Smithsonian Report. 1936. — Johnston 



PLATE 1 



Courtesy U. S. Department of Agriculture. 



1. Effect of length of day on Mammoth totjacco grown in greenhouse in winter. Plant on left shows 

 characteristic behavior under a short day length. Plant on right was grown under similar conditions 

 except that the day period was lengthened by use of electric light, which prevented it from flowering. 



M % 



Courtesy U. S. Department of Agriculture. 



2. Yellow cosmos, a short-day plant, grown with equal alternations of light and darkness ranging from 12 

 hours to 5 seconds. With decrease in the intervals of light and darkness there is progressive decrease in 

 height, size, and weight ol the plants and increase in etiolation and attenuation till the 1-minute interval 

 is reached. Further shortening of alternations causes marked improvement in growth and appearance 

 of the plants. All intervals from 1 hour downward are almost equally unfavorable for flowering. 



