DORMANCY IN SEEDS 73 



time to learning the method or methods of this water-proofing in the hope 

 of applying it to his needs. The seeds of any crop vary in perfection of 

 water-proofing, as we have already seen. This insures time-distributed 

 germination of any crop. 



The embryos of most kinds of hard seeds germinate readily and grow 

 vigorously when the hard coats are broken and germination conditions 

 furnished; but the embryos of redbud, Cercis canadensis, need several 

 weeks' after-ripening in a low-temperature germinator or stratification in 

 the swollen condition to prepare them for growth.^ The embryos of hard 

 seeds of the beach pea, Lathyrus maritimus, grow much more promptly 

 after the seeds have a period in dry storage.^^ 



In red clover seeds Saulescu ^"^ finds that percentage hardness increases 

 with diminution in size and with darkness in color from yellow to purple. 

 Grimm ^^ finds the same size-hardness relation for several clovers, but 

 believes that lighter-colored seeds have a higher percentage of hard seeds. 

 Lespedeza stipulacea seeds ^^ show the same relations between size and 

 hardness. In alfalfa seeds ^^ percentage hardness seems to decrease with 

 range in color from bright yeUow to green to brown. Hardness in Vicia 

 saliva seeds ^^ increases with depth in color. In commercial alfalfa and 

 clover seeds the percentage of hard seeds varies greatly with varieties, con- 

 ditions during ripening and storage, and with conditions in machine- 

 hulling such as closeness of cylinder and concave, and moisture content. 

 The persistence of hardness ^^' "''• ^^^- ^^^ increases in the following order: 

 alfalfa, red, white, and alsike clover. With spring sowing, hard alfalfa 

 seeds swell and germinate mostly the first season with few or none carry- 

 ing over. In the clovers some seeds carry over until the second spring and 

 a few still longer. In storage, hard alfalfa seeds ^^ soften faster than the 

 clover seeds, and low temperatures and high moisture favor softening 

 during storage. 



Lupines ^^ and hairy vetch seeds ^^^ become hard in high-temperature, 

 low-humidity storage, and soften in low-temperature, high-humidity stor- 

 age. Even a few days in dry laboratory air increases the degree and per- 

 centage of hardness in yellow and blue lupine seeds. ^- Careful attention 

 must be given to storage condition of these seeds during the ^vinter to 

 avoid hard-coatedness and failure to germinate when soun in the spring. 

 Temperatures at the freezing point or lower favor softening of hard clover 

 and alfalfa seeds whether wet or dry. 5°- ^^^ ^^ Busse ^^ found that freezing 

 dry hard seeds of sweet clover and alfalfa to — 190° C (— 310° F) softened 

 the coats \vithout injury to the seeds. Even repeated freezing at this 

 temperature did not injure dry alfalfa seeds. High temperatures, 60° C 

 (140° F) for 2 hours or 75° C (167° F) for 0.6 hour," soften hard alfalfa 

 seeds. Both high and low temperatures are probably factors in softening 

 hard seeds in the soil. This is especially true of low and variable tempera- 

 tures during the winter. 



High hydraulic pressures "■ 24. 99 have been used to soften hard coats of 



