Vernalization and Photoperiodism — 108 — A Symposium 



northern hemisphere. These marked the culmination of the great climatic 

 cycle of extreme refrigeration in northern latitudes. 



The Tertiary Period is unique in that the plant life of the world radically 

 changed its life form and distributions, if our geological evidence has been 

 correctly interpreted. It has been shown that through the Cretaceous Pe- 

 riod the woody forms of Angiosperms prevailed, and the herbaceous forms 

 constituted a minor element of the flora so far as known. With the great 

 world-wide climatic changes, which resulted in zonal distributions of tem- 

 perature similar to present conditions, the herbaceous life form rapidly 

 arose into prominence, and these Angiosperms, speaking in terms of geo- 

 logical time, soon dominated much of the world. That these events were 

 associated with the Tertiary refrigerations of climate cannot well be 

 doubted. The herbaceous plants, annual and perennial, proved to be more 

 successful under the new climatic conditions. Many of the woody plants 

 probably succumbed to the cold and other conditions, or were driven south- 

 ward, but the herbs became the dominant life form, mainly in the north 

 temperate regions, where formerly they appeared to have occupied a small 

 niche in the world's flora. 



Various theories have been advanced to explain the origin and dispersal 

 of the great hordes of herbaceous Angiosperms which appeared in Tertiary 

 time. As a result of this great change in floristics and life form the herbs 

 by the infinite variety of their specializations and adaptations, not only be- 

 came dominant in the Tertiary climate but have remained one of the most 

 successful groups of plants in the present flora throughout the world. 



Seasonal Cycles in Tertiary Climate and the Responses of Plant 

 Life : — The exact astronomic conditions which were responsible for any 

 of the great pan climates of former geological periods will probably never 

 be known. Reasoning from the dependence of our present seasonal cycles 

 upon an inclined axis, it can be assumed, however, that if there were no 

 marked seasonal cycles in Cretaceous time, there were only slight varia- 

 tions in length of day. Since the present seasonal cycles with their accom- 

 panying changes in length of day, depend upon the obliquity of the earth's 

 axis, it may not be unreasonable to assume that this axis which is now in- 

 clined nearly 23° 27' 3" had just begun to depart from a near zero obliquity 

 in early Tertiary time. If this is a correct assumption a very gradual de- 

 parture from the constant length of day of about 12 hours would have 

 manifested itself in high latitudes in early Tertiary time. With increasing 

 tilt of the axis progressively longer days would have obtained in higher lati- 

 tudes and the equatorial regions alone would have retained the original 

 short-day conditions of 12 hours as they do today. 



The early Tertiary climate must have become progressively cooler, so 

 that the warmth-demanding plants, formerly at home in Spitzbergen and 

 Greenland, found inhospitable conditions here. These gradually would 

 have given ground and perhaps have confined themselves only to tropical 

 and subtropical regions which alone remained favorable to their existence, 

 as it is at the present time for their descendants. 



Those plants which were day-neutral, and great hosts of short-day 

 plants, more especially those of annual behavior, together with hardy, 

 perennial types, would have been able to exist and to multiply in high north- 



