LIFE SPAN OF SEEDS 41 



well stocked mth seeds it takes years of cultivation for the complete ger- 

 mination and final destruction of the weeds. 



While the old imbibed seeds in the soil are of necessity dormant — or 

 else they would have germinated — the dormancy in the main is due to a 

 rather delicate equilibrium that is overcome by exposure to light, by fluc- 

 tuating temperatures of the top soil, or even by mechanical disturbance 

 or better oxygen supply. Disturbing dormant weed seeds in soil starts 

 them germinating.^" 



Barton^"^ finds that seeds of Amaranthus retroflexus that have remained 

 dormant in a germinator at 20° C (68° F) for more than a year can be 

 induced to germinate quickly by several different treatments: raising the 

 temperature to 35° C (95° F) gives prompt, complete germination ; rub- 

 bing them in the palm of the hand with the finger for three minutes and 

 plac\>'^- them back at the same temperature induced 83 per cent germina- 

 tion; partial desiccation and fluctuating temperatures are also effective. 

 Denny ^^ finds that dormant corms of gladiolus that have remained in 

 moist soil at room temperature for a year or more can be thrown out of 

 their dormancy and caused to grow by only a few hours of exposure to 

 5° C (41° F), whereas freshly harvested corms require several weeks at 

 5° C (41° F) to after-ripen. 



Dormancy and Delayed Germination in Seeds 

 OF Wild Plants 



In connection with the statements about buried seeds it has been men- 

 tioned that seeds of wild plants are much more commonly and more per- 

 sistently dormant than those of cultivated plants, and consequently their 

 germination is much more delayed. There are many records of long- 

 delayed germination in seeds of wild plants. Nobbe and Hanlein *^ placed 

 400 seeds each of 31 species of wild plants, mainly weeds, in Petri dishes 

 as germinators at room temperature and kept records on the germination 

 for 1173 days. All the species of seeds used in this experiment were said 

 to be soft-coated, that is, absorbed water. Table 8 shows the number of 

 germinations of the different kinds of seeds after various numbers of days 

 in the germinator, also the total germination and per cent germination for 

 each after 1173 days. It will be noted that there was no germination, also 

 no decay, of the Phyteuma spicatum, Primula elatior, and Verbascum 

 nigrum seeds. Lack of decay indicates that the seeds were alive, and the 

 failure to germinate indicates that room temperature in the germinator 

 failed either to furnish the conditions for after-ripening or for germination. 

 The evidence in the next chapter will indicate that it was the former. 

 Five other sorts of seeds gave less than 1 per cent germination; likewise, 

 these showed no decay, and the low germination is undoubtedly explained 

 on the same basis as the three mentioned above. In several of the seeds 

 that showed moderate or considerable germination, the germination is 



