Mar. 1, 1920 Effect of Length of Day on Plant Growth 595 



suited, until about August i. In this latitude, then, provided other 

 conditions did not intrude, flowering would be delayed until about 

 August I, and the chances of successfully maturing seed before killing 

 frosts interv^ened would be greatly lessened. If the seed were carried 

 still farther north, the plants might not blossom at all, owing to the fact 

 that even the shortest days of the summer growing period would exceed 

 those to which they were best suited in their normal habitat. Although 

 in such instances these failures naturally would have been explained in 

 the past on the basis of unfavorable temperature relations alone, it is 

 obvious that length of day, primarily, is the limiting factor which has 

 retarded the reproductive period so that unfavorable temperature rela- 

 tions have intervened to prevent the ripening of seed. 



Although the Arctic summers are very short, plants have become 

 successfully established under such conditions, largely by the develop- 

 ment of specialized perennial types, which find the extremely long days 

 favorable both to vegetative growth and to flower production. Although 

 it has been usually considered that the purely Arctic forms are confined 

 to Arctic conditions because of certain temperature requirements, etc., 

 it is possible that length of day, hitherto overlooked as a factor in plant 

 distribution, may have much to do with their restricted range apart 

 from other factors of the environment. 



In tropical regions it is probable that the success of many native plants 

 is more or less closely dependent upon the conditions of equal or nearly 

 equal days and nights which prevail there during the entire year. The 

 varieties of bean coming from Peru and Bolivia appear to be of this type. 

 It is evident that such plants, whose flowering conditions depend more 

 or less closely upon a length of day little if at all exceeding 12 hours, 

 can not attain the flowering stage attended by successful seed production 

 in higher latitudes, at least during the summer season, which would 

 necessarily be characterized by days in excess of 12 hours. It is indi- 

 cated by the beans in question, however, that some plants of this class 

 may grow and attain successful seed production under day lengths less 

 than 12 hours. This being the case, such plants could at least extend 

 their range beyond the Tropics in so far as the temperature conditions of 

 the winter months in these latitudes . were favorable to growth and 

 reproduction. 



In any study of the phenological aspects of different species of plants 

 the fact stands out that certain plants bloom at definite seasons of the 

 year. This is quite as marked in subtropical regions as in more northern 

 regions having a definite summer growing season. In this connection it 

 is probable that the relative lengths of the days and nights are of par- 

 ticular significance in many instances. The behavior of the composite 

 Mikania scandens, as observed under specially controlled conditions and 

 under winter conditions in the greenhouse, may be more critically con- 

 sidered in relation to its normal blooming season throughout its range. 



