60 GROWTH OF PLANTS 



Air conditioning — control of temperature and humidity of the atmos- 

 phere — is now used extensively and on a large scale in the United States. 

 The data given above indicate the importance of low constant moisture 

 content of seeds and of low temperatures in conserving the vitality of 

 seeds in storage. Already big seed firms are building large, air-conditioned 

 seed-storage houses, which can be used for drying the seeds by putting 

 them in loose fabric bags and cording the bags up, as well as to provide 

 low constant moisture and temperatures after the seeds are dried. It 

 might be desirable to use separate rooms for drying, so that higher tem- 

 peratures wth low humidities could be used for quickly drying the seeds 

 to the desired moisture content. Some rooms should also be run below 

 freezing for storage of certain seeds after they have been properly dried. 

 One large seed firm has consulted the Institute on the desirable specifica- 

 tions for such storage houses. With the storage houses described above 

 available, there need be no trouble in holding most farm, garden, and 

 flower seeds in full vitality for two, three, or four years. 



Why Do Seeds Degenerate with Age? 



Many theories have been offered to explain the degeneration of seeds 

 with age. Most of the explanations offered to date are highly theoretical 

 and have relatively little factual substantiation. We shall consider only 

 the more prominent of them and finish by giving the more probable one 

 along with the facts that tend to confirm it. 



It has been suggested that the enzymes in seeds degenerate with age, 

 with the result that the seeds become incapable of germination. This is 

 not at all probable, for dry seeds are relatively low in enzymes, and the 

 latter, both hydrolyzing and respiratory, are formed largely by the proto- 

 plasm of the embryo in the initial and later stages of germination. The 

 failure of enzymes to form in adequate amounts in older seeds must be 

 sought in changes in the protoplasm itseK. Auxins persist ^^ in Zea mays 

 seeds after 26 to 38 years of storage. 



It has been suggested that stored foods disappear in seeds mth long 

 aging, and that the embryos do not get sufficient nourishment in old seeds. 

 Most old seeds kept in dry storage contain large amounts of stored foods 

 long after vitality has been lost. For seeds in the soil under natural con- 

 ditions, if they absorb water readily, it is possible that exhaustion of the 

 foods by respiration determines the life span. Hard-coated seeds in the 

 soil will, of course, use little stored food in respiration and after many 

 years in the ground will have an abundance of stored foods. Jones and 

 Gersdorff ^^- ^^ find that three different types of changes occur in the 

 proteins of the grains of wheat and corn and the seeds of soybeans in 

 storage: (a) a decrease in solubility, (6) a partial breakdown of the pro- 

 teins indicated by decrease in true protein content, by decrease in the 

 amount of nitrogen precipitable A\ith trichloroacetic acid, and by increase 



