METHODS OF BREAKING THE DORMANCY OF SEEDS 635 



totally ineffective when used with seeds of another species, and sometimes 

 may even prolong dormancy. 



1. Scarification. — Whenever dormancy is due to any of the causes in- 

 herent in the seed coats it can be interrupted by scarification. We shall use 

 this term to apply to any treatment — mechanical or otherwise — which results 

 in rupturing or weakening the seed coats sufficiently to permit germination. 

 For example, machine-threshed legume seeds usually show a higher percentage 

 of germination than those that have been harvested by hand. The mechanical 

 treatment is sufficiently severe to scratch or crack many of the seed coats and 

 this permits ready ingress of water. Various types of mechanical treatments 

 have been devised for breaking the dormancy of seeds of this kind. Strong 

 mineral acids have likewise been used successfully to interrupt seed dormancy 

 caused by resistant or impermeable seed coats. It is essential, however, that 

 any method used to interrupt seed coat dormancy should not be injurious to 

 the embryo. Under natural conditions dormancy of such seeds is broken by 

 the slow decay of the seed coats or by the action of alternate freezing and 

 thawing. 



2. Low Temperatures. — After-ripening of many seeds occurs more rapidly 

 when they are stratified in moist peat at low temperatures than when stored 

 at higher temperatures. Temperatures between 5° and 10° C. for two or 

 three months are effective with conifer seeds (Barton, 1930) and greatly 

 increase the percentage of germination. Similarly, low temperatures com- 

 bined with moisture have been found to reduce the period of after-ripening 

 in seeds of mountain ash, basswood, elder, bayberry and many other species. 

 The effectiveness of low temperatures in breaking dormancy appears to be 

 associated in some species at least with a favorable relation between respira- 

 tion rates and the rate of oxygen absorption or carbon dioxide liberation. 

 Permeability changes in the seed coats may also be an important factor. 



3. Alternating Temperatures. — In some seed testing laboratories it is com- 

 mon practice to subject seeds alternately to relatively low and high tem- 

 peratures. The temperature extremes of such treatments may not differ by 

 more than 10° or 20° C. and both are commonly well above the freezing 

 point. The germination of seeds of Kentucky blue grass {Poa pratensis) , 

 for example, is greatly improved by subjecting the seeds alternately to tem- 

 peratures of 20° C. for 16-18 hours and 30° C. for 6-8 hours and the per- 

 centage germination of Johnson grass {Holcus halepensis) seeds is increased 

 by alternate treatments at 30° C. for 18-22 hours and 45° C. for 2-8 hours 

 (Harrington, 1923). The dormancy of some seeds may be interrupted by 

 alternate freezing and thawing though this is decidedly harmful to other 

 species. The action of the alternating temperatures upon the seeds is not 



