DORMANCY OF SEEDS 633 



seeds at the time of ripening since the seed coats are less resistant at these 

 temperatures. As the seeds age, the minimum temperature for germination 

 becomes lower. The embryos of these seeds have no dormant periods and 

 will grow readily if the seed coats are removed. Likewise any treatment which 

 weakens the seed coats increases the percentage of seeds that germinate. In 

 general, other seeds with mechanically resistant seed coats exhibit similar be- 

 havior. 



3. Seed Coats Impermeable to Oxygen. — The two seeds in a fruit of cockle- 

 bur {Xanthium sp.) are not equally dormant. Under natural conditions the 

 lower seed usually germinates in the spring following maturity while the 

 upper seed remains dormant until the next year. The dormancy of these 

 seeds has been demonstrated to result from the impermeability of the seed 

 coats to oxygen (Shull, 191 1). If the seed coats are ruptured, or if the 

 oxygen pressure is increased around intact seeds, germination occurs. The 

 oxygen requirements for germination are greater in the upper seed than in 

 the lower and this explains the more pronounced dormancy of the former. 

 During dry storage or under natural conditions the seed coats gradually be- 

 come more permeable to oxygen and the oxygen requirements of the embryo 

 decrease. Hence the intensity of the dormant condition gradually diminishes. 

 The seeds of a number of grasses and of many Compositae also have dormant 

 periods that seem to be due to the impermeability of the seed coats to oxygen. 



4. Rudimentary Embryos. — Many species of plants have seeds in which 

 the embryo does not develop as rapidly as the surrounding tissues so that 

 when the seeds are shed the embryos are still imperfectly developed. In 

 some species the ripened seeds contain embryos that have grown little bej^ond 

 the fertilized egg stage while in other species development of the embryos 

 may be nearly complete when the seeds are shed. The germination of such 

 seeds is necessarily delayed until formation of the embryo is complete. Ex- 

 amples of species in which dormancy of seeds is due to incompletely developed 

 embryos include ginkgo {Ginkgo biloba), European ash {Fraxinus excelsior), 

 holly {Ilex opaca) and many orchids. 



5. Dormant Embryos. — In many species although the embryos are com- 

 pletely developed when the seed is ripe, the seeds fail to germinate even 

 when environmental conditions are favorable. Dormancy of such seeds is 

 due to the physiological condition of the embryo. The embryo of such seeds 

 will not grow when the seeds first ripen even if the seed coats are removed. 

 Among the many species whose seeds exhibit dormancy of this type are apple, 

 peach, hawthorne, iris, lily-of-the-valley, basswood, ashes, tulip poplar, dog- 

 wood, hemlock, and pines. Germination of such seeds occurs only after 

 a period of "after-ripening." In many wild species after-ripening occurs 



