Allard —111 — Length of Day in the Past 



evolving from the original endemic Cretaceous flora, if it has been correctly 

 assumed that a world-wide distribution of related forms prevailed then. 



While many northern herbaceous elements may have entered the south- 

 ern hemisphere during Tertiary time, these plants would have found many 

 climatic barriers in their way. It is true that great mountain ranges and 

 high plateau lands afforded favorable altitudinal temperature zones which 

 would assist the cold-demanding elements in crossing the heated tropical 

 zones, but the factor of length of day seems not to have been considered 

 in these speculations. It has been assumed that with the suppression of a 

 dominant woody flora in cold mountain climates, the dispersal of the herba- 

 ceous element was an easy matter along these great natural highways. 



It is true that in North America the great western Cordilleran highlands 

 extended from Alaska through Central America to the southern tip of 

 South America. If cool temperature alone were the only factor to be con- 

 sidered, a northern herbaceous element could readily have worked along 

 these mountain highways across the hot equatorial regions, but it is not. 

 The length-of-day factor of climate exerts a rigid control over the north- 

 ward or southward distribution of many plants, and prevents their free 

 dispersal and successful colonization into lower or higher latitudes. Length 

 of day operates regardless of the temperature relations of the climatic 

 habitat. The long-day plants which are of necessity confined to high lati- 

 tudes having long days would be poorly equipped to traverse the short- 

 day equatorial regions in spite of cold temperate or Alpine conditions on 

 the mountain highlands. Presumably the day-neutral and the short-day 

 types would find less difficulty in traversing the short-day equatorial re- 

 gions into southern latitudes since they would be constitutionally adapted 

 to these conditions. It would be otherwise with a very large class of 

 herbaceous Angiosperms of long-day habit. These would find a suitable 

 home only near their latitude of origin in the northern hemisphere, since 

 favorable long days would exist nowhere except far south of the equator, 

 and intervening conditions of unfavorable short days would militate against 

 such migrations. 



These climatic barriers would greatly restrict the dispersal of the north- 

 ern herbaceous element into the ancient floras of Australia, New Zealand, 

 Patagonia, South Africa and Madagascar. These limitations to the south- 

 ward migration of important elements of the herbaceous Angiosperms 

 which originated in the northern hemisphere, together with less refrigera- 

 tion of the climate, and less continuous land areas for the ready migration 

 of plants from other areas, may in part explain why the floras of the land 

 areas of the southern hemisphere are not as rich in herbaceous elements as 

 many of the floras of the northern hemisphere. 



For these reasons one may reasonably question statements that a com- 

 paratively small body of herbs originated independently in Antarctic land 

 areas, and that the seeming afifinities of certain floras in the northern and 

 southern hemispheres must be due to a great migration of northern herba- 

 ceous Angiosperms into these regions. It is possible that endemic specializa- 

 tions of a similar type of primitive flora arising in response to similar con- 

 ditions could explain the presence of many of the genera and closely re- 

 lated species common to both the northern and the southern hemispheres. 



