Vernalization and Photoperiodism — 110 — A Symposium 



from the beginnings of the geological time scale as at present recognized, 

 and such elevations have continued to appear through all the great geological 

 eras. Even in a uniformly warm world climate such as is supposed to have 

 characterized the Cretaceous Period, these high, cold mountain lands would 

 have favored the development and preservation of the more adaptable her- 

 baceous vegetation rather than the woody life form. 



Subsequent Dispersal of the Tertiary Herbaceous Angio- 

 sperms : — Assuming that there has been a change from Cretaceous uni- 

 formities toward seasonal and zonal arrangements of warmth and cold in 

 the Tertiary accompanied by inconstant seasonal lengths of day, it must 

 naturally follow that the Angiospermous flora would be forced to undergo 

 profound redistributions. The prominence of the woody life form would 

 decline except in the warmer tropical and subtropical regions of the earth, 

 and the herbaceous life form would assume a striking dominance in the 

 colder Polar regions, and tend to follow the progressive extensions of these 

 cold areas away from the Poles. Since the culmination of this new seasonal 

 cycle appears to have brought glaciation only to the northern land areas, 

 a series of events which occurred in the Pleistocene, the rise and spread of 

 the herbaceous Angiospermous forms was especially favored in the north- 

 ern hemisphere. However, similar changes of seasonal and zonal expres- 

 sion would also have prevailed in the southern hemisphere, and to the extent 

 that continuity of the land masses allowed, the woody forms of plant life 

 would also have been forced to recede toward the equator, followed by the 

 rising tides of herbaceous plant life. 



Students of plant distribution have assumed that the great northern 

 herbaceous assemblages which appear to have arisen in the Tertiary, found 

 a means of spreading even across the equatorial regions into the southern 

 hemisphere far south of the equator. 



It is reasonable to infer that some members of the northern herbaceous 

 Angiosperms, in many instances, did find a means of dispersal into the lands 

 of the southern hemisphere. However, it is possible to show that certain 

 climatic barriers must have existed in Tertiary time to make this dispersion 

 a naturally slow and a highly selective process. If in the Cretaceous Period, 

 herbs also existed in these southern lands beside the dominant woody 

 Angiosperms, as it is reasonable to suppose, these were less affected by the 

 change toward seasonal progressions than were the northern herbs, for the 

 reason that great ice sheets did not overrun these southern land areas dur- 

 ing Pleistocene time. 



The occurrence in the land areas of the southern hemisphere of a very 

 large number of genera and species characteristic of the north temperate 

 and even of the arctic zones has served to excite the comment of many 

 botanists. Some botanists, have concluded that these elements could not 

 have originated in the southern hemisphere, but must have found a means 

 of migration into these areas from the great centers of distribution postu- 

 lated as existing in the northern hemisphere. 



The generation of similar climatic and seasonal cycles which must have 

 taken place in southern as well as in northern latitudes, however, could have 

 led to similarities of resemblance in the derivatives and their adaptations 



