634 GERMINATION AND DORMANCY 



during the winter while the seeds lie on the ground or just under the soil 

 surface. Such seeds will not germinate in the fall just after they are shed, 

 but will germinate the following spring if environmental conditions are favor- 

 able. In some species after-ripening occurs over a period of years, some 

 germination occurring each year. After-ripening involves principally a series 

 of changes in the physiological condition of the embryo which gradually con- 

 verts a dormant embryo into one which can resume growth. The nature 

 of these physiological changes is not clearly understood at the present time. 

 In some species after-ripening also involves changes in the properties of the 

 seed coats. The length of time required for completion of the after-ripening 

 process can be greatly modified by environmental conditions as subsequent 



discussion will show. 



Secondary Dormancy. — Some seeds which are capable of germinating as 

 soon as they are harvested lose this capacity after being kept in an unfavor- 

 able environment for a while. This induced dormant period is known as 

 secondary dormancy. Usually secondary dormancy can develop only when at 

 least one of the conditions essential for germination is unfavorable. For ex- 

 ample, if seeds of white mustard {Brassica alba) are exposed to high concen- 

 trations of carbon dioxide they fail to germinate, even under favorable condi- 

 tions for a long period after the removal of the carbon dioxide (Kidd and 

 West 191 7). Light sensitive seeds may pass into secondary dormancy if 

 kept in the dark and seeds that germinate only in the dark may become 

 dormant if exposed to light. Likewise secondary dormancy may be induced 

 in some kinds of seeds by exposures to low temperatures and in others by 

 high temperatures (Davis, i93o)- 



Secondary dormancy is often caused by changes in the seed coats since 

 in some species the embryos are able to grow immediately if the seed coats 

 are removed. In other kinds of seeds, however, the dormancy is produced by 

 physiological changes that occur within the embryo itself. Secondary dor- 

 mancy, Hke primary dormancy, may be interrupted by various treatments. 



Methods of Breaking the Dormancy of Seeds. — The dormancy of seeds 

 presents a practical problem of considerable economic importance. Plant 

 growers are often interested in securing seed which will germinate soon after 

 it is harvested. Ordinarily this would be possible only with seeds which 

 have a short dormant period or none at all. Methods have been devised, how- 

 ever, whereby the dormancy of many kinds of seeds can be broken, and whereby 

 the length of the dormant period in many other kinds can be shortened. The 

 methods employed for the breaking of dormancy vary depending upon its cause. 

 Methods which can be used for breaking the dormancy of one species may be 



