Vernalization and Photoperiodism — 116— A Symposium 



of the great geological eras. Length-of-day, also always an interrelated 

 condition of every climate, has never been recognized as specifically affect- 

 ing the behaviors and distributions of plants. For this reason older theories 

 have not been formulated upon a correct basis, and this has led the writer 

 to develop a new hypothesis, making use of length of day as well as un- 

 favorable temperature, in this approach. 



It is assumed that beginning in Tertiary time, changes in these dominant 

 conditions of climate made their appearance establishing marked seasonal 

 cycles which naturally would be accompanied by lowering temperatures at 

 or near the Polar areas, with attendant changes in length of day. While 

 the length of day remained at or near 12 hours in the equatorial regions as 

 before, the days poleward progressively increased in length as they do at 

 present during the summer season, the longest days being experienced at 

 the highest latitudes. These changes toward cooler temperatures and 

 seasonal cycles became more accentuated in later Tertiary time, with con- 

 tinued accentuation into the Pleistocene, when great ice invasions occurred 

 in the northern hemisphere. 



It is not necessary to conclude that these ice ages followed as a direct 

 result of a hypothetical change in the obliquity of the earth's axis and the 

 establishment of marked seasons with variations in length of day, since 

 these glaciations were confined to the northern hemisphere. However, 

 these great glaciation occurrences are of particular significance since they 

 favor the conclusion that a great herbaceous Angiospermous flora came 

 rapidly into prominence, geologically speaking, in Tertiary time, and ulti- 

 mately became dominant in the colder regions. Some world-wide change 

 in climate appears to have favored this dominance and the subsequent dis- 

 persion of certain elements of this herbaceous flora over all the earth, if 

 paleobotanists have correctly estimated the floral changes that actually oc- 

 cured at this time. 



There has been no satisfactory explanation which will account for this 

 major change from the Angiospermous woody life-form to the herbaceous 

 life-form following changes in the relatively uniform climate of the Cre- 

 taceous Period, which appears to have favored the woody life form over 

 all the earth. However, since the woody Angiosperms were dominant, and 

 the herbaceous forms of these appeared to be an insignificant element of the 

 vegetation, even in the Polar areas, there is reason to believe that weak 

 zonal distributions of temperature obtained, and that a warm climate pre- 

 vailed even in portions of the Polar regions. 



While SiNNOTT and Bailey and others have advanced the hypothesis 

 that the development of all herbaceous stems in the Tertiary Period re- 

 sulted from a simple reduction in the amount of secondary wood chiefly, 

 together with more or less increase of the ordinary parenchymatous tissue, 

 it seems just as reasonable to assume that some herbs had long been in 

 existence, playing a minor role in the great forests of woody forms in 

 Cretaceous time. When the Tertiary Period brought about conditions un- 

 favorable to these woody Angiosperms, the herbaceous Angiosperms, ow- 

 ing to their shorter life cycle, found better opportunities to increase and to 

 adapt themselves to the new conditions involving cold seasons, longer days 

 in summer, and sharp zonal distributions of warmth and cold. 



