DISTRIBUTION OF DISEASES. 3U DISTRIBUTION OF PLANTS. Diseases (Loiitloii, 11)00) ; THiiiiiig, Clinieal Jllusliatioiis of the More Important Diseases of lieiiijal and i'aleutta (London, 1S32-35) ; Morc- heiul, Researehes on the Diseases of India (Lon- tlon, ISoti). Thf older liti'iatuie of medical geography includes the e.cellenl works of Mnliry and Uouiict. Incidental illustrations of distriliu- tion may lie found under Dysk.ntkky ; K.nue.mic; GoiTKK: Lti'Uo.sY: M.l.ki.v: ri.iV(ii'E; and iu the articles on fivcr~. DISTRIBUTION OF PLANTS, Puytoceog- RAi'iiY, Plant (.ii;oc,i:Aniv, or (iEociU.viMiKAi. UorANY'. That division of Idological geography that deals with the distribution and with the causes of distribution of plants. The subject has been studied since the time of Linnaeus, but its modern perioil dates from the works of Al- phonsc Ue Candolle and of tirisebach, about the middle of the nineteenth century. More recently, the works of Warming. Drude, Engler, and Sehimpcr have added nmch to our knowledge of the sul>ject. Phytogeography may he divided into two main heads: First, ecological phytogeog- raphy, which seeks to accoinit for the distribu- tion of plant-forms (e. g. the forms peculiar to water-plants, to desert-plants, etc.) and the catises, mainly climatic and meteorological, of such distribution; .second. Horistic phytogeog- raphy, which has for its aim the subdivision of the world into lloral regions and districts, and the study of the distribution of jiliint-specics in these regions, thus dealing with the species as units, and seeking to determine the geological and similar causes for plant distribution. ECOLOGICAL PUYTOGEOGRAI'IIY". Schimper subdivides the world into tropical, temperate, and frigid regions, alpine districts, and the seas. Each of these divisions, especially the first two, is in turn redivided into forma- tions. These formations, if determined by cli- matic factors, arc termed climatic formations, if by local or soil factors, edaphic formations. For example, deserts and tropical rainy forests are climatic formations, their existence being determined mainly by the distribution of atmos- pheric moisture ; on the other han<l. the llora of a swamp or of a heath, being governed mainly by local soil factors, is spoken of as an edaphic formation. Climatic Formations. In general, the cli- matic plant-formations of the world belong to one of three types; viz. forests, grass-lands, or deserts. This classification points to three great atmospheric factors — wind, moisture, tempera- ture. Wind has acted as the great distributing agent, carrying seeds and spores from place to place. Moisture is not only necessary to the germination of seeds and the growth of ))lants, but its abundance or scarcity has a profound iiillucnce upon plant-growth: the moist belt bor- dering the equator is as remarkable for the luxu- riance of its vegetation as the arid districts — the great deserts of the worhl — under the Trojiic of Cancer, are noted for their barrenness; bar- renness proved, by o.ise.s at springs and by the results of irrigation, to be due mainly, if not solely, to the ;ibsi'ii(e of w:itcr. The range of ti'mperature in which plants will grow and re- produce varies widely with difTeront species; some arc capable of withstanding very low tem- pernture. others very high, and in many instances experiment has proved that members of either group will fail under conditions favorable to members of tlic other. A fourth factor, light, must be considered. Light is as essential to the growth of green plants as are moisture and heat, but the amount nece-sary for individual species varies greatly. .Some are able to thrive in even the dense shade of tropical forests; others are unable to exist except in direct siuilight. Again, the ell'ect of the amount of light is well illus- trated by the shortening of the period of growth required by cereals cultivated in the Canadian Northwest, due to the extra hours of sunshine during the growing season. Although, as yet, but little knowledge has l>een gained by experiment to aid in determining the e.act nature of Uio inlluence of these factors on the distribution of plants, there is no room for doubt that such inlluences do operate. Between the equator and the poles, between the base and the top of a mountain, between the moist coastal districts and the arid interiors of the continents, between the depths of a forest and the treeless plateau, there are gradual changes iu the vegetation that mark the striking parallelism that exists between the distribution of wind, moistiu-c, heat, and light, and the distribution of plants. KnAriiic FoRMATiox.s. .s already noted, the edaphic formations are determined by local causes, mainly those which ri'side in the soil. Much dispute has arisen among authors as to whether chemical or physical causes are upper- most in determining distributiim : dispute that obviously cannot be settled until knowledge based upon sullicient cx])erimental data, at present sadly lacking in most instances, shall be gained u])on this subject. It seems most in harmony with our luesent knowledge to hohl with Warm- ing that the water in the soil is the most ])otent factor in determining local distribution of forms. In passing from the centre of a pond to upland, plants may be encountered in more or less defi- nite succession whose structures bear close rela- tions to the water-supply. Forms, such as duck- weed, which have no root-anchorage, but drift with the wind on the surface of the water, have large air-containing chambers, which greatly in- crease the transpiring surface. A similar open structure is observed in water plants whose roots are anchored on the bottom and in those marsh plants growing on the margins, but as the ground becomes more and more dry the open structure which favors transpiration gradually disappears, and devices for checking tran- spiration take their plaie. Careful study has shown clearly that there are other elements, however, which must be considered. For ex- ample, the vegetation of an undrained swamp is radically difTerent from that of a similar moist situation on a river margin. Doubtless, certain conditions of drainage enter here to cause the dilTerence in plant forms. Again, fac- tors which are in a certain sense atmospheric often have a local influence. Plant forms found on the north side of a hill frequently difl'er from those on the south side because of possible dif- ferences in exposure to moisture and wind as W(dl as to difTerent exposure to the sun. .Ml of these factors may, .so to speak, be regarded as edaphic. since all are similar in being hwal rather than clinnitic. In general, the edaphic forma- tions may be subdivided into rnnstnl formations or societies, and inland formation/) or societies. The coastal societies mav be destnictivc, as illus-
