DORMANCY IN SEEDS 125 



for partially after-ripened seeds the percentage and speed of germination 

 was increased by two weeks' moist stratification at 5° C (41° F). Various 

 grass seeds respond similarly to both dry storage and low-temperature 

 stratification, or prechilling. 



Obviously, we would like to know the essential changes that occur in 

 dormant seeds in dry storage that enable them to germinate. We asked 

 the same question about low-temperature stratification and found the 

 answers only in small part satisfactory. Answers to this question are also 

 only partially satisfactory. There are, however, a number of hypotheses 

 and facts that throw some light on the question. On the basis of culture 

 of excised embryos from dormant and after-ripened barley grains on nutri- 

 ent gelatin and on the basis of transplanting embryos from dormant and 

 after-ripened grains on endosperm from dormant and after-ripened grains 

 in all possible combinations, Windisch ^-^ concluded that the embryos 

 from dormant grains themselves show dormancy, and that during dry 

 storage of the grains the epithehum of the scutellum is essentially modified. 

 There is much evidence that dormancy in many kinds of seeds of the grass 

 family is determined by intactness of the coats, and that breaking the 

 coats causes prompt, vigorous germination giving no evidence of embryo 

 dormancy (Crocker and Harrington ^^ for Johnson grass; Harrington®' 

 for wheats, oats, and barley; and Atwood ^ for wild oats). Many investi- 

 gators have found that dormant cereals mil germinate promptly if placed 

 in a germinator at lower temperatures, such as 10° to 12° C (50° to 54° F) 

 or 14° to 15° C (57° to 59° F). Flemion, in preliminary unpublished work 

 at the Institute, has found that wheat and oats gro\\Ti from dormant 

 grains, by opening the coats, show the same vigor in the later growth of 

 fhe seedlings as those grown from after-ripened grains. As we have seen 

 from Flemion's work cited above, dwarfishness or lack of dwarfishness in 

 the seedling is the best criterion for embryo dormancy. 



There is the possibility that freshly harvested seeds contain inhibiting 

 substances that volatilize or decompose during dry storage. Shuck '*'^* '*'* 

 finds that fresh lettuce seeds form inhibiting substances when put into a 

 germinator at a high temperature, 25° C (77° F), in darkness. Repeated 

 growth of batches of seeds on the same filter paper leads to the accumula- 

 tion of inhibiting substances on the paper that inhibit later sowings. 

 Water and soil are more favorable germination media for these seeds than 

 filter paper. He believes this is the case because they have greater power 

 to absorb the inhibitors. Even soaking in cold water seems to remove the 

 inhibitors. He also finds that light and low-temperature germinators over- 

 come the inhibitors. We have already discussed other cases where inhibit- 

 ing substances prevent germination. This hypothesis deserves serious 

 examination in all seeds that after-ripen in dry storage. 



Cereal grains are hastened in after-ripening by heating at 35° to 40° C 

 (95° to 104° F) ■* for 2 to 4 days; the heating is effective without drying, 

 although dry heat adds to the effectiveness. Of course, heat will drive off 



