DORMANCY IN SEEDS 71 



such as using 12° C (54° F) germinators instead of 20° C (68° F) or higher, 

 by breaking the grain coats near the embryo, by using high oxygen pres- 

 sure, etc. Grains are more persistently dormant when they ripen during 

 rainy weather. The Germans repeatedly report difficulty in converting 

 "rain barley" promptly into malt because of delayed germination of the 

 grains. Even for many seeds that have very long dormant periods, includ- 

 ing those ^\dth dormant embryos, there are means of making prompt 

 vitality and vigor tests, as we shall see later in the discussion of quick 

 vitality tests. 



Categories of Dormancy 



Now let us consider the types of dormancy in seeds based on the mech- 

 anism by which the dormancy or delayed germination is secured. As we 

 shall see, nature has several devices for securing delayed and distributed 

 germination of seeds. 



Hard Coats 



We have already shown the extreme importance of hard-coatedness 

 in seeds in increasing their life span in storage as well as in the soil. Hard 

 coats maintain a low constant moisture content in the embryos — an effec- 

 tive storage condition — by hermetically sealing them individually. There 

 are several families, some species of which produce seeds that will not 

 absorb water. Leguminosae ^^ are outstanding in this respect, but the 

 Malvaceae, Cannaceae, Geraniaceae, Chenopodiaceae, Convallariaceae, 

 Convolvulaceae, Solanaceae and other families have species that bear hard 

 seeds. Hard-coatedness is primarily determined genetically, but the appear- 

 ance or degree of hardness is also modified by environmental factors. In 

 white sweet clover "^ either hard or soft strains can be developed by selec- 

 tion and inbreeding. The same is true of hairy vetch.^^ On the other 

 hand, the author has observed at Yonkers that more than 98 per cent of 

 hand-hulled, white sweet clover seeds are hard when they ripen during 

 hot, dry weather and that 100 per cent are soft when they ripen during 

 rainy weather. 



Many investigators agree that the outside layer of cells of the coats 

 which are palisade in form prevent the entrance of water, and some claim 

 that, of this layer of cells, only the outer half or the portion outside the 

 "light line" is impermeable. This layer of cells in Nelumbo was illustrated 

 and described in the previous chapter. White ^^s claims that in small 

 leguminous seeds the cuticular layer over the palisade cells determines the 

 impermeability, while in the larger leguminous seeds the outer portion of 

 the palisade cells is involved as well. The question has also been raised 

 whether the physical character or chemical composition of this thin layer 

 gives it the remarkable resistance to water absorption. Raleigh ^® con- 

 cludes that as the seeds of the Kentucky coffee tree harden in ripening, 

 pectic substances change into water-resistant substances. Shaw ^"^ germi- 



