DORMANCY IN SEEDS .123 



in the various phases of after-ripening and growth of seeds, such a con- 

 stant intermediate temperature could be used to consummate the complete 

 process; but nature in the temperate zone does not deal in constant tem- 

 peratures and the practical grower cannot easily maintain such a constant 

 temperature with other necessary growth conditions, nor can he wait so 

 long for the plants. 



The facts of seed dormancy stated m this and in previous sections show 

 the inadequacy of ending germination studies with the mere growth of the 

 root. Studies thus terminated would entirely miss epicotyl dormancy and 

 the dw^arfishness in seedlings due to the growth of non-after-ripened epi- 

 cotyls. We always follow the growth of the seedling for many months or 

 even years to catch any pecuharities in the later development of the seedlings 

 that might result from seed dormancy. 



A Period or Dry Storage After-Ripens Many Seeds 



A period of dry storage may be quite as important a factor in after- 

 ripening dormant seeds as low-temperature stratification, especially if one 

 considers the number of species that respond to each treatment. Many 

 light-sensitive seeds -^ lose then- light-sensitiveness partially or completely 

 if kept for several months in dry storage. Viscum album and Arceuthobium 

 oxycedri seeds are notable exceptions, for the first is killed by continuous 

 darkness, and neither germinates under any conditions without light. 

 There are probably other exceptions 'among light-sensitive seeds. Most 

 seeds of culti\'ated grains and other grasses after-ripen m dry storage. In 

 the cereals the period of after-ripening varies ^\'ith species, varieties, and 

 races from a few days to several months. According to Pietruszczynski,^^ 

 it is longest in oats, shorter in barley and wheat, and shortest in rye. The 

 period is shorter in whiter than in spring cereals, in early than in late 

 ripening varieties, and in dry than in wet seasons. In wild grasses, several 

 months of dry storage are generally required for complete after-ripening. 

 Most weed seeds studied in this respect after-ripen m dry storage. Even 

 seeds ^^^th dormant embryos ~^' ^^ can be at least partially after-ripened by 

 dry storage, although this treatment is not nearly as effective as low- 

 temperature stratification. 



Kroeger ^^ has recently studied the progressive after-ripening of Impa- 

 tiens halsamina seeds with increasing periods of dry storage. An examina- 

 tion of lot 1 in Fig. 49 shows that, as the dry storage period mcreased from 

 weeks to 43 weeks, the speed and total percentage of germination m- 

 creased. The fresh seeds gave 32 per cent germmation after 20 weeks in 

 the germinator, while the seeds that had been dry-stored for 43 weeks gave 

 100 per cent germination in one week. Even four weeks of dry storage 

 raised both the speed and final germination, which reached 70 per cent 

 after 20 weeks. The speed and final percentage of germination rose as 

 the dry storage period was lengthened progressively to 9, 16, and 25 wrecks. 



