SEED DORMANCY AND DORMANCY BREAKING 185 



both the inhibitors. However, if we take the germination of the 

 controls (in water) as 100, and express germination in the 

 presence of the inhibitors as percentages of the water control, 

 we see that the inhibitors differ in their action. Higher concen- 

 trations of dinitrophenol depress germination equally in dark- 

 ness and after light treatment, while coumarin at all concen- 

 trations inhibits germination much more strongly in the dark, 

 showing that light can to a certain extent obliterate the effect of 

 coumarin. Since, as was mentioned earlier, the light mechanism 

 influences only the early steps in the chain of reactions leading 

 to germination, it would follow that coumarin acts on these 

 early steps, whereas dinitrophenol affects processes which are 

 not any longer under the control of the light mechanism. 2-4 D, 

 according to Evenari, behaves in a similar way to coumarin^^. 



Since the question of respiration occupies so eminent a place 

 in the symposium, let us say something about respiration in dry, 

 resting seeds and in germinating ones. Do resting seeds respire 

 or not? There seems uo be no doubt that the level of respiration 

 in 'dry' seeds is largely a matter of moisture content. In general, 

 down to a water content of approximately 10%, there always 

 seems to be a small but measurable CO2 output and O2 uptake. 

 When moisture is reduced below this level, the gas exchange is 

 dramatically slowed down, and very frequently cannot be 

 measured, although the seeds remain viable for a long time. As 

 has been pointed out frequently, this only means that no 

 measurable amount of CO2 can be detected and not that gas 

 exchange actually ceases. 



James^^ cites in this regard a comment of F. F. Blackman: 

 'It may well be that there will not be enough CO2 produced to 

 be detectable in ten years, but who shall say that change has 

 ceased? Our methods of analysis which demand a large aggregate 

 of molecules for any demonstration are incapable of settling 

 this philosophical question'. Even in those cases where small 

 traces of CO2 output can be measured, two important questions 

 arise. First, if this CO2 output is indicative of what usually is 

 defined as respiration and secondly, how far it may not be due 



References p. 190 



