420 Mr. F. Kidd. The Controlling Influence of 



Brassica alba it seems clear that the C0 2 has acted directly upon the tissues 

 of the embryo. On the removal of the CO2 the seeds readily germinate. In 

 the case of Brassica alba the action of the carbon dioxide may have been the 

 same, but on the removal of the CO2 from the atmosphere the seeds do not 

 germinate but continue dormant. A direct action of CO2 on the testa, 

 rendering it less permeable to the passage of gases, is suggested. Such a 

 change in the testa produced by C0 3 would have two consequences : (1) a 

 reduction in the amount of oxygen reaching the embryo, and (2) a relative 

 rise in the CO2 pressure in the embryo tissues. The possibility thus arises 

 that lack of oxygen produced by a change in the permeability of the testa 

 due to the action of CO2 has been the cause of inhibition in all the experi- 

 ments described. 



The following experiment was therefore made with Brassica alba seeds 

 from which the testas had been removed. 



Table VIII. — Experiment indicating that Increased Pressures of CO2 can 

 act Directly in producing Inhibition on the Naked Embryo of Brassica 

 alba. 



Percentage 

 CO, 

 in air. 



Time seeds lay 

 without germinating 

 in presence of high 

 partial pressures 

 of COo. 



Numbers and 

 condition of seeds 

 set in presence of 



high partial 

 pressures of C0 2 . 



Percentage 



of seed 

 germinating 

 on removal to 

 air. 



Percentage of 

 germinated seeds 

 showing injury 



to the radicle. 





days. 









60 



2 



10 without testas 



100 







80 



7 



10 „ 



100 



30 



The above experiment appears to demonstrate that the inhibitory action of 

 increased partial pressures of C0 2 may be direct upon the naked embryo of 

 mustard seeds. The phenomenon of prolonged after-inhibition did not occur 

 in these cases in the absence of the testa. Further experiments were made 

 with peas and white mustard with similar results. A conclusion, therefore, 

 which ippears to be justified is that, while the inhibiting effect produced on 

 the embryo is the result of the direct action of GO2 thereon, in the case of 

 Brassica alba an accompanying change in the testa plays an important part 

 in sealing the seed under the influence of CO2 in a special dormant phase of 

 life. 



Summary. 



Experiments were conducted showing that the germination of seeds is 

 retarded or inhibited by high partial pressures of C0 2 in the atmosphere. 



