powers of decomposing many poisonous substances such as carbolic acid, cresol, naphthalene, etc., and in some cases the decomposition proceeds so rapidly that the substance disappears before it has had proper time to act. This difficulty is being met by the introduction of stabilizing groups.

See E. J. Russell and H. B. Hutchinson, " Partial Sterilisation of Soil," Jour. Ag. Sci. 1909, iii., 111-144, 1913, v., 152-221; E. J. Russell and F. R. Pethybridge, Jour. Ag. Sci. 1912, v., 86-111; Jour. Bd. Agric. 1912, xviii., 809-826, 1913, xix., 809-827 and 1914, xx., 102; E. J. Russell, Jour. Roy. Hort. Soc. 1920, xlv., 237; D. W. Cutler and L. M. Crump, Annals of Applied Biology (1920).

(E. J. R.)

VI. Ecology. In the domain of ecology the most important work since 1910 has been the intensive study of the habitat conditions in a number of limited areas. It is on such data alone that broad generalizations can be safely based, but much more needs to be accomplished in this direction before the significance of the results obtained can be rightly estimated. Of these intensive studies it is possible here to indicate only a few. Of the numerous types of plant communities that have been investi- gated, forests and woodlands have received a large share of attention and well illustrate the chief lines of progress.

Descriptive or primary survey work has elucidated interesting points respecting the courses of the altitudinal and polar tree limits. The meteorological conditions above and immediately below the timber line have been shown to exhibit an abrupt change associated with the cessation of shelter, but the gradual upward extension of the tree zone is limited by climatic conditions of which the duration of the snow-free period is an important factor. Both in America and Switzerland a rise of the timber line has been noted on the larger mountain masses and, as Brockmann-Jerosch has pointed out, the polar tree limit approaches the poles on the great con- tinental land masses whilst it recedes from them in the oceanic regions of high latitudes. In other words, a continental climate is favourable to tree growth whilst an oceanic climate is unfavourable except at low latitudes. Modifications of the altitudinal timber limit by soil and aspect have brought out the different demands and toleration of individual species. As a consequence of such changes the successive timber zones rise to a higher altitude on south ex- posures and on dry warm soils whilst on north aspects not only are the zones lower but they may also be more numerous. In Switzer- land, for example, successive zones of chestnut, beech and larch occur on slopes facing southwards whilst on northern slopes zones of silver fir and spruce become interpolated, the lower limit of the latter being apparently determined by the diminishing rainfall. The study of the biology and physiology of the constituent forest species has served to demonstrate the adaptational character of many of their salient features. The periodicity of the herbaceous vegetation

is, for example, intimately related to that of the shrubs and trees above, the assimilation of the more specialized members of the former being chiefly carried on before the canopy of the latter is complete.

Again, it has been shown that the optimum assimilation of such plants takes place in relatively weak illumination whilst their osmotic pressure, as in the plants of other habitats, is intimately related to the humidity of the environment. This has been shown to obtain in the case of some prairie species; even for the different parts of the same individual and in the different seasons of the year. Investigations of the soil conditions in relation to the plant covering have yielded promising results. Thus, the distribution of natural vegetation seems to be largely associated with changes in such fac- tors as acidity, water content, humus content and proportion of bases. The study of the first named has received a great impetus during the past few years, and in Sweden Hessellman has shown that the absence of natural regeneration in many forests is connected with high acidity and deficiency of nitrates.

Another aspect of vegetation is the change to be observed when the environment is altered or the original plant covering removed. The investigation of such succession phenomena has already yielded important economic results in relation to the improvement of pasture. The work of W. G. Smith in Scotland, of Dr. L. Cockayne in New Zealand and Prof.-J. W. Bews in South Africa has drawn attention to the possibilities of artificial control of the natural suc- cession. This principle is capable of wide-spread application wherever natural vegetation has an economic value; but it demands as a preliminary an intensive knowledge of the ecology of the individual species. It is as an outcome of such knowledge that Oliver has sug- gested the use of the plastic plant in place of groynes in fore-shore control.

On the philosophical side of the subject more has been written than the present state of our knowledge or its usefulness warrants. An extensive literature has developed on the classification of plant communities but most of these centre around one or other of four main view-points. The first emphasizes the importance of soil conditions as the basis of classification and is exemplified by the system proposed by Gola in 1910 based on his theory of osmotic edaphism. The second lays most stress on the physiognomy of the constituents of plant communities and with this are associated the names of Brockmann-Jerosch, Gams, Raunkiaer, Rubel and Warming. The third, associated with the American school, lays especial stress on succession, and Clements, who has done most to develop this view, classifies plant associations according to the clima- tic climax of which they represent phases of development. The fourth regards floristic composition as of paramount importance and has been upheld by Braun-Blanquet, Du Rietz, etc.

The first two and the last tend to result in systems that are too artificial, whilst that of Clements demands a knowledge that we often do not possess and tends to segregate phases which, though developing along divergent lines, are, regarded as plant communities, more closely related to one another than to the other phases of the same succession. Ecology is in much the same position as taxonomy in the early days when systems were frankly artificial because of the inadequacy of the knowledge to establish a natural system. Doubtless in time we shall find that, as with plant groups, different sets of characters must be used for different communities; but, in the meantime, these systems, however deficient, have served as an inspiration for valuable research which is yielding that knowledge on which the classifications of the future must be based.

—The chief literature prior to 1907 is cited by Flahault (Progressus Rei Botanicae, 1907), whilst for the literature subsequent to 1913 reference should be made to the Journal of Ecology (1913 et seq.) and Ecology (1918 et seq.), the respective organs of the British and American ecological societies. The following may be consulted either as illustrating particular aspects or as furnishing extensive bibliographies; J. W. Bews, The Grasses and Grasslands of S. Africa (1918); J. Braun-Blanquet, Les Cevennes meridionales (1915); H. Brockman-Jerosch, " Der Einfluss des Klimacharakters auf die Verbreitung der Pflanzen und Pflanzengesellschaften," Englers Jahrb. (1913); " Baumgrenze und Klimacharakter," ''Ber. d. Schweilz. Bot. Ges.'' (1919); F. E. Clements. Plant Succession (1916), L. Cockayne, papers dealing with New Zealand grassland. ''N. Z. Jour. Agric.'' (1919), H. Hessellman. papers on nitrate formation in soils, Ur Meddelanden Fran Staten Skogsforsokanstalt (1917), C. E. Moss, Vegetation of the Peak District (1913); E. J. Salisbury, " The Significance of Calcicoly," ''Jour. Ecology (1920) ; W. G. Smith, " The Improvement of Hill Pasture," Scottish Jour. Agric. (1918); A. G. Tansley, "The Classification of Vegetation," Jour. Ecology'' (1920); Types of British Vegetation (ed. by A. G. Tansley, 1911). (E. J. S.)

VII. Horticultural Exploration.—Botanical exploration in relation to horticulture centred during 1910-20, as in the preceding decade, in S.-E. Asia, particularly in western China. The gradual acquisition from all parts of the world of species new to cultivation has proceeded on a steady course, but the novelties from the East so far surpass in number, and in some