VITALITY OF SEEDS IN SOIL 421 



years. We may recall, too, Darwin's and Guppy's experi- 

 ments and observations, which showed that seeds of many 

 plants could retain their vitality in salt water for as much 

 as a year. 



Retention of vitality in the presence of abundant water, 

 and particularly the failure to germinate, is more difficult 

 to understand than the dormancy of dry seeds. In seeds 

 with impermeable coats it may be due to failure to absorb 

 water while the coat is uninjured. In many cases this 

 explanation fails, and the chief factors must be lack of oxygen 

 and excess of carbon dioxide. The importance of the 

 latter has been emphasised by the work of Kidd (1914, 

 1917), and Kidd and West (1917, 1920) on Brassica alba. 

 They have shown that seeds saturated with water fail to 

 germinate if kept in an atmosphere with a suitable percen- 

 tage of carbon dioxide, the gas having a narcotic effect. 

 At low temperatures and low oxygen pressures narcosis is 

 secured with less carbon dioxide. Many seeds which 

 showed this effect, e.g. beans, cabbage, barley, peas, onions, 

 germinated as soon as they were removed from the inhibit- 

 ing atmosphere. In the mustard, and presumably it is not 

 unique in this, the inhibition continued indefinitely after 

 the seeds were replaced in normal air, and was only re- 

 moved by complete drying and re-wetting, or by removal 

 of the testa. Kidd further found that this type of dor- 

 mancy might be caused by natural conditions in the soil, 

 if abundant organic matter, the decay of which liberates 

 considerable quantities of carbon dioxide, is present. The 

 conditions of low oxygen and high carbon dioxide content 

 in the mud of ponds, or in forest soil, may thus induce 

 dormancy. When the mud or soil is turned up, partially 

 dried, and exposed to free aeration, the inhibiting con- 

 ditions are removed and germination may take place. We 

 may here recall Kidd's suggestion that the arrest of deve- 

 lopment of the embryo in the maturing seed is due 

 primarily, not to desiccation, but to the accumulation of a 

 narcotising concentration of carbon dioxide within the testa. 



Biological Effects. — The capacity of the dry seed to 



