Vernalization and Photoperiodism — 112 — A Symposium 



The flora of the latter regions, then, would tend to display a greater per- 

 centage of shrubby elements than the northern floras, where extreme re- 

 frigeration caused greater extinction of the woody elements, giving room 

 for a more abundant development and specialization in the surviving herba- 

 ceous elements. One may liken this hypothetical rise of the Tertiary herbs 

 following the destruction of the woody Cretaceous Angiosperms to the 

 noticeable inrush of herbaceous forms which follows the destruction of a 

 great climax forest by lumbering and fire. Almost at once a host of 

 adaptable weedy annual field forms are encouraged to colonize the denuded 

 areas and to thrive as never before, until the perennial forms and the forests 

 once more encroach upon this great temporary herbaceous assemblage. 



To show the length-of-day difficulties which plants are forced to meet 

 when suddenly transferred by man to regions where the length of day 

 conditions are unfavorable, the common potato and its introduction into 

 northern Europe where the summer days are very long, may be cited. Ac- 

 cording to VAN DE Plank (1946) the common potato Solanum tuberosum 

 could have been introduced from two sources. One group centering in 

 Southern Chile where the summer days are long ; the second assemblage cen- 

 tering in Colombia, Ecuador, Bolivia or Peru where the short tropical lengths 

 of day near 12 hours prevail. From the experiences of growers in the high 

 northern latitudes of Europe it would appear that the stock of the first Euro- 

 pean potatoes came from the short-day tropical source, since the plants did 

 not tuberize well until very late in the season and continued to grow far into 

 the autumn days or until killed by frost. This was a decided handicap, and 

 it was recognized that great advantage would be gained if earlier sorts 

 could be grown, in reality those better adapted to mature crops on longer 

 days. Earlier strains have now been developed. From about 1573 up to 

 about 1830, a period of about 260 years at least^ late varieties or those ma- 

 turing on short days were the only sorts grown in the British Isles and 

 Europe. It is indicated that had the original stock been obtained from the 

 long-day assemblage native to southern Chile, the problems of adaptation 

 to European conditions would have been simpler, and earlier maturing sorts 

 would have been available from the outset. These facts having to do with 

 a conscious and very recent distribution by man illustrate some of the prob- 

 lems and difficulties plants must always meet in their natural disseminations 

 and subsequent colonization in regions where summer lengths of day are 

 not suited to their requirements. These short-day potatoes of the tropics 

 not only could not have competed with the great forested assemblages of 

 more northern latitudes, but were not adapted to the longer days of north- 

 ern or southern latitudes. If at some past Cretaceous period the progeni- 

 tors of the short-day potato assemblage had been able to invade more north- 

 ern cool highland regions at a time when the days and nights were near 12 

 hours in length all over the earth, the strong zonal changes of climate in 

 the Tertiary especially if associated with an increased length of day and 

 intense refrigeration, would have driven the short-day assemblage into the 

 tropics where we find this group at the present time. 



Nothing at present appears to be known of the former distributions of 

 the potato, north of the tropical regions. The writer is of the opinion that 

 the short-day tropical group is the more ancient one, and that the long-day 



