CHAPTER 3 



Dormancy in Seeds 



Significance op Dormancy in Seeds 



In the previous chapter we showed that seeds of many wild plants will 

 lie in a germinator or in moist soil for years without germinating, or in 

 cases where some germinate, the germination of a single planting of a given 

 crop may spread out over a period of years, with now and then a few 

 seeds germinating. In short, in seeds of wild plants, delayed or distrib- 

 uted germination is very common both in a germinator and with nature 

 in the soil. While this is markedly true for seeds of wild plants, it is also 

 true to a lesser degree of seeds of many cultivated plants. In the latter, 

 the dormancy is often transient and extends over a period of days or 

 weeks rather than years. The seeds of some cultivated plants do have a 

 long period of dormancy. This is true of some leguminous and other 

 hard-coated seeds that are slow to absorb water. 



With the knowledge available during the last part of the past century 

 the explanation of dormancy and delayed germination of seeds was much 

 simpler than it is today; and it was correspondingly more vague. Hard- 

 coated seeds, of course, did not germinate because they did not absorb 

 water. Seeds that absorbed water and did not germinate were supposed 

 to need some special stimulus besides the three conditions ordinarily 

 thought necessary for germination — proper temperature, oxygen, and 

 water supply. The stimuli were supposed to act promptly and to give 

 immediate germination. This was thought of as a release response, and 

 workers sought for stimuli that gave such immediate release. True, during 

 the latter part of the last century, many workers showed that light was 

 necessary for the germination -" of some seeds and accelerated the germi- 

 nation of others, and early in this century some seeds were found to be 

 prevented from or hindered in their germination by light. Since the 

 response to light is rather quick and the light need not act for a long time, 

 it was interpreted as a stimulus or release response. Now it seems prob- 

 able that even light brings about biochemical or biophysical changes that 

 lead to germination, and that its effect can be interpreted on the basis of 

 definite chemical and physical changes rather than on the basis of the 

 vague conception of release or stimulus response. 



We have already seen how long some seeds must be held in a low- 

 temperatiu'e germinator to after-ripen the dormant embryos, and also in 

 a high-temperature germinator with restricted oxygen pressure in order 



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