NURSERY PRACTICE FOR PRAIRIE-PLAINS PLANTING 23 



Under natural processes, it would probably be safe to say that 

 anything which occurs in the seed after it falls to the ground in the 

 autumn, or through the winter while the ground remains frozen, is 

 afterripening, while with thawing, and in the presence of abundant 

 moisture, the seed moves definitely toward germination, the more 

 rapidly as temperatures rise. However, this is not always the case, 

 since some seed, like those of the junipers, do not germinate the first 

 spring or summer with any certainty. It then is decidedly doubtful 

 what degree of exposure makes it possible for the seed to germinate 

 during the second spring. 



It is probable, however, that by simulating somewhat closely the 

 temperature-moisture cycle of a single over-winter period, most seed 

 which show any dormancy or difficulty in germination can be brought 

 through satisfactorily. But, as has been pointed out, freezing is 

 rarely if ever a necessary step in the procedure. Temperatures from 

 32° to 40° F. accomplish the same ends. 



Some investigators recognize a number of types of dormancy but 

 three types are certainly by far the most common. All of these are 

 inherent within the seed. They are: (1) Seed coats which prevent or 

 delay the intake of water or restrict exchange of gases; (2) dormant 

 embryo; and (3) combination of impermeable seed coat and dormant 

 embryo. A fourth type of dormancy is found in a few species. This 

 is caused by lack of complete development of the embryo at the time 

 the seed falls and necessitates additional time after seed collection in 

 which to obtain full development of the embryo. Seed of the genus 

 Gingko are in this class. 



The type of dormancy determines the germination treatment. 

 Impermeable seed coats can in a relatively short time be rendered 

 permeable by acid or mechanical scarification, or by soaking in hot 

 water. Seed which have only a slight amount of seed-coat dormancy 

 often respond to soaking in cold water. The type of dormancy and 

 the best known pretreatment for seed of about 50 species of trees 

 and shrubs are given in table 6. 



For seed possessing a dormant embryo, a substantial rest period is 

 necessary, varying in length with the species. Since low temperature 

 and a modicum of moisture that does not prevent aeration or respira- 

 tion are essential for certain obscure chemical changes taking place 

 at this time, stratification in a moist medium at about 40° F. is usually 

 employed, until sowing in the nursery is possible. The same treat- 

 ment is generally effective for seed possessing both an impermeable 

 seed coat and a dormant embryo. Difficulties are increased, however, 

 if stratified seed becomes too warm and begins to sprout before 

 sowing. Practically all stony-coated seeds and nuts require 60 days 

 or more of stratification. 



Fall sowing will, under favorable conditions, effectively break the 

 embryo dormancy or the combination of impermeable seed-coat and 

 embryo dormancy, but it is not recommended for seed in which 

 dormancy is caused by an impermeable seed coat alone, unless uniform 

 moisture conditions can be assured. Such seed, which do not require 

 a long low-temperature treatment, are bladder-senna, smooth sumac, 

 skunk bush, ash, and sycamore (decidedly dependent on degree of 

 previous drying), all of the legumes (Siberian pea-tree, honeyloeust, 

 black locust, redbud, and coffeetree), persimmon, soapberry, lilac, 

 and Osageorange. 



