186 THE FLOWERING PROCESS 



Chapter 9 and below). Flowering hormone moves through the plant 

 only through living tissues, and it will not pass any sort of diffusion 

 boundary (see Chapter 9). We do not quite know what to make of 

 these observations, but it has been suggested that the flowering 

 hormone might be a very large molecule, such as a protein or even 

 a nucleic acid. This could conceivably account for its very slow rate 

 of movement and perhaps for some of the other phenomena which 

 are discussed below. 



High Intensity Light Following the Inductive 

 Dark Period (22, 32) 



For many years it has been known that cockleburs flower best 

 when the inductive dark period is followed by high intensity light. 

 An experiment shown in Fig. 10-2 illustrates the phenomenon. At 

 least two possible explanations for this finding come to mind. 



Light interruption 



I 



80 

 80 



100 - 



a: 



Id 



90 I 



40 h- 



O 



30 q: 



10 12 14 16 



HOURS OF DARKNESS 



Figure 10-2 



Effects on flowering of different dark period lengths (apparently under 

 sub-optimal conditions) or different dark period treatments following a 

 10-hr inductive dark period and a short light interruption (2 min of 

 fluorescent light). Data from James A. Lockhart and Karl C. Hamner, 

 1954, Botan. Gaz. 116, 133-142. 



