586 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



the same behavior as the soy bean and aster. By using electric light 

 to lengthen the short days of winter as already described Spanish 

 needles (Bidens frondosa L.), which was in a dying condition after 

 having fruited, was induced to send up new shoots and continued to 

 grow all winter. Thus it seems that whether a plant behaves as an 

 annual, dying soon after flowering and fruiting, may depend largely 

 on the length of day to which it is exposed, and even after the plant 

 has reached a dying condition it may be rejuvenated through the 

 action of a favorable day-length. It is interesting to note, also, that 

 in the above experiments the soy bean, ragweed, and aster were 

 caused to complete two cycles of alternate vegetative and reproductive 

 activity within a single season. 



NATURAL DISTRIBUTION OF PLANTS IN RELATION TO DAY-LENGTH. 



In the preceding pages evidence has been given that not only 

 different species but also varieties of the same species show marked 

 differences in their response to length of day. Some are able to 

 flower and mature seed under the influence of relatively short days, 

 while others are able to reproduce successfully only under the in- 

 fluence of relatively long days. At the Equator the length of the 

 day remains 12 hours throughout the year, so that in the Tropics, 

 with seasonal range in length of day reduced to a minimum, it 

 might be expected that there would be less tendency toward the 

 annual type of plant behavior in so far as an unfavorable day-length 

 is responsible for early termination of growth, followed by fruit- 

 ing and final death of the individual. In the case of plants which are 

 adapted to a day-length of approximately 12 hours, the natural ten- 

 dency in the Tropics would be toward the perennial type of develop- 

 ment; and since such change in length of day as occurs must take 

 place very gradually, the everblooming habit would be greatly 

 favored. These deductions are in full accord with observation, for 

 predominance of perennials and everblooming types are character- 

 istic features of the tropical flora. It is to be expected that many of 

 these tropical plants would not be successful in higher latitudes, for 

 the increase in length of day during the growing season would tend 

 to prevent successful ripening of seed before cold weather, while the 

 more or less precipitate seasonal change would not find the plants 

 prepared to withstand the cold of winter. In other words, these 

 plants would hardly find favorable conditions in higher latitudes 

 either for propagation by seed or for survival as hardy perennials. 



On the other hand, some species behaving as fruiting perennials 

 in the Tropics might successfully mature seed in considerably higher 

 latitudes before the advent of cold weather, and hence might prove 

 successful as annuals. The most essential condition is that the plant 



