DORMANCY IN SEEDS 85 



The oxygen pressure relation to germination is not as simple as it might 

 seem from the statements above. Thornton found that high percentages 

 of carbon dioxide, especially 40 to 80 per cent, lowered greatly the mini- 

 mum oxygen pressure needed for the germination of intact upper seeds at 

 25° C (77° F). He did not determine whether this increased the perme- 

 ability of the coats to oxygen or lowered the oxygen pressure needed by 

 the embryo for germination as does a rise in temperature. Harrington ^^ 

 found that 60 to 80 per cent carbon dioxide is effective in forcing dormant 

 Johnson grass seeds in which oxygen supply is not a limiting factor. Later, 

 Thornton ^-^ found that similar concentrations of carbon dioxide were 

 effective in forcing the germination of intact lettuce seeds at 35° C (95° F), 

 a temperature many degrees above the maximum germination tempera- 

 tures in absence of carbon dioxide. 



As we have seen above, the upper seed of Xanthium is almost unique 

 among seeds, in that the thin coat reduces the oxygen supply below the 

 minimum needed for germination at lower temperatures. It is like^\^se 

 peculiar in that it prevents the germination at low temperatures but not 

 at high temperatures, while in the other two seeds studied in this respect. 

 Ambrosia and lettuce, the thin coats prevent the germination at higher 

 temperatures but not at lower temperatures. 



Dormant Seeds that Respond to a Single Period of Moist Low-Tempera- 

 ture Stratification 



There is probably no other condition — unless it is breaking the coats, 

 which is mainly impractical — that will overcome the dormancy of as 

 many different kinds of temperate-zone seeds as placing them in germina- 

 tion conditions at a low temperature for periods varying from a few days 

 to many months, according to kind and condition of the seeds to be treated. 

 In the earlier and century-old practice, the seeds and sand were laid do^\Ti 

 in successive horizontal layers and the stratified mass exposed to low 

 temperatures during the winter; hence the term "stratification." In recent 

 practice the seeds are mixed ^\^th moist sand, granulated peat, or other 

 medium and exposed to low temperature for the desired time. Many 

 workers still use the term "low-temperature stratification" for the newer 

 practice. For seeds of many water plants, which need little oxj'^gen, water 

 is a good stratification medium. Low-temperature stratification imitates 

 nature's methods of after-ripening seeds in the temperate zone; the seeds 

 fall to the ground in the fall and are more or less covered in the cold soil 

 during winter. Artificial stratification has the advantage of making pos- 

 sible the holding of the several stratification factors (temperature, mois- 

 ture, and oxygen) at the optimum, which produces the quickest possible 

 results. In nature these factors, especially temperature, are at the opti- 

 mum only a portion of the time. Fall, winter, and early spring sowing of 

 many seeds that respond to low-temperature stratification is a fair substi- 



