MECHANICS OF DORMANCY IN SEEDS 
of Chloris ciliata by a period at 20° C. in a dark germinator; KinzeH° 
in the dark requiring seeds of Nigella saliva by a period in an illum- 
inated chamber at 20° C; Crocker and Harrington''^ in Johnson grass 
(Sorghum halepense L.) and Davis and Crocker^i in Amaranthus 
retroflexus by a time in the germinator at subminimal temperatures; 
Kidd in white mustard by exposure of imbibed seeds to sufficient 
partial pressure of carbon dioxide and Crocker and Davis^o in the acid 
treated seeds of Alisma plantago by treatment with almost any con- 
centration of copper sulfate. In all these cases except Nigella the 
authors find that the modification is in the seed coats and that the 
embryos are at no time dormant. This interpretation for Nigella 
accords with the known facts quite as well as Kinzel's interpretation. 
Except for Nigella, on which experiments relative to this point have 
not been made, removal or rupture of the coats gives immediate and 
vigorous germination and in white mustard drying produces the same 
effect. 
Some evidence has been gained regarding the nature of the changes 
involved in dormancy production, but there is need of much more work 
on this point. In some cases it involves such simple reversible proc- 
esses as the stoppage of free gaseous exchange by the filling of capillary 
spaces with water. in other cases pectic or other gel-like materials fill 
the spaces. 43 In Alisma planlago the copper ions harden the coat col- 
loids, increasing their breaking strength and rendering impossible the 
further osmotic and imbibitional swelling of the embryo. In white 
mustard Kidd believes the carbon dioxide lowers the permeability of the 
coat to gases, thus hindering the free elimination of carbon dioxide and 
absorption of oxygen. The carbon dioxide narcotizes the embryo and 
the reduced oxygen supply is important in lowering the amount of 
carbon dioxide necessary for narcosis. Kidd^ has shown that the 
carbon dioxide content of soils is sometimes sufficient to induce 
dormancy in white mustard. This factor is, therefore, operative in 
natural conditions. It is probably limited in its significance to 
relatively few seeds, for many seeds (cabbage, onion, barley, beans, 
and peas) cannot be thrown into dormancy by any partial pressure 
of carbon dioxide. 
In Amaranlhus relroflexus^^ I believe dormancy production is the 
Kinzel. Ber. Deutsch. Bot. Ges. 25: 269. 1907. 
Unpublished Work at the Seed Laboratory of U. S. Department of Agriculture. 
42 Haberlandt. Bot. Jhrb. 3: 857, 1875. 
4^ Windisch. Wochenschrift Brauerei, 22: 89. 1905. 
