GEOGRAPHIC DISTRIBUTION OF ANGIOSPERMS 265 



Upon examining such data, certain generalizations in refer- 

 ence to the distribution of Monocotyledons become apparent. 

 These will doubtless be modified by a fuller knowledge of the 

 distribution of families, but they will serve to illustrate certain 

 facts : 



1. Four great terrestrial families (Gramineae, Cyperaceae, 

 Liliaceae, and Iridaceae) of Monocotyledons are world-wide 

 in their distribution. This means that they have been able to 

 become adapted to every condition of soil and climate possible 

 to high-grade vegetation. 



2. The Monocotyledons include a remarkable number of 

 purely hydrophytic families which also have a world-wide dis- 

 tribution so far as fresh and brackish waters are concerned. 

 The families are Typhaceae, Potarnogetonaceae, Xaiadaceae, 

 Hydrocharitaceae, Lemnaceae, and Pontederiaceae, four of 

 them belonging to the Helobiales. In spite of this wide dis- 

 tribution, these families contain less than 200 species. When 

 this fact is taken in connection with the 10,000 species belong- 

 ing to the four cosmopolitan terrestrial families mentioned 

 above, it becomes evident that the very diverse conditions of the 

 land surface are far more favorable to the production of species 

 than the comparatively uniform aquatic conditions. 



3. There is a decided massing of monocotyledonous families 

 in the tropics. This is so marked as to suggest that Monocotyle- 

 dons as a whole are essentially tropical. 



4. As a corollary to the last statement, the entire absence 

 of boreal forms, excepting the few belonging to the families 

 of universal distribution, is noteworthy. 



5. The poor representation of Monocotyledons in the 

 southern hemisphere, exclusive of the world-wide families, 

 is remarkable. Especially is this true of Australia, a region 

 prolific in endemic forms among Gymnosperms and Dicotyle- 

 dons. 



6. Very few families are characteristic of temperate re- 

 gions, and these (Sparganiaceae, Juncaginaceae, Alismaceae, 

 and Juncaceae) are represented in both the northern and 

 southern hemispheres, and none of them are of the higher 

 petaloideous type. 



7. The tropical representation of Monocotyledons is ap- 

 proximately equal in the two hemispheres, not merely in num- 



