NATIVE WOODY PLANTS OF THE UNITED STATES 33 



species of the rose family will germinate readily if planted at once, 

 but if dried will fall into a state of secondary dormancy necessitating 

 stratification to assure germination (313) . Seed of birches and bass- 

 wood germinates much more poorly when permitted to dry out after 

 maturity. 



Temperature greatly influences the keeping quality of seed (351). 

 For most woody plant seed, 35-40 F. is considered the best tem- 

 perature. At these levels the moisture content may vary somewhat 

 without harm to the seed. Freezing should be guarded against. 

 Low oxygen content can be obtained by storage in airtight contain- 

 ers ; this should be watched, since high oxygen content reduces keep- 

 ing quality. 



"Stratifying," as here used, means placing the seed in layers of 

 peat and holding them at 35-40 F. or, in specific instances, at the 

 temperature noted in the list under the species. If seed is to be 

 stratified, it should first be treated with a disinfectant. Potassium 

 permanganate in 5 to 10 percent solution is useful for this purpose. 



All of these operations and also those that follow are treated ad- 

 mirably in The Modern Nursery (351). Seeding and Planting in 

 the Practice of Forestry (573) treats particularly of tree species. 



Seed may fail to germinate because of incompletely organized 

 embryos or hard seed coats, or because certain internal chemical 

 changes that must take place before germination have not occurred. 

 Additional food in the form of a dilute sugar solution may assist 

 embryo organization during stratification. Seeds with hard seed 

 coats are treated with hot water or agitated with sharp sand to assist 

 them in germinating. Seed requiring a rest period must be stratified 

 in the manner described. 



Such of these practices as are known to apply are indicated for 

 each species or genus in the list. In addition, data on the number 

 of seeds per pound, the germination percentage, and the approxi- 

 mate number of usable plants to be expected from each pound of 

 seed are given. These notes are intended to assist the technician in 

 large-scale operations by giving him some idea of the nursery space 

 he may require. Obviously, they can be only approximate. 



The number of seeds per pound depends on individual variation 

 in size of seed. This in turn depends on the age and vigor of the 

 plant, the fullness of the crop, differences in site of the individual 

 plants, and place of collection. Trees on good sites produce larger 

 seeds than those on poor ones, and seeds collected at the northern 

 extension of the tree range or from higher elevations are usually 

 smaller than those collected from the southern part of the range or 

 from lower altitudes (573). This is probably true also of shrubs. 



It can thus be seen that the reported number of seeds per pound 

 may vary tremendously. However, some indication is better than 

 none at all. 



Theoretically, the number of seeds per pound multiplied by the 

 percentage of germination should give the number of usable plants 

 to be expected per pound of seed. Actually, this is rarely true. Too 

 many other factors are involved and the number of usable plants is 

 often only one-third to one-fifth or less of the theoretical figure. For 

 several hundred species the actual approximate number of usable 

 plants per pound of seed, determined empirically in the Section of 



