5y6 Journal of Agricultural Research voi. xvni. no. h 



narrowly defined this maximum length of day capable of inducing sexual 

 reproduction may be. The outstanding fact is that it is quite dififerent 

 for the four varieties of soybeans. In all cases, however, it is in excess 

 of 12 hours. 



Coming to tobacco, the contrast in behavior of the Connecticut Broad- 

 leaf and the Maryland Mammoth varieties is very striking. Sexual 

 reproduction in the Connecticut Broadleaf is not materially affected by 

 changes in length of day within the seasonal range for the latitude of 

 Washington or southward. On the other hand, the Maryland Mammoth, 

 which is presumably a mutation from a very old variety of Maryland 

 tobacco and appears to be a typical example of gigantism, can not be 

 forced into blossoming during the summer months by any method now 

 known except artificial shortening of the duration of the daily exposure 

 to light, while the character of gigantism is completely suppressed when 

 the plant is grown during the short days of winter. A glance at Table 

 IV shows that shortening the daily light exposure has not materially 

 affected the Connecticut Broadleaf but has been effective in shortening 

 the vegetative period of the Maryland Mammoth. The Cuban type of 

 Mammoth was affected like the Maryland type, but it appears that the 

 former has a somewhat longer vegetative period than the latter under 

 similar conditions. The Maryland type blossoms readily under the 

 influence of a 12-hour light exposure; but there is a suggestion that a 

 time factor is operative here, for the plants seem not to blossom so 

 promptly as when under the 7-hour exposure. It seems probable also 

 that the Cuban Mammoth will blossom under a 12 -hour exposure to 

 light. The obser^^ation has been made by Lodewijks (17) that a giant 

 type of Sumatra tobacco — grown under the influence of the 12-hour 

 equatorial day — which may reach the extreme height of 24 feet, either 

 does not blossom at all or forms only a few flowers and seeds. Gigantism 

 in tobacco disappears when the plant is brought under the influence of 

 short days such as prevail in the temperate zone during the winter 

 months. Nicotiana rustica, so far as tested, behaves like the Connecticut 

 Broadleaf. 



Aster linariijolius, again, has given clean-cut results under the different 

 light exposures, as is shown in the summarized data of Table IV. Its 

 behavior is strictly comparable with that of the Biloxi soybean and the 

 giant type of tobacco. It is a typical "short-day" flowering perennial. 

 As with the Biloxi soybean, however, this maximum length of day 

 capable of bringing into expression the flowering and seed-formation 

 processes is in excess of 12 hours. Exposure to light twice daily was 

 without effect, for the vegetative period of the test plants, counting from 

 the beginning of the experiment, was 92 days and that of the controls 

 (not shown in Table IV) was 94 days. Here, again, attention is called 

 to the fact that the total daily exposure to light averaged only about 10 

 hours, and the plants were in darkness during the period of most intense 

 illumination, 10 a. m. to 2 p. m. 



