2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



APPARATUS 



The apparatus used in this experiment has been described in a paper 

 by Hoover, Johnston, and Brackett (1933). Although the apparatus 

 was rebuilt in order to improve the method of controlling the temper- 

 ature and humidity, its general features remain the same. The plants 

 are placed in a closed glass system in which a rapid recirculation of 

 air is maintained. New air at a constant rate enters the system at one 

 point and is drawn off at another for analysis. 



The design of the glass which holds the nutrient solution was 

 changed in order to permit a continuous flow of solution. The seal 

 between the flask, holding the plant, and the growth chamber was 

 made by the lower part of the growth chamber dipping into a cup of 

 paraffin oil sealed to the neck of the flask. A view of the apparatus 

 is shown in plate i. 



FILTERS 



The problem of obtaining radiation of sufficient energy in narrow 

 wave-length bands over an area large enough for even a small wheat 

 plant is a difficult one. A good monochromator gives the best means 

 of obtaining narrow bands, but in most cases the radiation intensity 

 is low except for very small areas. Glass, liquid, or dyed gelatin films 

 may be used to obtain bands of radiation of considerable intensity 

 over a large area, but these filters transmit rather broad bands. Thus 

 it is difficult to compare the results obtained with these filters. There 

 are, however, certain combinations of these filters called line filters 

 which do transmit narrow bands of radiation. The transmission of 

 these filters is low, but when used with line sources such as the 

 mercury arc, sufficient energy was obtained for this experiment. The 

 following filters were used to isolate a few of the strong mercury lines. 



Wave length (A) Filter 



5780 Corning monochromatic filter No. 4 



5461 " " " " 5 



4358 " " " " 7 



4048 " " " " 8 



3650 " " " " 10 



The Christiansen filter described by McAlister (1935), although 

 not an ideal filter, has certain advantages for an experiment of this 

 nature. By merely changing the temperature of the filters it was 

 possible to cover the entire spectral region of photosynthesis by using 

 two filters with different proportions of carbon disulphide and benzene. 

 Tn the first experiment with these filters, four filters 6 inches in 



