47 1 
Influence of Carbon Dioxide . IV. 
The conclusion indicated is that neither lack of oxygen nor accumulation 
of carbon dioxide in the embryo controls the phenomenon of secondary 
dormancy. This conclusion is confirmed by the results of experiments 
described in a following section, in which it was found that germination of 
inhibited Brassica alba seeds could not be forced either by increasing the 
oxygen pressure or by lowering the internal carbon dioxide concentration 
in the seeds by exposure to a vacuum (see Table XXII). 
Changes in the Mechanical Resistance of the Seed-coats. 
Crocker and Davis (5) have shown in the case of Alisrna Plantago , 
a water-plant, the seeds of which normally lie under water, that while the 
seed-coat is readily permeable to water, the factor responsible for the non- 
germination of these seeds is the mechanical resistance of the testa to 
physical swelling of the embryo, whereby the latter remains dormant owing 
to an insufficient water content for growth and germination. When the 
testa is weakened by treatment with acids the expanding force of the seed- 
contents is sufficient to rupture it. Further swelling and uptake of water 
by the embryo can then take place, and a point is finally reached at which 
growth begins. The same result is obtained by experimental rupture of the 
seed-coat. 
In the normal germination of White Mustard seeds it is clear that the 
rupture of the seed-coats by the radicle takes place as the result of growth, 1 
and not as the result of physical swelling of the embryo. This may be 
concluded from the fact that no rupture of the seed-coats occurs in seeds 
which are prevented from germinating by lack of oxygen. 
It is conceivable, however, that the action of carbon dioxide may 
toughen the testa, with the result that seeds set to germinate under inhibiting 
conditions of carbon dioxide do not swell to the point at which sufficient 
water is present for the growth of the embryo after removal from the 
inhibiting gas mixture. Direct experiment proves, however, that this is not 
the case. The following table compares the weight of fully swollen normal 
seeds, just before and just after germination respectively, with that of 
inhibited seeds. The latter are on the whole slightly heavier. 
Table XIV. 
Comparison between the Weight of Inhibited Seeds from Damp Sand and 
fully swollen (20 per cent, actually sprouting) N on-inhibited Seeds. 
Weight of 200 
inhibited seeds. 
Weight of 200 fully swollen non - 
inhibited seeds ( soaked 
for 20-25 hours'). 
2^41 grin. 
2*51 grm. 
2 ‘40 grm. 
Average— 2*42 grm. 
2*41 grm. 
2*41 grm. 
2*24 grm. 
Average = 2 *35 grm. 
2‘io grm. All germinated. 
1 Histological examination of seeds during the period immediately preceding germination 
showed that the root forces its way through the seed-coats by growth pressure. No enzyme action 
was indicated. 
