120 ANNUAL REPORT SMITHSONIAN INSTITDTION, 1950 



filter cell and makes it possible to operate a unit involving one or more 

 internal reflector lamps without contamination of the distilled-water 

 filter by dust in the air or replacement of it for at least 2 weeks of 

 continuous operation. This type of unit is proving useful for the 

 incandescent irradiation of small cultm-es of algae and germinating 

 seedlings. With this luminaire it is possible to obtain 3,000 foot- 

 candles over small areas without excessive temperature rise of the 

 irradiated cultures. 



Two large grating monochromators have been designed and are 

 now being constructed for action spectrum studies. One unit will be 

 used for recording absorption spectra and the other as an irradiation 

 monochromator for action spectrum studies. The soiu-ce for the 

 irradiation monochromator is a 12-kilowatt carbon arc. 



A new type of 60-cycle conductance bridge with a phase-detector 

 amplifier has been developed which records directly changes in con- 

 ductance of 1/R instead of some complex function of resistance, as 

 with conventional bridges. With this instrument it is possible to 

 follow changes in the concentration of dilute inorganic nutrient solu- 

 tions with a precision of better than 0.3 percent, and no replotting of 

 data is necessary. Continuous recordings of the uptake of single salts 

 by seedlings may be followed, as well as the loss of salts from roots 

 and other tissues. This instrument is being used for studies of the 

 effect of light and growth substances on ion exchange in plant tissues. 



During the latter third of the year experimental work was under 

 way in three general areas: First, the effect of native and synthetic 

 auxins on the water and ion exchange relations of potato-tuber tissue 

 and corn and bean roots. As this work is being supported by the 

 United States Army Chemical Corps, the results are not available 

 for this report. The second area of work pertains to an investigation 

 of the action spectrum and pigment systems involved in photomor- 

 phogenesis of seedlings. Seedlings are being grown under conditions 

 of constant light intensity and limited spectral range as obtained by 

 large dyed gelatin filters prepared in these laboratories in order to 

 separate photomorphogenesis from the other photochemical reactions 

 of phototropism, chlorophyll synthesis, and photosynthesis. The 

 third area of investigation pertains to the effect of light on the per- 

 meability of plant tissues and on the capacity of seedlings to absorb 

 nutrients from single salt solutions. These data are being obtained 

 by continuous recording of solution conductance. 



A paper entitled "Light as a Modifying Influence on the Mineral 

 Nutrition of Plants" was presented by the chief of the division at the 

 Symposia on Plant Growth Substances and Mineral Nutrition of 

 Plants at the University of Wisconsin in September 1949. 



