DISTRIBUTION OF PLANTS. 316 DISTRIBUTION OF PLANTS. i.iniilics vi iiiuiKnutyledons are wurldwido in tlieir distribution, their number of species is remarkably small, beiiiy about 160. In eontrast- iu}; this with the 10,0(10 sjafies of the four terrestrial families above referred to, it beeomes evident that the relatively uniform eondilions of aquatic life do not result in the evolution of ffieeies as do the extremely varied eomlitions of terrestrial life. Tropieal Plants. — The remark- able massing of monocotyledonous families in the tropics, far in excess of any normal ratio of tropical increase, makes it evident that the monocotyledons as u whole must be considered the leading tropical group of angiosperms. This tropical massing is indicated by the fact that, excluding the ten families of world-wide dis- tribution, twenty-three out of the thirty-tliree remaining families are tropical. It is even more strongly indicated by the fact that, excluding the 10.000 species that were noted above as belonging to the four worldwide families, all but a few hundred of the remaining 10.000 arc tropi- cal. The approximately eipial distribution of tropical forms between the two hemispheres is noteworthy. This is true not merely in the number of families, but in genera and species as well. There are four or live families peculiar to the Orient, seven or eight peculiar to the Occi- dent, and perhaps ten or eleven common to both. The palms may be taken as an illustration of the distribution of tropical monocotyledons. There are approximately a thousand species of palms, almost exactly divided between the two hemi- spheres, but not a single native species is common to the two. The genera are about l;iO. approxi- mately, 75 of them being Oriental and .5.5 Occi- dental, the apparent inequality being accounted for by the more numerous monotypic genera in the Orient, due to the larger and more broken tropical area, but the two hemispheres have not a genus in common. The family is divided into great tribes, but not one of these tribes is com- mon to the two hemispheres. These tribes are groups of genera on the way to becoming fami- lies, and by some are regarded as stich already. The palms thus notably illustrate the ellect of long separation in plant adaptation to conti- nental diversities, temperate and arctic con- ditions long having cut off the connecting land between the tropieal regions of the two hemi- spheres, which are believed to have been united in prehistoric times and between which plant and aninul forms could migrate. The prepon- derance of epiphytic forms in the Occidental tropics is probably associated with the great development there of the rainy tropical forest. The two great epiphytic groups of flowering plants are bromelias and orchids, both monocoty- ledons, the former family being restricted to the Occidental tropics, and the latter much more abundant there than in the Orient. The general adaptation of the monocotyledons to tropical conditions is empha.sized by the extreme dearth of monocotyledonous species in arctic and alpine regions, only the few species belonging to the four families of universal distribution being present. The feeble representation of monocoty- ledons in the Southern Hemisphere outside of the tropics is noteworthy. There are only four small families belonging exclusively to this region: and in .ustralia. a continent prolific of endemic forms among gymnosperms and dicotyledons. there is but a >inglc endemic fauiily ul monocoty- ledons, containing only lour sjiecics. .V family of most peculiar distribution is the Stemonacea-, comjirising only seven or eight species, but serv- ing to illustrate how a family once widely dis- tributed may later occur only in widely separated areas. One genus, .S/cmo/iu, occurs from the Himalayas to southern Australia; another, Vroo- mill, in Florida, Georgia, and Japan; and the third, t<ticli>uiiiuii, in the Kast Indies. UicoTYLKDo.xs. There are approximately 80.000 species of dicotyledons, about e(|ually di- vided between the two great groups Anhiclila- iiii/ihw and f'liiiiprtiilir. These must be consid- ercil separately, since they have developed in- dependently and are quite distinct in the general features of the geographical distribution. AKcnicni..MYi>E.K. The most conspicuous facts in connection with the distribution of the Archichlamyde;e are as follows: In the repre- .senliition of tropical families, there is a notable pairing of continents, the American tropics usually being one mi'mlur of the pair, and Asia or Africa the other. For example, the gieat alliance to which buttercups, mustards, poppies, laurels, etc., belong is represented by tropical forms chiefly in America and .sia, the omission of Africa being notable: the alliance to which geraniums, balsams, flaxes, rues, etc.. belong has its tropieal forms ehielly in America and Africa; while the melastonias are massed in Itrazil. and the myrtles ;ire common to South America and Australia. Thus tlie predominance of .Vmlpriea appears in the display of tropical Archichla- n.ydeir. This becomes all the more clear from the statement that in .merica almost all the tropical and subtropical families are representetl, and two very large families, the cactus and mela- stoma families, are found as natives exclusively in .Vmerica. The Archichlamydca> are also peculiar in not containing any terrestrial family of world- wide distribution, as do the monocotyledons and Sympetabr: nor is there any distinctly boreal family as among the Sympetabe. There have been developed among them certain very charac- teristic north temperate families, the .smart- weeds, pinks, crowfoots, mustards, saxifrages, roses, evening primroses, and undiellifers. all of which, however, have their representatives in tropical and boreal regions; but the dis])lay of these forms is not to be compared with the mass- ing of the Sympetabe in the temperate regions. While there is a much larger disjilay of Archi- chlamyde;e in the north temperate than in the south temperate regions, two very large and characteristic families have been developed in the Southern Hemisphere, viz. the Proteaceip, trees that belong to the same general alliance n» do the oaks, elms, hickories, etc.. of the Northern Hemisphere; and the Tbymcla'acea'. character- istic of Australia and the Cape region of .frica, and represented in our . icrii;in flora by the leatherwood (Dirro). The .rchichlnniydea" con- tain nu>st of the hard-wond trees, and it is inter- esting to note that the trees of the tropics and the temperate zones belong to different alliances. I'or example, the oaks, hickories, elms. iM'Oches, birches, etc.. belong to a great alliance charac- teristic of north temperate regions, while the laurel, belonging to an entirely different alliance, is the cbarncterislie forest type of the tropics. B.v far the erreatest fnniil.v of the .-Xrehiehln- mydeie. in point of numbers and general success.
