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coats, even after the seeds are apparently mature. These changes are 

 referred to as after-ripening. Delayed gennination may result from a 

 number of factors, which may be considered in three groups: 



1. Delayed germination, the result of environmental conditions out- 

 side the seed. 



2. Delaved gennination, the result of conditions within the seed, but 

 outside the embryo. 



3. Delayed germination, the result of conditions within the embryo 

 itself. 



The delayed germination in the last two groups is the result of true 

 dormancv in seeds, since the cause of dormancy is within the seed. 



Germination delayed by the environment. Environmental conditions 

 profoundly affect the growth of the embryo, both before and after the 

 seed appears to be mature. In temperate regions winter temperatures are 

 too low for the germination of most seeds, and in our southwestern 

 deserts summer temperatures are too high for the germination of many 

 seeds, even though moisture conditions are favorable. Temperatures 

 that facilitate germination vary with the plant species, but comparatively 

 few seeds germinate below 50° F. Temperatures either unfavorably high 

 or low for a particular species result in no germination or in stunted 

 seedlings. At temperatures above 70° F. the root systems of some plants 

 develop very slowly, and vigorous plants rarely form. The embryos of 

 two water plants, wild rice and eel grass, die soon after maturity unless 

 stored at temperatures just above the freezing point in moist situations. 



The effects of environmental conditions on the growth of embryos 

 and seedlings are not always limited to this period of growth. They may 

 continue to influence the subsequent development of the plant, even if 

 the environment is changed. Rapid germination of seeds and growth of 

 seedlings are not necessarily beneficial to the subsequent growth of the 

 plants. The relative growth of shoots and roots and the length of the 

 vegetative period may be influenced by the temperature at germination 

 and early growth of seedlings ( Chapter XXXI ) . 



Prolonged droughts obviously restrict germination, and are often a 

 source of great agricultural losses when they follow the planting of 

 seeds of crop plants. Gennination may be stopped at any stage from 

 the swelling of the seed to the emergence of the seedling above the soil 

 surface. 



The seeds of such plants as willow, cottonwood, and elm will germi- 



