

EFFECT OF LIGHT ON PLANTS — GARNER AND ALLARD. 577 



it throws a great deal of light on the underlying cause of difference 

 in varieties as to their adaptation to different latitudes. To test this 

 point a series of plantings at approximately three-day intervals 

 throughout the spring and summer were made of the Mandarin, 

 Peking, Tokyo, and Biloxi varieties of soy bean, and careful watch 

 for the first appearance of blossoms on each planting was maintained. 

 It is obvious that as the summer advances the successive plantings 

 will be exposed to shorter day-lengths and the time elapsing between 

 germination and the arrival of a length of day of, say, 12 hours, will 

 become progressively less. The fact that the duration of the vegeta- 

 tive period which precedes flowering is progressively shortened as 

 the season advances has already been sufficiently emphasized in the 

 case of the very late variety, Biloxi, but consideration of the com- 

 parative behavior of early and late varieties under different lengths 

 of day leads to such important conclusions as to justify further dis- 

 cussion. The remarkable differences in the response of the Mandarin, 

 Peking, Tokyo, and Biloxi varieties to external factors connected 

 with advance of the season are graphically shown in figure 2. The 

 outstanding feature is that the curves showing the number of days 

 required by the different varieties to reach the flowering stage show a 

 striking tendency to converge as the plantings are made later and 

 later through the season. The time required by the Biloxi to reach 

 the flowering stage is shortened most by advance of the season while 

 the Mandarin is affected least. It appears from a study of figure 2 

 that if plantings could be made late enough in the season without 

 interference by cold weather or other factors a point would be 

 reached where all varieties would require the same length of time to 

 reach the flowering stage, namely, about 25 days. Now, by referring 

 back to page 575 it will be seen that this is exactly the result obtained 

 in midsummer when temperature and light intensity are at their 

 highest, simply by exposing all varieties to a day-length of 12 hours 

 or less. Thus there seems to be no doubt but that speeding up of 

 flower and fruit formation with advance of season is due to the 

 decrease in length of day, and changes in temperature and the in- 

 tensity of the light play only a subordinate role in this phenomenon. 

 If earlier flowering and fruiting of the later plantings are to be 

 considered as due to approach of cold weather, it must at least be 

 admitted that the plants may be easily fooled in the matter. 



Returning to the differences in behavior of early and late maturing 

 varieties, we reach the important conclusion that if grown in the 

 Tropics where the length of day is not far from 12 hours through- 

 out the year these differences between early and late varieties wtftrM 

 largely disappear. Conversely, varieties or strains which ^m\tu¥$ 

 at the same time in the Tropics might readily show markefd ^IrnW-* 



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