572 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



we have in the Maryland Mammoth tobacco a plant which shows a 

 nonflowering type of development during the summer months, at- 

 taining giant proportions, while {luring the winter months this 

 tobacco invariably flowers and does not grow any larger than other 

 tobaccos. Again, the question may well be raised as to the cause of 

 this striking difference in behavior of the Maryland Mammoth 

 tobacco when grown in the summer or in the winter, a difference not 

 shown by most varieties of tobacco. Differences in temperature can 

 not be regarded as a factor, for the greenhouse may be kept as warm 

 during the winter as the outside air of the summer months, but this 

 in no wise interferes with the flowering of the tobacco, so that one 

 must seek elsewhere for the cause of the difference in the winter and 

 summer behavior of the Mammoth tobacco. 



There is really no fundamental difference between the Biloxi soy 

 bean and the Maryland Mammoth tobacco as regards their behavior 

 when grown at different seasons of the year. Normally, neither 

 flowers during the summer months in the latitude of Washington, 

 while both flower readily during the fall and winter months. Hav- 

 ing excluded temperature difference as a possible cause of this be- 

 havior sunlight naturally presents itself for consideration. It has 

 long been known that sunlight is one of the indispensable factors 

 in plant growth, and it is obvious that outside the Tropics the light 

 conditions change decidedly as the season advances. In midsummer 

 when the path of the sun across the sky is at its highest the total 

 intensity of the light to which the plant is exposed during the middle 

 of the day may reach 10,000 foot-candles, but in the winter the mid- 

 day light intensity is scarcely half as great. A series of experi- 

 ments was carried out with soy bean, using specially constructed 

 shades of cloth to reduce the intensity of the sunlight falling upon 

 the plant. Two types of shade were used in the tests, as shown in 

 plate 1. Loosely woven cotton netting of five different weaves 

 was employed in shading the plants in order to secure suit- 

 ably graduated reduction in light intensity. The five grades of net- 

 ting, one of which was the standard cheesecloth extensively used for 

 surgical dressings, are shown in natural size in plate 2. The de- 

 gree of the shading effected varied, of course, with the changing 

 angle of the sun at different hours of the day. The reduction in the 

 intensity of the direct sunlight at noon ranged from about 30 per 

 cent of the total for the most open-mesh cloth to more than 65 

 per cent of the total for the closer woven netting. These values do 

 not take into account the diffuse light reaching the plants, but it is 

 obvious that the total light intensity was greatly reduced where the 

 denser grades of netting were used. Though the soy bean plants 

 were affected in other particulars by the shades, the date of bloom- 

 ing as compared with that of plants grown without any shade was 



