4§3 
Influence of Carbon Dioxide. IV. 
can be induced by acid concentrations which are actually toxic to the 
growing radicle, or by concentrations which closely approximate to this. 
In other words, the conditions of acid concentration on the threshold of 
injury to the growing radicle will cause germination. 1 
In view of the results obtained with the above acids, and of the 
previous results in which it has been shown that following treatment with 
high concentrations of carbon dioxide secondary dormancy does not occur, 
experiments were conducted to test whether treatment with a high con- 
centration of carbon dioxide would terminate the secondary dormancy of 
seeds inhibited by a lower concentration. Positive results were obtained. 
Treatment of seeds which had been inhibited by 25 per cent, carbon 
dioxide and 20 per cent, oxygen for twelve days, and which had lain 
dormant for two weeks subsequently in air, by an immersion for seven 
days in 100 per cent, carbon dioxide was followed by 95 per cent, 
germination in two days. The seeds not treated with 100 per cent, carbon 
dioxide remained dormant. 
§ 5. Summary. 
The presence of carbon dioxide inhibits the germination of seeds, and 
the concentration of carbon dioxide necessary is correlated with temperature 
and with oxygen supply as previously described (2). 
In the case of Brassica alba , the primary effect of carbon dioxide in 
causing inhibition of germination is followed by a secondary effect of 
prolonged dormancy after the carbon dioxide has been removed. This 
phenomenon has been termed secondary dormancy. 
Changes in the seed-coat occurring during the period of primary 
inhibition have been suggested as the immediate cause underlying the 
phenomenon of secondary dormancy in White Mustard seeds, namely, (a) 
a decreased permeability of the seed-coats to carbon dioxide or to oxygen 
occurring either as a result of a process of hysteresis in the colloids of the 
seed-coats (cf. Crocker, 3 ) or as a result of the specific action of carbon 
dioxide (Kidd, 1 ), or (< b ) an increase in the mechanical resistance of the seed- 
coats. No evidence was found in support of either of these hypotheses. 
It was found, on the other hand, that the embryos of secondarily 
1 The interesting fact is to be recorded here that the ungerminated seed is far more resistant to 
acid injury than the growing radicle. Our experiments with carbon dioxide constantly demonstrated 
this fact ; at ordinary temperatures (20° C. circa), any germinations that occur in the presence of 20 
per cent, carbon dioxide (20 per cent, oxygen present) show marked injury to the radicle. Further, 
embryos, removed from their seed-coats, which can sprout in the presence of this concentration of 
carbon dioxide, invariably show injury after ten days’ immersion. Again, the radicles of seeds which 
are just sprouting when placed in 20 per cent, or in higher concentrations of carbon dioxide (20 per 
cent, oxygen present) always suffer injury. In contrast, seeds inhibited in 20 per cent, carbon dioxide 
(20 per cent, oxygen present), when finally induced to germinate, show no signs of injury to the 
radicle. Concentrations of carbon dioxide up to 100 per cent., provided that the period of immersion 
does not exceed six days, and longer periods in the case of lower percentages, cause no injury to 
ungerminated seeds whether previously inhibited or not. 
