126 



THE AMERICAN BOTANIST 



The northern vegetation thus developed, proved ex- 

 tremely hardy and aggressive, and was able not only to over- 

 spread the great continental area of the north temperate zone, 

 but to invade as well, the tropics and even the antipodes. The 

 presence of a large number of typically northern genera of 

 plants in Australasia, southern South America and South 

 Africa, often separated from their related forms by the whole 

 width of the tropics, has long been recognized as one of the 

 most fascinating problems of plant distribution. It is im- 

 portant to note, that this invasion of northern plants ( nearly 

 200 genera are known) which has been so successful in pene- 

 trating far southern regions and which display so well the 

 ''wonderful aggressive and colonizing power of the Scandi- 

 navian flora" tO' which Wallace and others have called atten- 

 tion, has in reality been an invasion of herbs, for almost none 

 of the northern trees and shrubs have participated in it. 



Herbaceous plants have also been developed in the south 

 temperate zone, apparently in response to the refrigeration of 

 climate there in the late Tertiary. Antarctic herbs were doubt- 

 less among the very last plants to^ leave the polar continent as 

 the glaciers advanced. They are still almost all alpine or cold- 

 loving perennials, and have as yet failed to give rise to the ag- 

 gressive lowland annual type. 



Refrigeration of climate was doubtless not the only factor 

 in the development of an herbaceous vegetation. A large body 

 O'f such plants seem to have originated in the arid regions, 

 where they spring up rapidly and produce seeds during the 

 rainy season, thus bearing- precisely the same relation to ex- 

 tremes of moisture that arctic and alpine herbs do to extremes 

 of temperature. The assumption of a rapidly climbing habit, 

 especially in the tropics, has also resulted in the development 

 of an herbaceous type of stem in such families as the melons, 

 milkweeds, and passion flowers. 



