LIFE SPAN OF SEEDS 33 



it impossible to ship them A\ath assurance from one sugar region of the world 

 to distant regions where seedlings are desired for breeding. Verret " found 

 that vitality could be lengthened materially by taking the seeds from the 

 thoroughly air-dried heads, placing them m cans with 9 grams of CaCb to 

 1 liter of space, displacing the air wnth carbon dioxide, hermetically sealing, 

 and storing at the freezing point. In these seeds, low and perhaps constant 

 water content and absence of oxygen seem to be necessary for retention 

 of vitality. It is possible also that carbon dioxide plays a positive role rather 

 than merely displacing oxygen. Kidd " finds that the life span of seeds of 

 Hevea brasiliensis can be greatly lengthened by sealing them in 40 to 45 

 per cent carbon dioxide. He assumes that this gas acts as a narcotic and that 

 it induces dormancy. 



There are many other seeds that retain their vitality for only a few weeks 

 or less than a year when stored in the au*. For most of these there is httle 

 information on the effect of the several factors involved in atmospheric stor- 

 age. It is generally assumed, however, that the seeds are injured by drying, 

 but the possible error m such a conclusion is made evident by the discussion 

 above. Among the seeds of tropical plants that have a short life span, Hevea 

 and sugar cane have been mentioned already. Others are Boea, Thea, Co- 

 cos, Oreodoxa, Sabal, Attalea, Mauritia, Thrinax, and Acrocomia. 



De Candolle " says seeds of most species of the families Rubiaceae, 

 Myrtaceae, and Lauraceae lose the germinative capacity soon after being 

 detached from the mother plants. There are, however, exceptions in the 

 first and second families. Coffea (Rubiaceae) seeds are used for planting 

 up to three years. Ewart ^ reports several Myrtaceae that retain their 

 vitality for considerable periods. Nearly all of 35 species of Eucalyptus 

 reported upon show some vitality in the seeds after 10 years of storage. 

 E. calophylla seeds 10 years old gave 96 per cent germmation, and those 

 32 years old, 5 per cent. Leptospermum scoparium 10 years old gave 8.2 

 per cent, and those 16 years old, 2.4 per cent. Callistemon lanceolatus seeds 

 16 years old gave 75 per cent germination, and C. rigidus seeds 22 years 

 old, 2.8 per cent. A thorough study would probably show many other excep- 

 tions to de Candolle's statement. The seeds mentioned by de Candolle as 

 short-lived generally are sown soon after harvest. 



While the studies mentioned above have thro\Mi much light on the factors 

 determining the life span of short-lived seeds, our knowledge would be 

 much fuller and more conclusive if the life span of each seed had been 

 studied under a wide range of intensity of each of the effective atmospheric 

 variables, singly and in combination. 



Life Span of Seeds in Soil 



In the literature there are hundreds of records of seeds that are supposed 

 to have lain in the soil for decades, still dormant but capable of germination. 

 These determinations are based on the appearance of plants, not common 



