190 PLANT GROWTH 



cies have a short dormancy. White oak acorns which can be 

 seen germinating almost immediately after they fall, while 

 closely related species, black or red oak acorns, under the 

 same conditions, remain dormant until early spring. 



Most of the seeds of the clover family have a dormant 

 period. Freshly hand-hulled sweet clover or alfalfa seed has 

 about 90 per cent of dormant seeds. They remain dormant 

 because a band of impervious matter in the seed-coat pre- 

 vents the entrance of water. These seeds are usually treated 

 in some manner to crack or remove the impervious layer 

 before planting, which causes about 90 per cent to germinate 

 at once. This treatment is called "scarification." In other 

 seeds the seed-coat may prevent the entrance of oxygen to 

 the embryo as in the common cocklebur {Xanthium). A 

 third condition, induced seed dormancy, is due to a tough 

 seed-coat. The swelling embryo cannot break the coat as in 

 case of the pigweed. Dormancy caused by the seed-coat can 

 be overcome by filing or by rubbing it with an abrasive. 



Other seeds remain dormant because of the condition of 

 the embryo. The embryo may be immature and unable to 

 germinate until it grows to a larger size. In other cases the 

 embryo appears to be mature but it requires internal changes 

 grouped under the heading of "after-ripening." These 

 changes have been studied but in most cases are not under- 

 stood. The breaking of this type of dormancy is more diffi- 

 cult than those caused by the seed-coat. 



The physiology of dormancy indicates that it is due to 

 some changes in those substances called growth regulators, 

 which are in small amounts but have profound influences. 

 In some plants a change in enzyme content occurs at the end 

 of dormancy. In other plants it appears to be a growth hor- 

 mone change. The change may be made in a branch or a 

 single bud while the remainder of the plant is dormant. 



Methods of breaking or overcoming the dormant period 

 have been found without knowing the nature of the change. 



