REPRODUCTION AND DISPERSAL gii 



their vitality for twenty-five years, when buried at a depth of fifty centi- 

 meters. It has been shown also that deep burial insures greater longev- 

 ity than does shallow burial. Under such conditions, longevity would 

 seem to depend largely upon the resistance of the seed coats to water. 

 If the seeds are deeply buried, the conditions are relatively favorable for 

 longevity, because of the uniformly low temperature and because of 

 comparative freedom from exposure to air and to alternations of wetness 

 and dryness in the soil. 



While the amount of water in seeds is small, a portion of this amount 

 is essential to life.' Hence, it is probable that any seed would die, if 

 it is exposed to evaporation for a sufficient length of time, but the time 

 may vary with the species from a few hours or days to hundreds of years. 

 If continued respiration takes place in dry seeds, however slowly, it is 

 obvious that death must sooner or later ensue. Conditions which are 

 fatal to most other plant organs often have no deleterious influence 

 upon seeds. For instance, dry seeds can be kept for some time with- 

 out injury at a temperature of — 210° C, even if the testa is perforated, 

 and a long sojourn in a vacuum or in an atmosphere of carbon dioxid or 

 nitrogen is not injurious. Extremely high temperatures also may be 

 withstood without harm, but with them there is a recognizable limit, 

 as is not the case with low temperatures. Most desiccated seeds can 

 withstand for one or two hours a temperature of 100° C, and alfalfa 

 seeds can withstand a short exposure to a temperature of 120° C, even 

 when placed in water. Perhaps the severest test yet made has been 

 with the seeds of alfalfa, mustard, and wheat, whose coats had been 

 perforated and thus made permeable; these seeds germinated after 

 having been subjected to desiccation for six months, and then placed in 

 a vacuum for a year, and finally subjected for three weeks to a tem- 

 perature of — 190° C, and for three days to a temperature of — 250° C. It 

 may well be wondered why seeds should ever die if they can withstand 

 such severe conditions. The seeds which have been reported to have 

 retained their vitality for more than two centuries were subjected during 

 this time to constant changes of humidity and temperature. It is im- 

 possible to conjecture how long they might have lived, had they been 

 stored under conditions of uniform desiccation and refrigeration. 



Seeds as organs of food accumulation. — Introductory statement. — It 

 has been seen elsewhere that foods accumulate in various organs, par- 



' When seeds are placed in a desiccator, they retain six per cent or more of their water 

 for weeks ; when at last this hygroscopic water evaporates, death ensues. 



