460 PLANT PHYSIOLOGY 



may be light or deep. Accordingly, many seeds 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 that has just been discussed in the 

 consideration of buds. In such cases, seeds acquire the capacity 

 to germinate rapidly only after having been stored for a suffi- 

 ciently long time, or else they require some external stimulation. 



A gradually increasing power of germination is often found 

 in winter wheat and other cereals, which sometimes appear 

 incapable of germinating immediately after harvesting. A few 

 weeks or months later, however, they can be germinated quickly 

 and uniformly. This phenomenon has been termed ''after- 

 ripening." This characteristic is exhibited by various varieties 

 in different degrees. In regions with a very wet autumn, varie- 

 ties of wheat with a long rest period may be of advantage in that 

 their grains will not germinate in the shock or stack in rainy 

 weather. Larson, Harvey, and Larson have recently made a 

 study of the length of the rest period of cereal varieties, which 

 shows that the time of cutting, the temperature of storage, and 

 varietal inheritance are important in determining the length of 

 the rest period. 



On account of the necessity of seeds to pass through a rest 

 period, there have been developed methods of stratification, 

 which are 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 cold place, for instance to a 

 cellar, the best temperature being +5°C., or they may be left 

 under snow. Toward spring, the rest period will be completed, 

 and the seeds will germinate. Instead of being buried 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 afterripening does not take place. 



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

 refuse to germinate wdthout exposure to special stimulation 

 even after prolonged storage. The seeds of the gooseberry, 

 Grossularia sp., of Lunaria, of Impatiens, and other plants require 

 more than a simple stratification. They must be frozen at 



