18 MISC. PUBLICATION 4 3 4, U. S. DEPARTMENT OF AGRICULTURE 



The extraction factor, or the weight percent of good seed obtained, is 

 essential in computing in advance the quantity of fruit which must be 

 collected to obtain a given weight of dry, clean seed. Factors for 

 different species, computed from records which the Forest Service has 

 maintained and assembled over several years, are given in table 6, 

 page 34. 



Storage and Afterripening of Seed 



Seed collected in the fall may receive any of the following treat- 

 ments, which should govern in a large measure the final cleaning 

 processes, especially the degree of drying to which it is subjected: 



1. The seed may be sown immediately or within a few weeks; 

 whereupon such prompt-germinating seed as Siberian pea-tree may 

 make a small amount of growth at once, but those species which are 

 necessarily sown late in the fall or must have a definite period of 

 afterripening will lie dormant over the winter. Afterripening is 

 almost invariably associated with exposure to at least a moderate 

 degree of cold; but freezing is an unavoidable, rather than a necessary 

 part of the process. 



2. The seed may be kept only over winter in a shed with dry air 

 and a wide range of temperature, in a cold-storage room in sacks or 

 bins, or stratified in peat, sand, or other damp medium, either outdoors, 

 in a cellar, or in regulated cold storage. For none of these methods 

 is severe preliminary drying necessary, and with some seed it is both 

 useless and detrimental. One must be certain, however, that low 

 temperatures govern from the start, since under even mild tempera- 

 tures freshly treated seed may sweat considerably in sacks, bins, or 

 other mass storage, and mold and spoil very quickly. If the seed is 

 properly protected and then sown early in the spring, the low tem- 

 peratures will be sufficient guarantee against spoilage. 



3. The seed may be set aside for fall sowing after approximately a 

 year, or for insurance against seed-crop failures within several years. 

 Any seed which has to be kept over a summer must be treated as for 

 longer storage, and it is for this kind of storage that speciaL precautions 

 must be applied at the time of extraction. However, there are many 

 species which cannot be kept for any great length of time, and those 

 which are known not to be long-lived must be handled with especial 

 care if any large proportion is to remain germinable after a year or 

 more. 



THE KEEPING QUALITY OF SEED 



Much is yet to be learned, in most cases, as to the length of time the 

 seed of deciduous species will retain its viability under the best storage 

 conditions. No comprehensive study covering the range of species 

 has yet been made. Tourney (38) reports no germination of mulberry 

 and 21 percent germination of black locust seed stored 10 years at 

 room temperature in unsealed glass jars. Giersbach and Crocker (13) 

 report that American ("wild") plum seed stored in a laboratory cup- 

 board improved in germination up to 26 to 30 months and retained 

 more than half their viability up to 46 months. Seed stored at 45° to 

 50° F. retained good germinating capacity after 53 months of storage. 



Among nurserymen in the prairie-plains region it is general knowl- 

 edge that seed of the legumes, green ash, boxelder, and hackberry will 

 keep in storage at ordinary variable temperatures for 2 or more years. 



