LIFE SPAN OF SEEDS 



45 



For such inhibiting chemicals to be effective in holding seeds dormant 

 in the soil for years, two conditions \\ill have to prevail: (1) the inhibiting 

 chemicals must be very stable in the soaked-up seeds, and (2) they must 

 be prevented from diffusing out of the seeds by semipermeable membranes 

 of either the living protoplasm or of the non-li\'ing seed coats. Both these 

 conditions may be met by alkaloids and glycosides. On the whole, how- 

 ever, much more investigation will be needed before we can attribute to 

 inhibiting chemicals an important role in holding many seeds of wild plants 

 dormant in the soil for years. We have learned already that many of them 

 do stay dormant for years in the soil in the imbibed condition. 



Secondary dormancy. It has long been kno^^'n that unfavorable germi- 

 nation conditions often throw seeds into dormancy so they will not germi- 

 nate when shifted to a favorable condition. Kinzel ^^ showed that seeds 

 of Nigella sativa, which are prevented from germinating by light, if placed 

 in an illuminated germinator, soon change so they will not later germinate 

 in darkness. They become "light-hard." This holds for other light- 

 inhibited seeds. Many seeds that need light for germination when placed 

 in a dark germinator become "dark-hard," so they will not germinate later 

 in light. Kidd ^^ has sho^^^l that high partial pressures of carbon dioxide, 

 24 per cent more or less, will inhibit germination of certain seeds and in 

 time throw them into dormancy. Reduced pressures of oxygen render the 

 carbon dioxide effective in lower concentrations. There are many other 

 cases of bad conditions in a germinator that throw seeds into dormancy. 

 Most of these conditions are active in the soil and may, in part, account 

 for the long rest period of seeds in nature. In 1916 the author ^° spoke of 

 dormancy in seeds induced by bad conditions in a germinator as "second- 

 ary dormancy." 



B 



C D 



Figure 12. Ambrosia trifida fruits and seeds: A, the fruit; B, the seed; C, the embryo; 

 D, section of the two-layered seed coat. 



