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ECOLOGY] with secondary results on organs or parts of organs, may be brought together under this heading. No sharp line can be drawn between these diseases and some of the preceding, inasmuch as it often depends on the external conditions whether necrosis is a dry-rot, in the sense I employ the term here, or a wet-rot, when it would come under the preceding category. The “dying back” of the twigs of trees and shrubs is a frequent case. The cortical tissues gradually shrink and dry up, turning brown and black in patches or all over, and when at length the cambium and medullar ray tissues dry up the whole twig dies off. This may be due to frost, especially in “thin-barked” trees, and often occurs in beeches, pears, &c., or it may result from bruising by wind, hailstones, gun-shot wounds in coverts, &c., the latter of course very local. It is the common result of fires passing along too rapidly to burn the trees; and “thin-barked” trees—hornbeam, beech, firs, &c.—may exhibit it as the results of sunburn, especially when exposed to the south-west after the removal of shelter. The effects of frost and of sunburn are frequently quite local. The usual necrosis of the injured cortex occurs—drying up, shrivelling, and consequent stretching and cracking of the dead cortex on the wood beneath. Such frost-cracks, sun-cracks, &c., may then be slowly healed over by callus, but if the conditions for necrosis recur the crack may be again opened, or if Fungi, &c., interfere with occlusion, the healing is prevented, in such cases the local necrosis may give rise to cankers. The dying back of twigs may be brought about by many causes. General attacks of leaf-diseases invariably lead to starvation and necrosis of twigs, and similarly with the ravages of caterpillars and other insects. Drought and consequent defoliation result in the same, and these considerations help us to understand how old-established trees in parks, &c., apparently in good general health, become “stag-headed” by the necrosis of their upper twigs and smaller branches: the roots have here penetrated into subsoil or other unsuitable medium, or some drainage scheme has deprived them of water, &c., and a dry summer just turns the scale. Such phenomena are not uncommon in towns, where trees with their roots under pavement or other impervious covering do well for a time, but suddenly fail to supply the crown sufficiently with water during some hot summer.

7. Monstrosities.—A large class of cases of departure from the normal form, depending on different and often obscure causes, may be grouped together under this heading, most of them are of the kind termed Teratological, and it is difficult to decide how far they should be regarded as pathological if we insist that a disease threatens the existence of the plant, since many of these malformations—e.g. double flowers, phyllody of floral parts, contortions and fascinations, dwarfing, malformed leaves, &c.—can not only be transmitted in cultivation, but occur in nature without evident injury to the variety. In many cases, however, monstrosities of flowers have been shown to be due to the irritating action of minute insects or Fungi, and others are known which, although induced by causes unknown to us, and regarded as internal, would not be likely to survive in the wild condition. This subject brings the domain of pathology, however, into touch with that of variation, and we are profoundly ignorant as to the complex of external conditions which would decide in any given case how far a variation in form would be prejudicial or otherwise to the continued existence of a species. Under the head of malformations we place cases of atrophy of parts or general dwarfing, due to starvation, the attacks of Fungi or minute insects, the presence of unsuitable food-materials and so on, as well as cases of transformation of stamens into petals, carpels into leaves, and so forth. Roots are often flattened, twisted and otherwise distorted by mechanical obstacles, stems by excess of food in rich soils, the attacks of minute parasites, overgrowth by climbing plants, &c. Leaves are especially apt to vary, and although the formation of crests, pitchers, puckers, &c., must be put down to the results of abnormal development, it is often difficult to draw the line between teratological and merely varietal phenomena. For instance, the difference between the long-stalked and finely-cut leaves of Anemone attacked with rust and the normal leaves with broad segments, or between the urceolate leaves occasionally found on cabbages and the ordinary form—in these cases undoubtedly pathological and teratological respectively—is nothing like so great as between the upper and lower normal leaves of many Umbelliferae or the submerged and floating leaves of an aquatic Ranunculus or Cabomba. When we come to phenomena such as proliferations, vivipary, the development of “Lammas shoots,” adventitious buds, epicormic branches, and to those malformations of flowers known as peloria, phyllody, virescence, &c. while assured that definite, and in man cases recognizable, physiological disturbances are at work, we find yourselves on the borderland between pathological and physiological variation, where each case must be examined with due regard to all the circumstances, and no generalization seems possible beyond what has been sketched. This is equally true of the phenomena of apogamy and apospory in the light of recent researches into the effects of external conditions on reproduction.

This sketch of an enormous subject shows us that the pathology of plants is a special department of the study of variations which threaten injury to the plant, and passes imperceptibly into the

study of variations in general. Moreover, we have good reasons for inferring that different constellations of external causes may determine whether the internal physiological disturbances induced by a given agent shall lead to pathological and dangerous variations, or to changes which may be harmless or even advantageous to the plant concerned.

.—General and Historical.—Berkeley, “Vegetable Pathology,” Gardener's Chronicle (1854) p. 4; Plowright, British Uredineae and Ustilagineae (1889); Eriksson and Henning, Die Getreideroste (Stockholm, 1896); De Bary, Comparative Morph. ''and Biol. of the Fungi, &c. (1887); Frank, Die Krankheiten der'' Pflanzen (1895-1896); Sorauer, Handbuch der Pflanzenkrankheiten (1906); Ward, Disease in Plants (1901). Causes of Disease, &c.—Pfeffer, Physiology of Plants (Oxford, 1900); Sorauer, Treatise on the Physiology of Plants (1895); Bailey, The Principles of Agriculture (1898); Lafar, Technical Mycology (1898); Hartig, Diseases of Trees (1894); Marshall Ward, ''Proc. Roy. Soc.'' (1890) xlvii. 394; and Timber and some of its Diseases (London, 1889). Fungi.—See and ; also Marshall Ward, Diseases of Plants (Romance of Science Series), S.P.C.K.; Massee, Text-Book of Plant Diseases (1899); Tubeuf, Diseases of Plants (London, 1897). Insects.—Ormerod, Manual of Injurious Insects (1890); C. V. Riley, Insect Life, U.S. Department of Agriculture (1888-1895); Judeich and Mitsche, Lehrbuch der mitteleuropaischen Forstinsektenkunde (Vienna, 1889). Healing of Wounds, &c.—Shattock, “On the Reparatory Processes which occur in Vegetable Tissues,” ''Journ. Linn. Soc.'' (1882) xix. 1; Richards, “The Respiration of Wounded Plants,” ''Ann. of'' Bot. (1896), x. 531; and “The Evolution of Heat by Wounded Plants,” ''Ann. of Bot.'' (1897), xi. 29. Enzymes.—Green, The Soluble Ferments and Fermentation (1899). Chemotaxis, &c.—Miyoshi, “Die Durchbohrung von Membranen durch Pilzfaden,” Pringsh. Jahrb., B. (1895), xxviii. 269, and literature. Parasitism, &c.—Marshall Ward, “On some Relations between Host and Parasite, &c.,” ''Proc. Roy. Soc.'' xlvii. 393; and “Symbiosis,” ''Ann. of Bot.'' (1899), xiii. 549, with literature. Specialization of Parasitism—Salmon, in Massee's Text-Book of Fungi (1906), pp. 146-157. Statistics.—See Wyatt, Agricultural Ledger (Calcutta, 1895), p. 71; Balfour, The Agricultural Pests of India (1887), p. 13; Eriksson and Henning, Die Getreideroste; the publications of the U.S. Agricultural Department; the Kew Bulletin; Zeitschrift für Pflanzenkrankheiten, and elsewhere. Spraying, &c.—See Lodeman, The Spraying of Plants (1896), and numerous references in the publications of U.S. Agricultural Department, ''Zeitschr. f. Pflanzenkrankheiten'', the Gardener's Chronicle, &c. Etiolation, &c.—Pfeffer, Physiology of Plants, and other works on physiology. Albinism, &c.—Church, “A Chemical Study of Vegetable Albinism,” Journ. ''Chem. Soc. (1879, 1880 and 1886); Beijerinck, “Ueber ein Contagium,” &c., in Verhandl. d. kön. Acad. v. Wet. (Amsterdam, 1898); Koning in Zeitschr. f. Pflanzenkrankh.'' (1899), ix. 65; Baur, ''Ber. deutschen bot. Ges.'' (1904), xxii. 453; ''Sitzungsber. berlin. Akad.'' (Jan. 6, 1906); Hunger, ''Zeitschr. f. Pflanzenkrankheiten'' (1905) xv. Heft 5. Wounds, &c.—Marshall Ward, Timber and some of its Diseases, p. 210; Hartig, Diseases of Trees (London, 1894). Cecidia and Galls.—Küster, “Beiträge zur Kenntniss der Gallenanatomie,” Flora (1900), p. 117; Pathologische Pflanzenanatomie (1903); Molliard, Revue générale de bot. (1900), p. 157. Canker.—Frank, Krankheiten der Pflanzen, and papers in ''Zeitschr. f. Pflanzenkrankh.'' Rotting, &c.—Migula, ''Krit. Uebersicht derjenigen Pflanzenkrankheiten,'' welche angeblich durch Bakterien verursacht werden (1892); Smith, “Pseudomonas campestris,” ''Cent. f. Bakt.'' B. iii. 284 (1897); Arthur and Bolley, Bacteriosis of Carnations, Purdue Univ. Agr. Station (1896), vii. 17; A. F. Woods, “Stigmonose, a Disease of Carnations,” ''Vegetable, Physiol. and Pathol. Bull. 19 U.S. Department of Agriculture (1900); Sorauer, Handbuch der'' Pflanzenkrankheiten (1905), 18-93. Frost, Drought, &c.—Hartig, ''Lehrbuch der Anat. und Phys. der Pflanzen; Fischer, Forest Protection'', iv. of Schlich's Manual of Forestry. Teratology, &c.—Masters, Vegetable Teratology, Ray Society (1869); Molliard, “Cécidies florales,” ''Ann. Sci. Nat.'' sér. 8, i. (bot.) p. 67 (1895). (H. M. W.)

Introduction.—The word ecology is derived from, a house (habitat), and , a discourse. As a botanical term, ecology denotes that branch of botany which comprises the study of the relations of the individual plant, or the species, or the plant community with the habitat. Following Schröter (Flahault and Schröter, 1910: 24), the term autecology may be used for the study of the habitat conditions in relation to the single species, and the term synecology for this study in relation to plant communities.

From the phytogeographical standpoint, ecology is frequently termed ecological plant geography. Thus Warming (1901: 1 and 2)