126 



GROWTH OF PLANTS 



volatile inhibitors if they are present. Gadd ^^ thinks that at first seed 

 coverings are alive and thus deprive the embryos of oxygen, and that when 

 the coats die oxygen gets to the embryo. We have found that freshly har- 

 vested lettuce seeds that will not germinate at 30° C (86° F) in air also 

 will not germinate at this temperature under one or two atmospheres of 

 pure oxygen. They will, however, germinate promptly at this temperature 

 if the coats are broken. It might seem that a 10-fold increase in oxygen 

 pressure would be sufficient to allow some to reach the embryos. Death 

 of living cells in the coats might also favor the outward diffusion of in- 

 hibitors. 



Table 17. Percentage of Water Held by Seeds of Impatiens balsamina after Different 

 Periods of Soaking; Freshly Harvested Seeds; Seeds Dry-Stored 8 Weeks. Dry Weight 

 Basis. Temperature of Soaking 20° C. Weighings Made on DupUcate Lots for Each 



Soaking Period. 



* Maximum water absorption. 



Barton, in unpublished work at the Institute, has found that certain 

 seeds that after-ripen in dry storage increase greatly in the initial rate of 

 water absorption and somewhat in the final total amount held by them 

 when fully imbibed as the dry storage period increases. Table 17 shows 

 this relation for Impatiens seeds freshly harvested and dry-stored in the 

 laboratory for eight weeks. It will be noted that freshly harvested seeds 

 bore 12.1 per cent water while the dry-stored seeds bore 6.2 per cent. In 

 spite of this initial difference, after three hours' soaking, the dry-stored 

 seeds contained 23.0 per cent water against 20.9 per cent for the freshly 

 harvested; in three hours of soaking the dry-stored seeds had absorbed 

 16.8 per cent of their dry weight while the fresh ones absorbed 8.8 per 

 cent. She got similar results with Rumex, Amaranthus, and lettuce seeds. 

 The initial rate of water absorption and the total amount absorbed increase 

 with the period of dry storage as after-ripening progresses, and is above 

 that which occurs from drying without after-ripening. 



While the statements above throw some light on the changes involved 

 in after-ripening of seeds in dry storage, the great number and range of 

 species and varieties of seeds in tliis category demand that many of these 

 seeds, including several representatives from every family of plants in- 

 volved, be examined in the light of all the hypotheses and facts mentioned 

 above, and in the light of new hypotheses that will be suggested by such 

 investigations. 



