PTERIDOPIMES. In number the ferns SO far exceed all other pteridophytes 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.
GYMNoSPERMS. In presenting the distribution of gymnosperms, the three principal living groups must he 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 Cyea: ranges throughout the Oriental tropics, and Zamia throughout the Occidental tropics, one genus is strictly Austral asian, two are African, two and one is Cuban. The conifers form the largest group of gymnosperms. 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 Northern 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 and by a northern genus, Lihocedrus, 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 Oeean, the chief areas being the China-Japan region. the Australasian region. and western North America. The most remarkable displays of endemic genera are in the China-Japan and the Australasian regions., the former containing
eight such genera. and the latter five. The other regions of endemic genera are North Ameriea, with its redwood ( Sequoia bald cypress (Taxodium), and South America, with a peculiar genus in the mountain: of l'atagonia. Through out the north temperate regions the dominant and NN idely distributed genera are the pine (Films). juniper (.1uniperus), lir (Abies), spruce i Picea I, cypress ICupressus). and larch (1.arix) ; time order of citation indicating their relative abundance. There is also a remarkable pairing of western North Americo and en .tern Asia in the display of certain genera, no less than six genera being common to these two regions and occurring nowhere else. The distrom tion of the conifers of the Southern Ilemisphei is modified by the temperate conditions that occur in three great isolated areas. The dominant genus, Podoca•pus, the Tine' 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 continent-, the Australasian region always being one member of the pair. and, with 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 Gnetale:, constituting the third prominent group of gym nosperms, embrace three genera of very distinet characters and distribution. Ephedra occurs under both tropical and temperate conditions in the arid regions of Europe and adjacent Asia, and in arid parts of America: Gnetum ranges through the moist tropics of both hemispheres; while the monotypie Timilmon ( witsehia) is narrowly restricted to certain ex arid regions of Western Africa.