GEOGRAPHIC DISTRIBUTION OF ANGIOSPERMS 263 



resulted not only in distinct genera, but almost every tribe is 

 either oriental or occidental. Furthermore, the much larger 

 number of monotypic genera in the orient must be associated 

 with its larger and more broken tropical area. 



Synanthaxes. — The Cyclanthaceae are as restricted to 

 the American tropics as the Pandanaceae are to the oriental 

 tropics. 



If this general " palm " type, comprising these three alli- 

 ance-., was once connected in the two hemispheres by a northern 

 distribution, the palms alone found both hemispheres congenial 

 in the tropics, while the Pandanaceae disappeared from the 

 western and the Cyclanthaceae from the eastern hemisphere. 



Helobiales. — This primitive series is very widely dis- 

 tributed and contains relatively few species, probably on 

 account of its aquatic character. Three of its families (Pota- 

 mogetonaceae, Naiadaceae, and Ilydroeharitaeeae) have a 

 world-wide distribution. The remaining five families are some- 

 what restricted as follows : Aponogetonaceae in the Indian 

 Ocean region, Triuridaceae in the tropics of both hemispheres, 

 Butomaceae extending from the tropics into temperate regions, 

 while Juncaginaceae and Alismaceae are mostly outside of the 

 tropics in the northern and southern hemispheres. 



Aeales. — The possible relationship of this group to the pre- 

 ceding one has been mentioned. The aquatic Lemnaceae are 

 universally distributed, but 92 per cent of the Araceae are 

 within the tropics, being massed chiefly in South America, 

 India, and the East Indies. This family, as the palms, affords 

 a good illustration of the development of continental diversi- 

 ties. In this case, however, the diversity has not reached so 

 extreme a stage as in the palms, in which even the tribes of 

 the orient and Occident are for the most part distinct. Among 

 Aroids the tribes of the two hemispheres are by no means dis- 

 tinct, at least two tropical genera (Cyrtosperma and Homalo- 

 mena) have species in both hemispheres, and the monotypic 

 Pistia is found in every tropical region. The species are more 

 numerous in the American tropics, but the number of genera 

 is nearly twice as great in the oriental tropics. The Aroids 

 differ further from the palms in having at least six genera 

 characteristic members of north temperate vegetation, and these 

 for the most part are common to both hemispheres. 

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