..)IONOCOTYIEDONS. The most gen eral facts in connection with the distribution of monocotyledons are as follows: Cosmopol t , groups.—Four great families (grass, lily. and iris). including almost 10.000 species or about one-half of the monocotyledons, are world wide in their distribution. This means that they have succeeded in adapting themselves to every condition of soil and climate possible to flower ing plants: and in this feature the grasses load not only monocotyledon:. but all other seed. hearing Beyond the natural massing toward the tropics. the distribution of these four families is fairly uniform. Aquatic plant R.—The monocotyledons include an unusual number of purely aquatic families, although among the other families the number of water-species is remarkable. A distinct water habit is associated with world-wide distribution, not merely of fami lies. but often of species. For example. the common mat-tail rush of the States is also found in Europe. Asia, and Afriva. But it must be noted that although these aquatic I. 'tithe- of ilono•otyledons are world wide in t4•411 distribution, their number of is r( mark:Oily small, being about Intl, 111 rontrast nig this with the 111,11110 -.peek, of tile t( rrtstrial families above referred to, it ident that the relatively uniform conditions of aquatic life do not result in the eNollition of ,pet It the extremely varied conditions of tern..trial life. 7 rupien I l'ht remark able massing of iininocotyledonous families in the tropics, tar ill excess of any normal ratio of tropical increase, makes it evident that the monocotyledons as a must be considered the leading tropical group of angiosperms. 'Phis tropical massing is indicated by the fact that, excluding, the ten families of world wide dis tribution, twenty-three out of the thirt:. -three remaining families are tropical. It is even more indionted by the faet that. excluding the 10.000 species that Were noted above as belonging to the four world-wide families, all but a few hundred of the remaining 141,000 are tropi cal. The approximately equal distribution of tropical forms between the two la test orthy. '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. semen or eight peculiar to the Peci (lent. and perhaps tell or eleven common to both. The palms may be taken as an illustration of the distribution of tropical monocotyledons. There are approximately it 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 aro about 131), approxi
mately, 75 of them being Oriental and 55 occi dental. the apparent illeglialify !wing aecounted for by the more numerous nionotypic genera in tl e 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 (opi nion to the two hemispheres. These tribes are groups of genera on the way to becoming fami lies, and by stone are regarded as such already. The palms thus notably illustrate the effect of long separation in plant adaptation to ronti nental diversities, temperate and arctic eon ditions long having cut nIl' the connecting land between the tropical regions of the t WO Lwuli sphere,. which are believed to have been united in time, and between which plant and animal forms could Migrate. The prepon derance of epiphytic forms in the occidental tropics is probably a.so •i:1led with the great deNelopinclit there of the rainy tropical forest. The two great epiphytic groups of flowering plant. a re bromelias and orchid•, both 14-41ons, the former family being restricted to tl e I lee jilt Ilt al tropics, :11141 the latter much more daindant there than in the Orient. The general .(1410. 1 it a of the monocotyledons to tropical conditions is emphasized by the e XI ilearth 4f mono(otyledonous 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 ledon- in the `southern hemisphere outside of the tropics is noteworthy. There are tots four belonging exclusively to this region; and in \u•tralia. a continent prolific of endemic ft rills among gyinzu spern-s and there is but a single endemic family of nicalocoty letIons, containing only lour species. A family of most peculiar distribution is the :stemonacele, comprising seuen or eight species, but sers tug to illustrate how a lamily once dis tributed may later occur only ill widely separated areas. (int. geniis. St(.441041(1, twetirs. front tile to southern .1ust•alia; another. rrou mia, in Florida, Georgia, and Japan; and the third, Ntie/4444 anon, in the East DIco•rvi.taioNs. There are approximately S0.4/011 species of dicotyledons. about equally di vided betWeell the two great groups Archie/I/a piy(b a. and .•1/444144 /Oa% These niust be consid ered separately. since they have developed in dep•ndently and are Tilte distinct in the general features of the geographical distribution.