Page:The New International Encyclopædia 1st ed. v. 06.djvu/371

DISTRIBUTION OF PLANTS. 317 DISTKIBUTIVE LAW. is the pea family (Leguminosa), represented by abuut 7000 species. Althougli it seems to be abuniiauUy displayed iu the temperate regions, it is far more e..teusively represented in the tropies, being the greatest tropical family of the group. The lianas or climbers are ehielly repre- sented among Archiehlamyde;e by such forms as the grape-vines and their allies. Naturally, they are ehielly displayed in tropical forests and are most largely developed in the moist forests of South America, as are the epiphytic orchids and bromelias among the monocotyledons. Their next largest display is in Africa. SY>iPET.vL.t. The Sympetala; are the ranking plants of the plant kingdom and are also those most recently evolved. While they include a number of shrul)S and trees in the tropics, they are by no means so extensively represented in temperate regions by shrubs and trees as are the Archichlamydeae. The prominent facts of their distribution are as follows: the earlier groups of angiosperms (Archichlamydeie and mono- cotyledons) were massed in the tropics, the tem- perate and boreal regions being relatively poor in true flowering plants. The SympetaliB, there- fore, while largely represented in the tropics, found the temperate and boreal regions compara- tively free for occupation, and it is in these regions that they have become peculiarly domi- nant. This does not mean that there are not more Sympetahe in the tropics than in the tem- perate regions, for the proportion of species in favor of the former is about three to two; but it does mean that this is about the normal ratio of tropical increase, which is far exceeded by the other groups. Accordingly, of the Sympe- talse alone can it be said that their distribution throughout tropical and temperate regions is about uniform. The families of Svmpetala; are not only better defined structurally than those of the .Vrchichlaniydeae, but their geographical dis- tribution is much more definite. This follows the fact that they are more highly specialized and more definitely related to environment than are the Archichlamydea-. Among the Sv'mpe- talse, three families, the plantains, mints, and composites, have a world-wide distribution. The Compositae, as the ranking family of the plant kingdom, and also the youngest and largest, de- serves special mention. It includes at least 12,- 000 species, which, although found everywhere, are most numerous in the temperate regions, where they constitute from one-tenth to one- seventh of all of the seed-bearing plants of every flora. They are especially characteristic of tem- perate .merica, where asters, goldenrods, sun- flowers, etc., abound. Two great north temperate families, the fig^vorts and the borages, have also been developed; and the heaths form a family characteristic of boreal conditions. The impress a peculiar region may have upon the general aspect of its' vegetation, resulting in the a.ssumption of the same general appear- ance by plants of the most diverse relationships, is illustrated by the fact that many sympetalous families of South America are characterized by a heath-like habit, the region being known as one of the heath-like plants. There is no such notable continental pairing as is shown by the .Archichlamydejp and the monocotyledons, due probably to the fact that extensive forests and boreal distribution permit free intermingling of continental flora. .Among tropical Sj-mpetalie, however, the pairing is somewhat evident, and iu tropical display America is distinctly domi- nant. It is also worthy of note that there is a remarkable paucity of aijuatic plants among Symi)etala;, as compared with Archichlaraj'desB and with monocotyledons. This seems to be re- lated to the fact that the two latter groups hail already become well established in fresh and brackish waters, while it was the temperate and boreal land surface that was especially free for occupation by the Sympetahe. t'o.NCLUSiox. Through the lapse of time since the advent of plant life upon tlie earth, the meteorological factors, wind, moisture, heat, and light, have been at work as plant distributors. At divers times and in many parts of the world their work has been interrupted, assisted, or ren- dered nugatory by geological agents — glaciers, especially of the ice age, which have driven vege- tation before them and exiled many plants from their pristine homes; .seismic upheavals or sink- ings, the former interposing tall mountain ranges, the latter wide oceans, beyond which certain species could not migrate, and which to a greater or less degree isolated these species, sometimes genera, families, and even tribes. The efforts of these factors and agents have been and are assisted by man, by other animals, and by plants; the first either wittingly carrying use- ful or ornamental species from country to coun- try, or more often unwittingly introducing use- less or even noxious ones ; the second in a more limited range transporting seed either in or on their bodies; the third breaking down and dis- integrating rock, becoming incorporated with the detritus and thus aflFording a habitation to a succeeding plant society. Far from having ceased their labors, these meteorological factors and geological agents, working in conjunction or in conflict, still profoundly influence the ceaseless change in the distribution of plants. BiBLiOGB-A Pii T. Conteiean: Gvofirapliie iota- nique (Paris, 1881); Drude, Udiidhiich der Pflanzengeographie (Stuttgart, 1890) : id., Man- uel de geographie hotanique (Paris, 189") ; De Candolle, Geographie botanique rainonnee (Paris, 1855) ; Engler, Versuch einer Enttcick- lutigsgeschichte der extrafropischen Floren- gebiele (Leipzig, 1879) ; Engler and Drude. Die Vegetation der Erde (first 4 vols., Leipzig. 1806- 1901) ; Grisebach, Yegetation der Erde (Leipzig, 1884); Sehimper. Pflanzengeographie' aiif physi- ologischer Grundhige (.Jena, 1898) : Wallace, Island Life (New York, 1895) ; Warming. Ocko- logische Pflanzengeographie (Berlin. 1890). The ecological aspects of distribution are touched upon under the following heads: Ecology: Ai,- nxE Plants: Akcttc Plants; Benthos ; Clay Plants ,• Cliff Plants ; Cosmopolites : Desert : Dune Vegetation: Dur.tion; Enuemism; Forest; Grasslands: IL^lophytes: Heath; Hydrophytes; Meadow; Mesophytes: Praiiue: Rock Plants; Pvderal Plants: Savannas; Sva.mp; Thicket; Tundra,- Xehophytes. De- tails of floristic phytogeography may be found in the articles on various genera and. species of plants mentioned above. DISTMBTITIVE LAW. In its elementary form tliis mathematical law may be expressed thus: To multiply any compound expression by a simple expression is to nuiltiplv each term of the compound expression by the multiplier and