DISTRIBUTION OF PLANTS. 315 DISTRIBUTION OF PLANTS. trated by the erosion of clilVs where they break down the rocks; or they may be eoustruetive, as illustrated by swamps and beaches, where they act in building up the land. Inland societies may be diviiled into those associated with river activities and those that are not. In the life of a region all areas will ultimately be worked over by rivers, so that the stages which are not associated with rivers may be regarded as more or less temporal. The ultimate condition of a region is that of the base-level (the Hat lowland resulting from completed erosion) toward which the other conditions are approaching, and which in a favorable climate will be covered in most instances by a mesophytic forest. I^LOKISTIC PIIYTOGEOGBAPHY. In a general presentation of the distribution of plants, the algie, fungi, liverworts, and mosses may be dismissed with a few statements. These groups are found everywhere. It should be noted, however, that the brown alg* (kelps) reach their greatest development in the colder waters ; and that the licliens and mosses are more abundantly displayed in temperate and arctic than in tro])i- cal regions. PTERiDOPiiyxES. In number the ferns so far exceed all other pteridopliytes that they may be taken as representing the group. Their greatest display is in the tropics, where they often assume the tree form and develop extraor- dinarily large fronds. In temperate regions they are neither abundant nor conspicuous: while in arctic and alpine conditions they are hardly at all represented. It will be noted that the distri- bution of ferns is almost in direct contrast to that of mosses. GYMXOSPER5IS. In presenting the distribution of gj-mnosperms. the three principal living groups must l)e considered separately. The cycads are strictly tropical forms, being distributed about equally between the Oriental and Occidental tropics, the genera for the most part being defi- nitely restricted to certain regions. For example, while the genus Cycas ranges throughout the Oriental tropics, and Zamia throughout the Occidental tropics, one genus is strictly .ustral- asian, two are African, two Mexican, and one is Cuban. The conifers form the largest group of gymnospenns. and their distribution contrasts sharply with that of the cycads, being entirely absent from the tropics and massed in the temperate regions, especially of the Xorthern Hemisphere. The broad tropical belt separating the conifers of the north and the south temperate regions is traversed in only two places, namely by a southern genus. Podocarpus. that reaches China and .Japan through the East Indies, and by a northern genus. Liboeedrus, that reaches into temperate South America by way of the Andes. By far the greatest conifer display, with respect to number of genera and of species, is found in the districts that border the Pacific Ocean, the chief areas being the China-Japan region, the . stralasian region, and western North America. The most remarkable displays of endemic genera are in the China-.Tapan and the .uitralasian regions, the former lontaining eight such genera, and the latter five. The other regions of endemic genera are North America, with its redwood ( Sequoia ) . ,and bald cypress (Taxodium), and Soiith America, with a peculiar genus in the mountains of Patagonia. Through- VOL. VI.— 21. out the north temperate regions the dominant and widely distributed genera are the pine (Pinus), juniper (Juniperus), fir (Abies), spruce (I'icea), cypress (Cupressus) , and larch (Larix) ; the order of citation indicating their relative abundance. There is also a renuirkable pairing of western North Amerjca and eastern Asia in the display of certain genera, no less than six genera being common to these two regions and occurring nowhere else. The distribu- tion of the conifers of the Southern Hemisphere is inodilied by the temperate conditions that occur in three great isolated areas. The dominant genus, Podocarpus, the 'pine' of the Southern Hemisphere, is the only one represented in all of these regions: but in the display of certain other genera there is a pairing of the continents, the Australasian region always being one member of the pair, and, with one exception. South America the other member. In conifers, therefore, there is much more in common be- tween Australia and South America than be- tween either of them and Africa. The Gnetales, constituting the third prominent group of gym- r.osperms, embrace three genera of very distinct characters and distribution. Ephedra occurs under l)ot!i tropical and lem])erate conditions in the arid regions of iledilerrancan Europe and adjacent Asia, and in arid parts of America: Gnetum ranges through the moist tropics of both hemispheres: while the nionotypic Tumboa I Wel- witschia) is narrowly restricted to certain ex- tremely arid regions of Western Africa. AxGiosPERMs. The immense number of angio- sperms. or true flowering plants, makes their distribution a difficult subject to present in a few statements. It must be understood, there- fore, that the following presentation is very gen- eral, considering large masses of plants rather than species or even genera. It must be remem- bered, also, that in general the conditions of plant life are most favorable in the tropics, where there is apt to be massing; and that under Arc- tic conditions, which are very unfavorable, the vegetation is apt to be scant. The three promi- nent grou|)s of angiosperms, viz. the monocotyle- dons and the two divisions of dicotyledons (Ar- ehichlamydea- and Sympetahie) must be consid- ered separately. JIoxocoTYiEDOXS. The most conspicuous gen- eral facts in connection with the distribution of monocotyledons are as follows: Cosmopolitnn groups. — Four great families (grass, sedge, 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 easily lead not only monocotyledons, but all other seed- bearing plants. Beyond the natural massing toward the tropics, the distribution of these four families is fairly uniform. Aijunlir plnxts. — The monocotyledons include an uiuisual 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 cat-tail rush (Ti/phn) of the United States is also found in Europe. .sia. and .frica. But it must be noted that although these aquatic
