32 GROWTH OF PLANTS 



to 17 per cent. Rate of drying as well as degree may be important, or it is 

 possible that the temperature at which the drying occurs is a determining 

 factor. Sour orange and rough lemon seeds also retain their vitality well 

 when stored open at 5° C (41° F). They also endure more drying than the 

 other two sorts of citrus seeds. Storage at — 5° C (23° F) was injurious to 

 all four sorts of citrus seeds because of their necessarily high water content. 



Other seeds of the temperate zone that lose their vitality readily when 

 stored in open air are oaks, beeches, horse chestnuts, walnuts, hickories, 

 and chestnuts. These are generally stratified Avith moisture at a low tem- 

 perature in the fall, which prevents germination until spring and after- 

 ripens such as are dormant. Delavan,^^ working on seeds of three species 

 each of hickories and of the white oak and the black oak groups, found that 

 the seeds kept well until the following spring in an ice box and in a pit out- 

 side, but lost their vitality in a few months in dry storage. They also gradu- 

 ally after-ripened at the low temperatures, as shown by quicker germina- 

 tion with the lengthening of the period of storage. Barton " found low- 

 temperature stratification necessary for the after-ripening of seeds of 

 hickory, walnut, and butternut. 



Seeds of willows also lose their vitality quickly when exposed to the air. 

 This has been assumed to be due to excessive drying, but Nakajima's work 

 disproves this assumption. He found ^^ that seeds of Salix opaca, S. japon- 

 ica, and S. Reinii retained their vitality much better in closed tubes over a 

 solution of 50 per cent by volume of H2SO4 in water than they did in open 

 air. In later work he found that seeds of Salix Pierotii and S. japonica in 

 open air lost their vitality completely within a week, but when stored over 

 the sulfuric acid solution mentioned above and kept in an ice chest they 

 still gave 53 per cent germination after 360 days of storage. Such a solution 

 gives a relative humidity of only 13 per cent, which is much lower than the 

 average humidity of the atmosphere at ripening time of the seeds. Evidently 

 the injury in open air is not caused by excessive drying. Valuable informa- 

 tion might have been obtained if he had also tried low oxygen pressure and 

 absence of oxygen, as Busse did for aspen seeds. 



It is a well known fact that poplar seeds lose their vitality within a few 

 weeks when left in the air. Busse " believes this is due to the injurious 

 action of oxygen, also that higher temperatures hasten the degeneration. 

 Storage of these seeds in a vacuum in a cellar preserved 90 per cent viable 

 after 22 months. 



Seeds of the English elm lose their vitality almost completely within six 

 months of open storage and seeds of the American elm keep little better." 

 Barton ^ finds that sealed storage of American elm seeds at low tempera- 

 tures, 5° and — 5° C (41° and 23° F), prolongs the life of these seeds greatly. 

 There seems to be little difference whether the moisture is 2, 3, or 7 per cent. 

 In sealed low-temperature storage these seeds retained full vitality for five 

 years, with the experiment still running. 



Sugar-cane seeds degenerate rapidly when stored in open air. This makes 



