466 Kidd and West . — The Controlling 
a critical value in relation to oxygen supply and temperature at which 
germination can take place. 
(iii) Resulting in increased mechanical resistance, owing to which 
either ( a ) the full swelling of the embryo by physical water uptake to the 
critical point at which germination and further growth become possible is 
prevented, or ( b ) owing to which the radicle is unable to burst its way 
through the seed-coats by growth although the water supply is not the 
limiting factor. 
B. Changes in the embryo during the period of primary inhibition in 
the inhibitory gas-mixtures, owing to which the embryo becomes less 
sensitive to growth conditions ; in other words, there is a rise in the 
threshold value of the necessary growth stimulus, so that the same value in 
the case of some critical growth condition (e. g. of oxygen supply, C0 2 
tension, moisture, temperature, hydrogen ion concentration, or other internal 
factor), which in the case of the untreated seeds was sufficient, although 
near the critical minimum, to cause germination, becomes subminimal after 
treatment with carbon dioxide. 
Changes in the Permeability of the Seed-coats to Gases . 
In order to test whether any change in the permeability of the testa to 
gases takes place during the immersion of the seeds in atmospheres of 
carbon dioxide, direct experiments were carried out. These were conducted 
in two series. In Series A the permeability of the testa to carbon dioxide 
was determined, whilst in Series B the object of the experiments was to 
ascertain the permeability of the seed-coats to oxygen. 
Series A. 
The principle of these experiments was as follows : 
A number of fully swollen White Mustard seeds, inhibited and non- 
inhibited respectively, were brought suddenly into an atmosphere of pure 
C0 2 ; then the rate of C0 2 uptake was compared in either case. When no 
further uptake occurred, the atmosphere of C0 2 was quickly replaced by 
an atmosphere of nitrogen and the rate at which the C0 2 escapes was 
measured. 
Fig. 2, a , represents the apparatus employed. The seeds are placed in 
the wide-bore tubes A and B, which are then connected to the capillary tubes 
C and D by means of rubber connexions as is shown in the diagram. The 
stop-cocks E and F are opened and a rapid current of C0 2 is turned on 
from G. The stop-cocks are then shut and the mercury bath H is raised 
into position. When absorption of C0 2 is complete, a current of nitrogen 
is passed through the apparatus in the same way, but after bringing the 
mercury bath into position an initial negative pressure is created by suction 
before shutting the stop-cocks. A modified form of the -apparatus, in which 
the rubber connexions are avoided, is also shown in Fig. 2 , b . 
