458 Kidd and West . — The Controlling 
gas-mixtures. This induced dormant condition of White Mustard seeds 
following as an ‘after-effect of carbon dioxide inhibition is here termed 
‘secondary dormancy J (cf. Crocker (3), p. 114 et seq.). 
Normally, Brassica alba seed germinates quickly and uniformly in 
twenty hours at ordinary temperatures with a high percentage of germina- 
tion. 1 The behaviour of Brassica alba seeds showing secondary dormancy 
as an ‘ after -effect of short C 0 2 -treatment was in striking contrast to this. 
The typical behaviour of a sample of Brassica alba seed removed from an 
inhibiting partial pressure of carbon dioxide is shown by the following 
table : 
Table I. 
Number of days after removal from Number of 
the inhibiting gas-mixture.* germinatio 7 is. 
6 17 
16 49 
44 73 
68 103 
365 135 
Number of seeds used= 153. 
* 20 % C0 2 ; 20 % 0 2 for 19 days. 
In this experiment, which was extended to twelve months, during the 
whole of which period the seeds lay on damp sand in a moist atmosphere, 
it is seen that a few seeds are constantly germinating in a sporadic manner. 
This slow sporadic germination is typical of Brassica alba seeds showing 
secondary dormancy. After twelve months, in this particular case, eighteen 
seeds out of an original sample of 153 inhibiting seeds still remained un- 
germinated. These eighteen seeds finally gave rise to perfect seedlings. 
Occasionally it was found that the secondary dormancy produced was 
not complete. A rush of germinations took place during the first few days 
after the removal of the seeds from the inhibiting gas-mixtures, but after 
this period, however, characteristic sporadic germination set in. For 
example, in ten days 41 per cent, of a sample of eighty inhibited seeds had 
germinated, whilst only 5 per cent, more germinated in the following 
twenty days. 
The object of the work described in the present paper has been to 
discover, if possible, the controlling causes of this secondary dormancy 
which follows as an ‘after-effect of carbon dioxide inhibition in the case 
of Brassica alba. 
§ 2. The Production of Secondary Dormancy in Seeds of 
Brassica alba . 
In view of the fact that the degree of secondary dormancy had been 
observed to vary considerably, a complete series of experiments was 
conducted in order to determine the optimum gas-mixtures with regard 
UThe Brassica alba seeds used in this investigation gave germination tests of 98 % to 100 %. 
