PHOTOPERIODISM 



487 



single leaf, a single leaflet, or a whole plant. In this way duration of 

 treatment wdth radiation of known intensity in a given w'ave-length band 

 can be much shorter than required for extension of the photoperiod. 

 Where the response is control of floral initiation, the plants are irradiated 

 near the middle of several successive long nights. They can be returned 

 to noninductive photoperiods for development prior to dissection. 



z 

 9 



f- 



<L 

 CC 

 O 



< 



IE > 200 



5 

 S 



o 





-150 - 



UJ 



< 

 (r 



H 

 Z 

 U liJ 



> o 



li 



< 

 UJ 



a: 



100 



50 - 



BARLEY 



C-PHYCOCYANIN 



4000 



5600 



6200 

 LENGTH, 



6600 7000 

 A 



Fig. 10-2. Composite action curves for control of floral initiation of soybean, cockle- 

 bur, barley, and Hyoscyamus and for regulation of leaf size in etiolated peas, together 

 with a curve showing the relative concentrations of C-phycocyanin for a given 

 absorption. 



The wave-length bands used in measurement of the action spectra 

 must have a very low level of extraneous radiation, since a given response 

 for different wave-length regions might vary a thousandfold in its energy 

 requirement. In the design of the radiation source a balance must be 

 effected between area irradiated, wave-length regions isolated, and energy 

 per square centimeter per second. A spectroscope with two large prisms 

 (ibid.) was used in obtaining the action spectra shown in Figs. 10-1 and 2. 

 Results for floral initiation in Soja max var. Biloxi for the red portion of 

 the spectrum are shown in Fig. 10-1 and throughout the region of effec- 

 tiveness at wave lengths longer than 4000 A in Fig. 10-2, above. The 



