MECHANICS OF DORMANCY IN SEEDS 
III 
of the two seeds with the coats intact and the forcing action of high tem- 
peratures.'^^ As time elapses, either in dry storage or in natural condi- 
tions, the coats become more permeable to oxygen; also the rate 
of oxygen absorption by the naked embryos falls. These facts 
explain in part the timing of the delay in nature and the greater efficiency 
of increased oxygen pressure as a forcing agent at mid-winter than 
immediately after ripening. In nature, however, frosts and many 
other conditions modify the testas; also the embryo may have its 
oxygen minimum lowered by sugar accumulation. For the latter, 
however, we have no evidence. 
Upon the whole the mechanics of the dormancy and of its duration 
is fairly well elucidated for the seeds of the cocklebur. 
Since the discovery of this method of delay in Xanthium, many 
other seeds, especially composites^^ and grasses, 28 have been found 
to possess similar characters. It has been found that in unafter- 
ripened seeds of Chloris ciliata high temperature forces germination 
even in normal oxygen pressure as it does in the intact seeds of cockle- 
bur; while the unafter-ripened seeds of wild oats, Avena fatua, and 
"rain barley" (barley ripening during rainy weather) germinate in 
normal oxygen pressure only at relatively low temperatures. As after- 
ripening progresses the minimum temperature falls in the first and 
the maximum rises in the last two. In Chloris ciliata light and certain 
salts will substitute for increased oxygen pressure. No experiments 
have been performed to elucidate most of these phenomena. It is not 
known for instance whether the vicarious action of light and salts upon 
Chloris is due to effects upon the coat increasing its permeability to 
oxygen, or upon the embryo, lowering its minimum oxygen pressure 
for germination as high temperature apparently does in the cocklebur. 
The disposition of the workers to philosophize rather than to out- 
line and perform searching experiments has proved a great hindrance 
to progress. 
It has been shown that the presence of glucose^^ may substitute 
for oxygen in the growth of certain plant organs as well as in the ger- 
mination of seeds. It is possible that the amount of sugar present 
2^ It is possible that permeability changes in the testa may also be induced by 
temperature changes. 
2^ Becker. Inaug. Diss. Miinster. 191 1. 
2^ Gassner. Jahrb. Ham. Wiss. Anstalten Beih. i. 29: 191 1. 
28Atwood. Bot. Gaz. 57: 386. 1914. 
2^ Lehmann. Jahrb. Wiss. Bot. 49: 61. 1911. 
