DORMANCY IN SEEDS 79 



Brown ^^ estimates from his experiments with barley grains that embryos 

 in partially imbibed grains floating on water are in equilibrium with 10 per 

 cent of oxygen, whereas excised embryos are in equilibrium with the full 

 percentage of oxygen of the air. The rate of uptake of oxygen and release 

 of carbon dioxide was much higher in the excised embryos. This no doubt 

 is partly due to the higher water content of the excised embryos as well 

 as to higher oxygen pressure. Brown ^^ has also shown that the imbibed 

 seed coat of Cucurhita pepo permits carbon dioxide to diffuse through it 

 several times as fast as oxygen. StMfelt "^ shows that the amount of 

 oxygen taken up by white mustard seeds in a germinator is increased if 

 the partial pressure of oxygen is increased from 20 to 50 per cent. The 

 increase is much greater for the cotyledons than for the root. 



The researches just cited, as well as a number of others that might be 

 cited, show that some imbibed seeds are limited in their use of oxygen 

 from the air because of the low permeability of the coats to oxygen. Are 

 there cases in which seeds stay dormant because oxygen does not pass 

 through the coats sufficiently fast to permit germination of the embryos 

 even when they have good aeration? If this is true, two conditions must 

 be fulfilled in such seeds: (1) the embryo must have a certain oxygen pres- 

 sure demand in order to grow, and (2) the low permeability of the coats 

 must limit the oxygen pressure to the embryo below the minimum neces- 

 sary for growth. 



It is well established that embryos of various kinds of seeds vary greatly 

 in the oxygen pressure or oxygen supply needed for germination. One 

 might expect seeds of water plants that normally germinate under water 

 to have low oxygen requirement for germination. Crocker and Davis -^ 

 found that embryos of seeds of Alisma Plantago, with coats broken, will 

 germinate in absence of oxygen. They heated the seeds in water in special 

 flasks at 35° C (95° F) for 30 minutes under reduced air pressure of 0.1 mm 

 of mercury and sealed the flasks at this temperature and vacuum. In the 

 vacuum cultures in water the embryos grew in length 1100 to 1200 per 

 cent in 21 days, while in the checks in water the growth in length was 

 1800 to 2200 per cent. In the vacuum cultures no leaf branches and no 

 chlorophyll formed. Both developed in the controls. About 5 mm of air 

 pressure were required for chlorophyll formation, and more than 5 cm of 

 air pressure for leaf differentiation. While germination occurs in absence 

 of oxygen, the growth is limited and differentiation and chlorophyll forma- 

 tion do not occur. Takahashi ^^^ claims that the plumule in rice, another 

 water plant, will grow in absence of oxygen, but the root will not. Taylor ^'^ 

 determined the effect of various oxygen pressures on the germination of 

 rice, as a water plant, and wheat, as a land plant. He says (p. 736): "In 

 the absence of O2 the germination of rice seeds was reduced less than 

 1 per cent below that in air and was accomplished at more than half the 

 normal rate. No germination of wheat occurred under similar conditions. 

 Significant reduction in the extent and rate of germination of wheat 



