Mar. 1, 1920 Effect of Length of Day on Plant Growth 575 



out here that the progressive decrease in the length of the vegeta 

 tive period of all varieties apparent in the very early plantings which 

 germinated during the early part of May is probably due to a gradual 

 reduction in the retarding action of relatively low temperatures which 

 prevailed at the time. Again, there is distinct evidence of the retarding 

 influence of lower temperatures on the very latest plantings of the 

 Peking and Biloxi varieties. Eliminating these portions of the curves 

 from consideration, it is evident that the graph for the early variety, 

 Mandarin, is practically horizontal, while there is a marked downward 

 trend in the graphs for the remaining varieties which increases in pitch 

 as we pass toward the later varieties, the drop being quite precipitate 

 in the curve of the very late variety, Biloxi. There is, in short, a marked 

 tendency for the graphs to converge toward a common point as the sum- 

 mer season advances, a' fact which is in full accord with the results of the 

 tests under controlled conditions. Another interesting feature of these 

 curves is that for the period around May 25 to June 15 there is a more or 

 less well defined "hump" which is most strongly developed in the curve 

 for the Peking, less prominent in that for the Tokyo, and hardly appar- 

 ent in the curves for the Biloxi and the Mandarin. A possible explana- 

 tion of this relative lengthening of the vegetative period of the Peking 

 and Tokyo plantings which germinated during the close of May and early 

 June is to be found in the fact that these plants received the longest 

 possible average light exposure. This would not affect the Mandarin or 

 the Biloxi, since the length of the day is well above the "critical" for the 

 Biloxi and below it for the Mandarin. Apparently field plantings can 

 not be extended through the season in such a way as to bring the plants 

 throughout the vegetative period under a light exposure below the critical 

 in length and at the same time secure throughout the period a suffi- 

 ciently high temperature (and possibly other favorable factors) to 

 reduce the length of the vegetative period to that which experiments con- 

 ducted under controlled conditions have established as apparently the 

 physiological minimum requisite for sexual reproduction. There can be no 

 doubt that decreasing temperature, within limits, will retard vital activities 

 of the plant; and the fact should be emphasized that, as a rule, the action 

 of decreasing temperatures as fall approaches must be retarding rather 

 than accelerating in its influence on the attainment of the flowering 

 stage by the plant. It should be pointed out here that the hastening 

 eff"ect of the shorter days on the final maturation of the seed of the soy- 

 beans is shown by the fact that in the late plantings there is an evident 

 tendency for the early Mandarin and the later Peking varieties to pro- 

 gress toward maturity at the same rate. 



As regards the critical length of day required for furnishing the stim- 

 ulus which brings into expression the processes of sexual reproduction 

 mentioned above, it should be stated that this has not been determined 

 as yet for any of the plants under study, and it is not possible to state how 



