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ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 2 



effective rays? What differences are created in plant gro^Yth when 

 the rays are changed either in color, in intensity, or in time of 

 exposure per day? What are the chemical processes which go on 

 in the laboratories of the plant leaves by which cellulose, sugars, 

 odoriferous materials, fruit and nut substances, poisons, and the host 

 of organic chemicals which plants produce are built up? Finally, 

 how does radiation cause plants to bend in those interesting ways 

 illustrated by the sunflower and nasturtium, and bj^ the twining stalks 

 of the beans and peas ? 



All of these cjuestions are being studied at the Smithsonian Insti- 

 tution. The investigation is stiil young, so that little as yet has been 

 published. Plate 1 and Figure 8, however, give some idea of the 



aa«r 5 0:7 0.9 M "i^ rs~ tt r§ 21 j2 



W/\v£ Length 



Figure 7. — Wave leugrth and intensity in the solar spectrum. Smithsonian observations 



ingenious apparatus and interesting work which have been developed 

 already under the direction of Dr. F. S. Brackett. I will but mention 

 the phototropic experiments in which a little oat sprout is being used 

 as an indicator. It is situated between two lights of different colors, 

 whose intensities ma}^ be graduated until the oat sprout grows ver- 

 tically. Then, of course, the two lights are equal in their tendencies 

 to produce bending. It is found that green is one thousand times 

 more active than yellow, and blue thirty times more active than green 

 to produce bending. Red and infra-red are like darkness, having no 

 bending influence at all. 



Another very promising line of investigation is that illustrated in 

 Figure 8. Pure organic chemicals such as benzene, chlorobenzene, 

 and others of greater and greater complexity are introduced in a 

 spectroscope between the source of light and the recording thermopile 



