RESPIRATION AND OXYGEN. 77 



respectively. The seeds of cocklebur can not germinate without 

 relatively large amounts of oxygen, being opposite in this respect 

 to seeds of Alisma and rice. They do not support the view that 

 the seeds of higher plants can grow in the entire absence of oxygen. 



Babcock (1912 : 99) determined that corn containing 6 to 8 per 

 cent of water kept for a year in C0 2 germinated more slowly than that 

 kept in the air, while corn with 30 per cent of water kept in carbon 

 dioxid for 8 to 12 months entirely failed to germinate. Kernels of 

 corn immersed for one or more days in water boiled free from air did 

 not germinate until oxygen was supplied. 



Becker (1912 :21) demonstrated that seeds of Dimorphotheca 

 germinated more readily in oxygen than in air, especially those from 

 the ray-flowers. Exposure to oxygen for 30 hours was found to pro- 

 mote germination in the air. When the seed-coats were removed, 

 germination was favored by 10 hours' exposure to oxygen, but de- 

 layed by exposure for 13 hours. The normal seeds of Calendula 

 germinated much more readily in oxygen, while those of Atriplex 

 were injured by increased oxygen-pressure. 



Atwood (1914 : 386) concluded that the delay in the germination 

 of the seeds of Avenafatua was due to the oxygen supply as a limiting 

 factor. This is indicated by the results obtained by breaking or 

 searing the seed-coats or the removal of the embryo, by the amount 

 of germination occurring in different concentrations of oxygen, and 

 by measuring the rate of oxygen absorption in seared and intact 

 seeds. The increased permeability of the seed-coat to oxygen was 

 regarded as a factor in the process of after-ripening. 



Pack (1921 : 41) has shown that the catalase activity of seeds 

 stored 45 days at 25 C. rises steadily with the increase of oxygen 

 from 30 to 100 per cent and that the activity at 80 per cent equals 

 that in air. 



Kidd (1914) has found that C0 2 in relatively small quantities in 

 the atmosphere inhibits germination, the actual percentage varying 

 with temperature and oxygen supply. Inhibition was produced by 

 2 to 4 per cent at 3 C., while 25 to 30 per cent was required at 20 C. 

 With 5 per cent oxygen, 9 to 12 per cent C0 2 brought about inhibi- 

 tion, while 20 to 25 per cent was required with 20 per cent oxygen. 

 In all seeds tested, except Brassica alba, germination followed nor- 

 mally when CO 2 was removed. 



Kidd and West (1917 : 457) have shown that the effect of C0 2 

 in inhibiting germination in Brassica alba is followed by a secondary 

 effect of prolonged dormancy after the gas is removed. If the con- 

 ditions during the primary period of C0 2 inhibition are injurious, due 

 to lack of oxygen or excess of carbon dioxid, secondary dormancy 

 does not occur. A high percentage of the latter can only be produced 

 by a mixture of 20 to 30 per cent C0 2 and not less than 15 per cent 

 oxygen. Secondary dormancy in the seeds of Brassica alba is not 



