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germinate as soon as they are placed under favorable conditions of 

 moisture and temperature, while others will not germinate under 

 the same conditions for many weeks, months, or even years. 



Various causes are responsible for this delay in germination, 

 just as there are manifold methods by which germination may be 

 hastened. Very often there is exhibited the same phenomenon of 

 deep rest or dormancy which has just been discussed in considering 

 buds. In such cases, seeds acquire the capacity to germinate 



rapidly only after having been 

 kept for a sufficiently long 

 time, or else they require some 

 external stimulant. 



Ah increased power of 

 germination is often observed 

 in winter wheat and other 

 cereals which sometimes ap- 

 pear incapable of germinating 

 immediately after harvesting. 

 A few weeks or months later, 

 however, they are able to 

 germinate quickly and uni- 

 formly. The phenomenon has 

 been termed " after-ripening." 

 This characteristic is exhibited 

 by various varieties in different degrees. In regions with a very 

 wet autumn, varieties of wheat with a long rest period may be of 

 advantage in that their grains will not germinate in the sheaves 

 in rainy weather. 



Because of the necessity of seeds to pass through the rest period, 

 there have been developed methods of stratification, so widely used 

 in horticulture. The mature seeds of the apple, pear, and stone 

 fruits are usually planted in layers with moist sand and transferred 

 to a cool place, the best temperature being 5° C, for instance to a 

 cellar, or they may be left under snow. Toward spring the rest 

 period will be completed, and the seeds will germinate. Instead 

 of burying in moist sand, the seeds of apples, for instance, may be 

 kept simply by leaving them in the flesh of the fruit. In dry 

 seeds of these plants the process of after ripening does not take 

 place. 



The rest period of mature seeds may be so deep that they will 



Fig. 116. — Nettle kept during the winter 

 (1) under snow, (2) in a warm greenhouse. 



