484 
Kidd and West . — The Controlling 
dormant seeds have become more stable and less responsive to the condi- 
tions under which growth is usually initiated. The bare embryos of secon- 
darily dormant seeds continue dormant under appropriate conditions after 
the removal of the seed-coats. This change in the embryo appears to 
be due to the action of carbon dioxide during the period of primary 
inhibition, as it cannot be induced by restraining germination by other 
means, e. g. by submitting the seeds to low temperatures. 
The embryo enclosed within the seed-coats is not completely swelled, 
but the rupture of the seed-coats when germination takes place, both in the 
case of normal seeds and in the case of seeds showing secondary dormancy, 
is due to a process of growth. 
The dormant condition of the growing tissues of the embryos of seeds 
showing secondary dormancy is broken up by treatments which are injurious, 
but not fatal. Treatments which actually kill the meristematic cells of the 
root-tip whilst still enclosed within the seed-coats cause the adjacent cells 
of the hypocotyl to start growing, and germination, in which the root-tip is 
absorbed, results. 
As a corollary, if the conditions during the primary period of carbon 
dioxide inhibition are injurious, either owing to lack of oxygen or to excess 
of carbon dioxide, secondary dormancy does not ensue. In consequence, 
a high percentage of secondary dormancy can only be produced by a 
limited range of carbon dioxide and oxygen mixtures (i. e. 30 per cent, to 
30 per cent. C 0 2 and not less than 15 per cent. 0 2 ). 
Embryos in their completely swollen condition still showing secondary 
dormancy after the removal of their seed-coats are more sensitive to growth 
stimuli than those still enclosed in their seed-coats and consequently not 
completely swelled. While in the latter case germination can only be induced 
by conditions which just fall short of producing visible injurious effects, in the 
former case it is only possible to maintain the secondary dormancy by 
removing the testa with the greatest care in avoiding as far as possible 
pressure torsion or abrasion. 
It will be seen that the main interest of this communication centres 
round the causes underlying the initiation of growth rather than in the 
condition of dormancy. In considering this question of growth in the case 
of Brassica alba seeds, our experiments show clearly that there is no ques- 
tion of limiting factors. We have been able to trace no limiting factor re- 
sponsible for the non-germination of White Mustard seeds showing secondary 
dormancy. We find ourselves rather in the presence of facts which empha- 
size a conception of stimulus. It has been seen that widely different treat- 
ments, quite unclassifiable in any feature other than that they all result in 
injury and death if carried too far, excite germination and growth of 
dormant White Mustard seeds. It appears to us probable that some return 
will have to be made to this conception of stimulus in plant physiology 
