910 ECOLOGY 



seeds live longer in the soil than when dried, possibly because, unlike 

 most seeds, they are unable to withstand prolonged desiccation. 



It is not unlikely that in some cases longevity is due to much less obvious features 

 than to seed coats. Seeds similar in structure and with envelopes equally imper- 

 meable vary widely in longevity. Still more striking in this respect are the minute 

 asexual spores of the seedless plants. While the spores of Equisetum die if they 

 fail to germinate almost immediately, moss spores that have lain dry in a her- 

 barium for fifty years have been known to retain their viability; it seems improb- 

 able that such differences can be accounted for by differences in the spore coat, 

 which is not noticeably dissimilar in the two cases. In the liverworts, however, it 

 has been observed that the spores of xerophytic species may withstand desiccation 

 for two years, whereas the thin-walled, green spores of semi-hydrophytic species 

 lose their vitality very quickly. 



The causes of the death of seeds are in part known and in part open 

 to question. While water is necessary for the initiation of germination, 

 it often is absorbed by seeds under conditions that are unfavorable for 

 the continuance of the germinative processes. This is the case with 

 many seeds which fall into the water, or which are subjected to low 

 temperatures or to desiccation, immediately after the absorption of 

 water has begun. Such seeds soon decay, or at any rate lose their 

 vitality. Submergence in water for a month results in the death of the 

 seeds of many land plants, such as rye, oats, and maize. However, the 

 seeds of many water plants (such as Alisma and Sagittaria) can with- 

 stand submergence for some years, probably because of the extreme 

 resistance offered by the seed coats to the penetration of water. Even 

 when seeds are kept in ordinary rooms, the changes in atmospheric 

 humidity probably are sufficient to reduce longevity seriously, because 

 of the hygroscopic properties of the integuments. Most seeds die 

 within three months if they are continuously exposed to saturated air, 

 the longevity increasing somewhat regularly as the percentage of hu- 

 midity is reduced. Parsnip seeds die within two months at a humidity 

 of 70 per cent, although they may be kept alive for three years when 

 desiccated and placed in a vacuum. Apparently, then, the exemption 

 of seeds from conditions that tend to incite water absorption, respira- 

 tion, or activity of any kind is a necessity for longevity. It is probably 

 for this reason that most seeds retain their vitality best when they are 

 stored where conditions are uniformly cool and dry. Experiments show 

 that certain seeds retain their longevity for a very long time when they 

 are buried in the soil, though not so long as in dry storage. For ex- 

 ample, seeds of mustard, dock, and purslane have been known to retain 



