128 AISriTUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



intensity, the time required for the first observable response. That 

 these curves determined by time of initial response should be almost 

 identical to those determined by quantitative intensity ratios strongly 

 points to a possible time-intensity product as the effective factor in 

 controlling the phototropic response. This is particularly interest- 

 ing, as such a relation is found to hold to a first approximation in the 

 case of photographic plates on the one hand and the erythema 

 dosage for the human skin on the other, as well as in most simple 

 systems. 



PHOTOSYNTHESIS 



Special all-vitreous growth chambers have been developed wherein 

 the carbon dioxide assimilated by wheat plants can readily be de- 

 termined. The accompanying illustration (pi. 1, fig. 1) indicates the 

 type of chamber developed ; the plants are inserted through holes in 

 the cork stopper and held in place by cotton, the roots being 

 immersed in a nutrient solution contained in the Erlenmeyer flask: 

 the leaves extend upward in a special tubular compartment. This 

 tubular compartment is double walled, permitting the circulation 

 of water for the maintenance of temperature. Illumination is 

 secured through these lateral walls. For experimentation with the 

 blue and ultra-violet similar containers have been made of corex. 

 Air is conditioned by a humidifier and introduced through an air- 

 flow regulator into the base of the leaf chamber. It is expelled at the 

 top and a portion passed through a conductivity cell. The variation 

 in carbon-dioxide content is thus determined by changes caused in 

 the conductivity of a potassium hydroxide solution. The record is 

 made continuously by a Leeds and Northrup automatic bridge. 



In later experiments eight 300-watt lights mounted upon adjustable 

 arms were substituted for those shown. Thus 2,400 watts could be 

 placed at any distance from 20 centimeters to a meter, the illumina- 

 tion being lateral and strictly symmetrical. A thermocouple with a 

 cylindrical receiver is introduced through the top in order to deter- 

 mine accurately the relative intensities for different adjustments. 

 The accompanying diagram (fig. 3) shows a typical run carried out 

 during a single day, showing the carbon dioxide assimilated for each 

 different light intensity. 



To a first approximation the curve is apparently made up of two 

 straight-line segments. While this appears to support the classical 

 theory of Blackman concerning limiting factors, no such conclusion 

 should be drawn until more rigid control can be maintained. The 

 small changes in values which maj^ result may be sufficient to obliter- 

 ate the apparent linearity. 



