82 



GROWTH OF PLANTS 



meable membrane is slowly decomposed by organisms in the soil. In other 

 types of seed dormancy to be discussed later, we shall see that microorgan- 

 isms in the soil play an important part in eliminating coats as factors in 

 seed dormancy; moreover, Thornton ^'^° shows that the seed coat of Xan- 

 thium is rendered permeable by very slight injury. Whether the higher 

 temperature increased the permeability of the coats to oxygen, or reduced 

 the oxygen supply or pressure needed for the germination of the embryo 

 of the upper seed and thus forced the germination, was later partly an- 

 swered by Shull and still later by Thornton. 



o 



z 



5 



80 



60 



40 



20 



< 



o 



UJ 



a. 



60 



40 



20 



EMBRCO OF 



LOWER SEED 



AT ^I'C 



EMBRYO OF 



UPPER SEED 



AT 21" C 



EMBRYO OF 



LOWER SEED 



AT 30° C 



EMBRYO OF 



UPPER SEED 



AT 30' C 



0.2 



0.b 0.6 0.7 0.8 0.9 LO 



OXYGEN. PERCENTAGE BY VOLUME 



1.2 



Figure 24. Minimum oxygen required for germination of the naked embryos of cockle- 

 bur during six days at 21° and 30° C (70° and 86° F). 



Shull,!"-'' 1"^ using Schaible's method of reduced atmospheric pressure, 

 determined the minimum O2 pressures under which the naked embryos of 

 the upper and lower seeds of Xanthiuvi would germinate at 21° and 31° C 

 (70° and 88° F); for the upper embryos it is 12 mm at 21° C (70° F) and 

 7 mm. at 31° C (88° F) and for lowers 9.5 and 3 mm at the respective 

 temperatures. The thin coats are extremely effective in reducing the O2 

 absorbed by the embryos in upper seed at 21° C (70° F); 12 mm of O2 

 pressure is required with naked embryos and 760 mm, or 63 times as much, 

 for the intact seeds. Naked lowers as against intact lowers absorb 2)4 times 

 as much O2 and naked uppers 5 times as much as intact seeds. 



Thornton, 120 using full atmospheric pressure with reduced or increased 



