LIFE SPAN OF SEEDS 



47 



completely after three months in a germinator at 1° to 10° C (34° to 50° F). 

 After the seeds are after-ripened they will germinate slowly but nearly 

 completely in a germinator at 20° C (68° F) and more rapidly but more 

 sparingly in a germinator at 30° C (86° F) . At the higher temperature 

 many of the seeds go back into the dormant condition. The secondary 

 dormancy is induced by the seed coats restricting the oxygen supply to 

 the embryo in a germinator at a high temperature. 



Fig. 12 pictures the fruit, seed, embryo, and seed coat structure of Am- 

 brosia. Fig. 13 shows the behavior of isolated embryos of Ambrosia when 



/ 



/ 



A ^ 



Figure 14. A, seeds of cocklebur in clay in a high temperature germinator to produce 

 dormancy; B, imbedded in agar for the same purpose. 



placed in a germinator for three to four days at room temperature after 

 three months' storage of fruits m three different conditions. Note that the 

 embryo of intact fruits in a germinator at low temperature, 1° to 10° C 

 (34° to 50° F), after-ripened so that they germinated promptly when the fruit 

 and seed coats were removed and the embryos were placed in a germinator 

 at room temperature. Those which had been kept in a germinator at a 

 low temperature gave prompt, complete germination; those in dry storage 

 a small percentage germination; and those in a high-temperature germi- 

 nator no germination. The embryos of the latter were in deep dormancy. 

 The embryos of the cocklebur -^ are also enclosed in very thin seed coats 

 that interfere with oxygen diffusion to the embryos. They differ from 

 Ambrosia in three ways: the involucral fruit coat (bur) encloses two seeds, 

 an upper and lower; the embryos are not dormant in the mature seeds, but 

 the thin seed coats hinder germination by limiting oxygen supply to the 

 embryos at certain temperatures, that is, below 20° C (68° F) for lower 



