Influence of Carbon Dioxide . IV. 485 
generally, and that in any experimental analysis of the living plant, as a unit 
and in relation to its life-cycle, the idea of limiting factors, which has for so 
long dominated the minds of plant physiologists, will have to be modified. 
§ 6. Conclusions. 
Secondary dormancy in seeds of Brassica alba is not due either to 
increased mechanical restraint of the seed-coats or to decreased permeability 
of the seed-coats to gases. It is due to a stable condition of the embryo 
tissue, which becomes slowly established during the period of primary 
inhibition under the influence of carbon dioxide. This condition appears 
to be comparable to that of mature organs and of embryos maturing on 
the parent plant. The embryos of White Mustard in this stable condition 
(secondary dormancy) do not respond to the ordinary environmental factors 
under which germination and growth will proceed. 
For the initiation of growth (by cell division) a change in the state of 
tissue equilibrium must occur, and this requires a definite stimulus. This 
change in the case of secondarily dormant White Mustard seeds is brought 
about by various treatments which cause injury and death when carried too 
far. The processes involved in the initiation of growth seem to be of the 
same kind as those which produce injury. 
Imperial College of Science and Technology, 
London, 
May, 1917. 
Appendix. 
Structure and Microchemistry of the Testa. 
The complete testa of the young green seed of Brassica alba consists of 
the following layers of cells : 1 
The layers numbered i-iv in Text-fig. 5, A and B, constitute the so- 
called outer testa or seed-coat, which, in the ripe seed, can readily be 
removed from the inner testa or seed-coat. 
i. An epidermis of large cells, which, in the mature seed, undergo 
complete mucilaginous degeneration. 
ii. A layer (1-3 cells in thickness) of large thin-walled cells which also 
undergo complete mucilaginous degeneration when the seed 
ripens. 
iii. A layer of large thin-walled cells which, in the ripe seed, undergo 
partial mucilaginous degeneration. The cell walls of layers i, ii, 
and iii give the staining reactions characteristic of pectin. 
1 For further details regarding the structure of the seed-coat in the genus Brassica the reader is 
referred to Holfert, J. : Die Nahrschicht der Samenschalen, in Flora, Bd. lxxiii, 1890; and to 
Schroeder, J. : Untersuchung der Samen der Brassica- kx\.zw und Varietaten, in Landw. Versuchs- 
Stationen, Bd. xiv, 1871. 
