Vernalization and Photoperiodism — 118 — A Symposium 



nish a convenient cool temperature bridge across the equatorial regions to 

 mobile, migrating, herbaceous Angiospermous forms, because they afforded 

 every degree of favorable temperature to the most selective and fastidious 

 temperate region and boreal types, unfavorable length of day still remained 

 always a permanent barrier to many plants even here. This was especially 

 true for the long-day class of plants which had arisen in the temperate and 

 polar regions of both hemispheres. The short-day and day-neutral plants 

 were the only types which could have migrated freely northward and 

 southward in both hemispheres. The eastward and westward dispersals 

 would be limited only by the unfavorable temperature and moisture relations 

 associated with high mountains, and by ocean barriers. 



It is natural that in the warmer and more discontinuous land areas of 

 the southern hemisphere, the primitive woody Cretaceous Angiosperms 

 should have suffered less extinction and change, thus making conditions less 

 favorable for the rise of the herbaceous Angiosperms. For unknown rea- 

 sons these areas were less affected by cold than land areas in the high lati- 

 tudes of the northern hemisphere, subjected to the great Ice invasions. 

 Furthermore, there was less opportunity for the herbaceous descendants 

 of this ancient flora to migrate elsewhere or for immigrant forms to enter 

 these antipodean land areas from the reservoirs of the northern hemisphere. 

 This may in part explain why, at the present time, there is great prepon- 

 derance of the woody life forms in the southern hemisphere embracing 

 many families and genera found also in the northern hemisphere. If the 

 cold had been less severe with no marked differences in length-of-day the 

 original Cretaceous woody Angiospermous flora would have changed less 

 and a high degree of endemicity of woody forms would have been favored. 

 For this reason this southern flora, long ago isolated by island conditions, 

 and suffering less extinction of the more ancient forms, would have re- 

 tained a more primitive facies even to the present day. 



These considerations of the changes which took place in the Cretaceous 

 flora followed by the rise of a great Angiospermous herbaceous flora more 

 especially in high northern latitudes in Tertiary time are merely to be re- 

 garded as speculative in nature. The writer has put forward these hy- 

 potheses because they approach the problem from a new climatic angle, 

 length of day, which hitherto has been an overlooked factor of the climatic 

 complex affecting all plant life, past and present. If this factor is of such 

 great importance at the present time exerting as it does a powerful selective 

 action upon plants and confining these to certain climatic zones, then it is 

 reasonable to assume that it has so operated in all the climates of the past. 



No theory purporting to present the effects of ancient climates upon 

 plant life can be taken too seriously which emphasizes the temperature fac- 

 tor only, and completely ignores the selective factor of length of day that 

 also enters into every climatic complex. If palms, bananas, breadfruit, and 

 other tropical plants which at present show definite limits of distribution 

 formerly freely intermingled in high northern latitudes with a temperate 

 region flora, these interminglings may not be entirely explainable on a tem- 

 perature basis alone. One may assume conditions of uniformly warm cli- 

 mate, from the equator to the Poles which would bring many dissimilar 

 types of warmth-demanding plants into a common association. However, 



