KENNETH J\ THIMANN AND C.EOR('.E M. CURRY 619 



"TT 



NODE 



24 



26 



2 3 4 8 12 \^ 20 



DISTANCE FROM TIP IN MM 



Fio;. 2. Distribution of growth rate, expressed as per cent elongation per U)0 



minutes, along the Ai'ena seedling. Solid line, plants kept in darkness, given red 



light onlv in first 2 hours of germination; dashed line, plants exposed to red light 



ior one hour just before measurement. (From Curry, Thimann, and Ray, 1956). 



posure of the experimental plants to red light; where possible, weak 

 gTeen light, which is phototropically inactive, was used, and such 

 exposure to red as was unavoidable was carefully standardized. The 

 treatment of the plants was of course standardized in all other re- 

 spects; they were used at an age of about 74 hours from the initial 

 soaking and selected for uniform height and straightness. 



When such standard seedlings are illuminated from one side with 

 measured dosages of monochromatic blue light, results like those in 

 Fig. 3 are obtained. As was shown as long ago as 1909 by Blaauw, 

 curvature is proportional over a certain dosage to the product 

 / X ^ (see especially Arisz, 1915) . In order to cover a sufficient range 

 of energies, log It is plotted here, which means that the curve should 

 flatten out asymptotically at the left. But over a satisfactory dosage 

 range there is linear proportionality to log It. Lange (1927) found 

 that the minimum light dosage for threshold curvature was Yz 



