EFFECT OF LIGHT ON PLANTS — GARNER AND ALLARD. 587 



be able to withstand the unfavorable conditions of winter (or of the 

 summer) in some resistant form, whether simply as a typical ever- 

 green, or in some resting condition, as deciduous woody perennial, 

 herbaceous perennial, or as viable seed. In passing northward or 

 southward from the Equator both the total annual range and the 

 daily amplitude of change in the length of day must be considered 

 in their relation to adaptation and natural distribution of plants. 



It remains to consider briefly the manner in which a species adapted 

 only to a given range in latitude because of its requirements as to 

 length of day may eventually extend its range into new regions. 

 For example, how could a species native to the Tropics successfully 

 invade the temperate regions? The answer seems to be that new 

 strains or varieties must appear which are better adapted to the 

 changed length of day prevailing in temperate regions. That new 

 strains and varieties are constantly coming into existence in nature 

 is well known, and it is equally certain that some of these differ from 

 the parental type in their light requirements, an example being found 

 in the Maryland Mammoth tobacco, which has been fully discussed 

 in the preceding pages. It may be well to recall in this connection 

 the observation previously made to the effect that a mixture of early, 

 medium, and late maturing varieties of soy bean as grown in northern 

 latitudes would behave as a single early-maturing variety when 

 grown in the Tropics. Plants which require a comparatively long 

 day to attain the flowering and fruiting stage, such as the radish, 

 apparently would need to produce strains capable of flowering under 

 a shorter day-length in order to extend their natural range toward 

 the Equator. That such strains, differing in their requirements as to 

 duration of the daylight period, do actually exist in some species has 

 been shown in this paper, and it seems reasonable to assume that 

 appearance of new forms thus differing from their parental types 

 has furnished the means for the species to extend its range northward 

 or southward. 



CONCLUSION. 



In the light of the experiments which have been described in the 

 foregoing paragraphs, it seems certain that the seasonal range in 

 length of day exerts a profound influence on the flowering and fruit- 

 ing habits of plants. This may seem less surprising, perhaps, when 

 it is considered that temperature, rainfall, and intensity of sunlight 

 as affected by cloudiness are subject to great variation in most locali- 

 ties independently of the normal seasonal change, while the length of 

 the daylight period, on the other hand, follows with mathematical 

 precision a definite seasonal change year after year. It is to be 

 expected, therefore, that any feature of plant development which 

 shows such sharply defined periodicity as does flowering and fruiting 



