712 



SCIENCE. 



[N. S. Vol. XII. No. 306. 



Lower Sonoran Zone ; (c) semi-tropical 

 xerophilous forms of G-ulf Zone distribution. 



. THE GULF ZONE NEO-TKOPICAL ELEMENT. 



The tei'ritory embraced within the Gulf 

 Zone includes those regions which have 

 had a common history in the development 

 of their flora during the fluctuating geolog- 

 ical conditions of the Gulf area. While 

 this zone is but a part of the greater ISTeo- 

 tropical, its association with a common 

 sequence of geological changes has, as Eng- 

 ler* thinks, given it a degree of distinctness 

 from the Brazilian region. The regions so 

 associated are : The coast lands, plains and 

 sub- Andean parts of Gaiana, Colombia and 

 Venezuela ; the Central American region 

 except the tierra templada, the tierra frias 

 of Guatemala and the isolated elevations 

 (above 8,000 feet) in Nicaragua and Costa 

 Rica; the tierra caliente of Mexico vphich 

 on the west reaches northward to include 

 the lower Colorado Valley in California and 

 Ai-izona,and embraces the point of the lower 

 California peninsula, and on the east coast 

 is a narrow belt extending northward to the 

 lower Eio Grande Valley in Texas, the 

 lower third of Florida and the greater and 

 Lesser Antilles. On the west, the tropical 

 elements pass vertically rather gradually 

 into the vegetation of the tierra templada 

 of Mexico and Guatemala, and at the north 

 a semitropical Gulf strip from the mouth of 

 the Rio Grande to and including upper 

 Florida, marks the transition to the subtrop- 

 ical flora of the Gulf States which, though 

 distinctly a part of the Atlantic Coast Plain 

 or Austro- riparian flora, has numerous 

 elements of tropical extraction, as, for ex- 

 ample, the Palmce, the Tillandsias, some Eu- 

 pJiorbiacece as Argithamnia, Acalypha, Sebas- 

 tiana, Stillingia and Hippomane ; Bignonia, 

 Phoradendron, Persea and many others. 



At the west, the northward extension of 



* Entwickelungsgeschiohte der Pflanzenivelt, II., 

 p. 197. 



tropical flora is checked byxerophytic con- 

 ditions, so that a very meager tropical ele- 

 ment reaches the United States in that 

 quarter. On the other hand, the free 

 northward extension to the Florida prov- 

 ince, whose physical conditions favor a 

 purely tropical flora, has been retarded by 

 interruptions in the continuity of land 

 masses, so that while the flora of South 

 Florida is not a part of the Austro-riparian 

 and sub-tropical, it is comparatively meager 

 in South American species. It has, however, 

 many elements in common with the Antilles. 

 The sharp distinction between South Flor- 

 ida and the remaining Gulf States and 

 North Florida, is shown in the following 

 data compiled by Drude* from Chapman's 

 Flora. " There are 360 species in Florida 

 which do not extend north of the 29 th 

 parallel ; of these 169 belong to 132 genera 

 which have no distribution further north- 

 ward, or 16 families reach a northern limit 

 in this peninsula." 



It is interesting to note that some of the 

 genera cited above and others, as marking 

 the transition from tropical to sub-tropical 

 United States, also extend into extra- trop- 

 ical South America, namely, to Argentine. 

 Those cited by Englerf are Argithamnia, 

 Bignonia, Lippia, Chaptalia and Galphimia, 

 to which may be added many Amaranths 

 and others. But as this element consists so 

 largely of xerophytic and halophytic spe- 

 cies, I have discussed it under the head of 

 semi-tropical xerophilous forms of Gulf 

 zone distribution. 



^. ALPINE AND ARCTIC- ALPINE FLOEAL 

 ELEMENTS IN SOUTH AMERICA. 



As previously stated, the extension of an 

 elevated continental axis from Alaska to 

 Cape Horn makes an approximately con- 

 tinuous boreal zone across the equatorial 

 regions. This continuity has fluctuated 



* Pflanzengeographie, 511. 



f Entwickelungsgeschiohte II., p. 189. 



