465 
Influence of Carbon Dioxide. IV. 
Table VIII. 
Time Factor in the Production of Secondary Dormancy . 
Number of days in the gas- 
mixture (30 % C 0 2 = 
14% o„). 
Germinations 10 days after 
removal from the inhibiting 
gas-mixture. 
100 % 
60% 
40 % 
4 ° % 
6% 
Mean temperature of laboratory, 14 0 
Approximate percentage of 
secondary dormancy. 
0% 
4 Q% 
60% 
60% 
94 % 
C. 
It has been shown that oxygen is necessary for the production of 
secondary dormancy in seeds of Brassica alba , since in the absence of 
oxygen during the primary period of inhibition injury occurs which prevents 
the phenomenon. Carbon dioxide is necessary because in the absence of 
this gas the primary period of inhibition in the presence of oxygen does 
not occur at normal temperatures. But it is not clear whether secondary 
dormancy is due to the specific action of carbon dioxide or simply to slow 
secondary changes occurring in the tissues of the fully swollen seed 
independently of the action of the carbon dioxide. In order to decide 
this point, fully-swollen seeds were prevented from germinating for a period 
of seven days by exposure to a subminimal temperature. On return to 
normal temperatures a full percentage of germination resulted, thus showing 
that carbon dioxide exercises a specific action in the production of secondary 
dormancy. 
§ 3. Changes in the Seed-coat or Embryo of Brassica alba 
ACCOMPANYING THE PRODUCTION OF SECONDARY DORMANCY. 
It is clear that the causes underlying the persistence of the dormant 
condition of Brassica alba seeds must be looked for in changes produced 
either in the testa or in the embryo during the primary period of inhibition 
in the presence of carbon dioxide. 
It has already been shown that when such changes, due to the lack of 
oxygen or to toxic concentrations of carbon dioxide, amount to injury to 
the radicle, secondary dormancy does not occur. 
It is convenient at this point to analyse, on the lines of previous research 
work on dormancy in seeds, the possible causes controlling the phenomenon 
of secondary dormancy in uninjured seeds of Brassica alba. 
A. A change in the seed-coat during the period of primary inhibition 
in the inhibiting gas-mixtures : 
(i) Resulting in decreased permeability of the testa to oxygen sufficient 
to cause the prolonged secondary dormancy observed. 
(ii) Resulting in decreased permeability of the testa to carbon dioxide, 
owing to which the tension of respiratory carbon dioxide in the tissues of 
the embryo during the period of secondary dormancy does not fall below 
