LIFE SPAN OF SEEDS 31 



albus, 51 years old, 78 per cent germination; Entelea arhorescens, 51 years 

 old, 47.3 per cent; Indigofera cytisoides, 51 years old, 51.2 per cent; and 

 Melilotus gracilis, 58 years old, 28.8 per cent. 



Seeds of Short Life Span 



Some seeds lose their vitality in a very short time if they are kept in 

 open air after harvest. Until recently this rapid loss of vitality was supposed 

 to be due mainly or solely to the drying effects of the air, that is, the proto- 

 plasm of the embryo was killed by partial desiccation. No doubt this is 

 the case with some short-lived seeds, but other factors determine life span 

 in other short-lived seeds. A consideration of some of the later more critical 

 work on various short-lived seeds will show the significance of several factors 

 in the loss of vitality. 



According to Jones,'^ the seeds of the river maple (Acer saccharinum) 

 are killed by relatively slight drying. When they fall from the tree in June 

 they bear about 58 per cent water. Regardless of the temperature of expo- 

 sure (0° to 35° C [32° to 95° F]), they were killed when the moisture content 

 reached 30 to 34 per cent. In his experiments it required six days at 35° C 

 (95° F) and 92 days at 0° C (32° F) to reach this water content, or the 

 death point. When these seeds were stored in a closed vessel over water 

 at the freezing point and provision made for preventing carbon dioxide 

 accumulation, they retained full vitality for 102 days, which was the limit 

 of the test. The low temperature prevented germination and reduced the 

 rate of metabolism. The latter is an important consideration, for these 

 seeds are fleshy and have rapid respiration at higher temperatures. They 

 should be sowed as soon as they fall. If this is impossible, because of the 

 necessity of shipping or for any other reason, they should be kept near the 

 freezing point, and water loss prevented. River maple seeds are not dor- 

 mant but begin germination in nature as soon as they reach the moist 

 ground. The seeds of the fall-fruiting sugar maple {A. saccharum) showed 

 very different behavior. They endure complete air-drying and respond to 

 several weeks' low temperature stratification for eliminating dormancy. 



Duvel " finds that wild rice (Zizania aquatica) seeds lose their vitality if 

 they are allowed to dry in the air for even a few days, but that they retain 

 their vitality perfectly until spring if stored in water at 0° to 1° C (32° to 

 36° F). In the spring they must be transferred from storage water to the 

 water in which they are to grow, without being allowed to dry. Seeds of 

 wild rice are dormant when mature, and storage in water near the freezing 

 point after-ripens them as well as maintains their vitality. 



According to Barton,^" various citrus seeds endure only partial drying in 

 the air. Grapefmit and sweet orange seeds are injured by drying to 52 

 and 25 per cent moisture content, dry weight basis, respectively, at labora- 

 tory temperatures but grapefruit seeds retain their full vitality for more 

 than a year when stored in the open at 5° C (41° F), where the moisture falls 



