84 GROWTH OF PLANTS 



for the lower; that the minimum oxygen pressure needed for the germina- 

 tion of the naked embryos falls decidedly with, a rise of 9° to 10° C (16° to 

 18° F) in temperature; that the intact coat of the upper seeds, through its 

 low permeability, increases the required oxygen pressure for germination 

 more than 60-fold ; and finally, that one factor leading to the germination 

 of the intact upper seed in air at higher temperatures is the lower minimum 

 oxygen pressure required for the germination of the embryo. Crocker, 

 Shull, and Thornton all agree that the failure of the upper intact seed to 

 germinate in air at temperatures below 30° C (86° F) is due to the low 

 permeability of the seed coat to oxygen; hence the upper seed remains 

 dormant in the soil at lower temperatures. It is of interest but of no known 

 significance in dormancy that on an equal dry weight basis the embryo of 

 the lower seed is richer in catalase "^ than that of the upper seed. On the 

 basis of his data, Shull agrees with Crocker in concluding that oxygen 

 has its function in the germination of cocklebur seeds in producing sufficient 

 aerobic respiration to furnish the necessary energy for growth and that it 

 does not act merely as a. stimulus, as Becker, Lehmann, and others have 

 assumed. The cocklebur embryos probably rank high among seeds in their 

 need for the energy from aerobic respiration for growth. 



A few other dormant imbibed seeds have been found in which increased 

 oxygen pressure will force them to grow. Atwood ^ and Johnson ^^ have 

 found that dormant, recently harvested wild oat grains are forced to ger- 

 minate by increased oxygen pressure, and Harrington *^ has found the 

 same for freshly harvested cereals. Spaeth,"- for American basswood, and 

 Stier,"^ for freshly harvested potato seeds, find that the portion of the 

 seed coat derived from the nucellus inhibits the passage of oxygen. A 

 number of other similar cases could be mentioned. For years we have 

 been studying the mechanics of dormancy in seeds in the seed laboratory 

 at this Institute and we always try increased oxygen pressure for forcing. 

 We have found very few cases where this is effective in contrast to the 

 many kinds of seeds that are dormant because of hard coats, because of 

 dormancy of the embryo, or because of the coats limiting the absorption 

 of water. The findings for the cocklebur, as interesting and definite as 

 they are, may not explain the dormancy of any considerable number of 

 different kinds of seeds. 



There is another way in which oxygen pressure is involved in dormancy 

 of seeds. As described in a previous chapter, Davis -^ and Thornton i-" 

 have shown that Xanthium and Ambrosia embryos can be thrown into 

 dormancy by keeping them in germinators at higher temperatures with 

 subminimal oxygen pressure for germination. In the case of after-ripened 

 seeds of Ambrosia, the thin seed coats reduced the oxygen supply to the 

 embryos sufficiently at high temperatures to induce dormancy. In Xan- 

 thium reduced oxygen pressure was necessary, in addition to the intact 

 coats. As mentioned in the last chapter, other unfavorable factors in the 

 germinator also induce secondary dormancy. 



