NURSERY PRACTICE FOR PRAIRIE-PLAINS PLANTING 



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Where a longer period of stratification is possible, especially for seed 

 occupying great space, such as walnuts and acorns, larger containers 

 or pits placed out-of-doors may be used. Stratification out-of-doors 

 must be maintained longer than the periods required indoors, and 

 below-freezing temperatures should not be permitted for the whole 

 period. Moisture absorption and chemical changes in the seed 

 practically cease while the seed is in a frozen condition. It is, there- 

 fore, customary to make pits reasonably deep and to insulate them 



Figure 8. — Seed in stratification in ice-freezing containers. 



against a constant frozen condition during the whole winter by using 

 straw or other mulch over the top of the mounded earth. 



Considerable variation exists in the length of the stratification 

 period required by the different species and even for different seed lots 

 within the species. The degree of dryness, previous storage method, 

 and age of seed are the important factors involved. Nurserymen and 

 seed investigators have, however, arrived at the fairly definite average 

 durations for most species, which will be presented later on. 



It has been observed frequently in some species possessing the com- 

 bination type of dormancy, such as plum and chokecherry, that in a 

 short time the seed will be pitted, i. e., the seed coat will split open 

 leaving the embryo exposed. This, however, does not mean that the 

 seed is ready to germinate, and longer stratification must usually be 

 employed to break the embryo dormancy. 



Seed of some species, such as plum, chokecherry, Russian-olive, and 

 lilac, may sprout in stratification as spring approaches, even though 

 the temperature has risen very little above 40° F. Frequent inspection 

 of stratified seed is therefore necessary, and if sprouting is observed 

 the medium should be placed in colder storage and so maintained 

 until the seed can be sown. 



Stratified seed ready for sowing is highly sensitive and is readily 

 susceptible to injury, especially through diving. Extreme caution is 

 necessary in the spring when removing the seed for sowing, to guard 



