38 GROWTH OF PLANTS 



It will be noted that nine of the 20 kinds of seeds buried were still germi- 

 nating at the 40-year period, five at the 50-year period, and three at the 

 60-year period, and that 8.2 per cent of the seeds were still alive at the 

 first two periods and 4.8 per cent at the 60-year period. Darlington ^^ 

 states that Verhascum Blattaria seeds were evidently placed in the bottles 

 tested for the 50- and 60-year periods instead of V. Thapsus, which 

 appeared in the earlier records. 



Not a single species of seed germinated at every test. This may be 

 explained either on the basis that certain seeds kept better in some bottles 

 than in others, or perhaps the germination conditions used in the various 

 tests were not equally effective in forcing those that were dormant to 

 germinate. The following did not give any germination for any of the 

 periods: Bromus secalimis, Erechtites hieracifolia, Chamaesyce maculata, 

 Agrostemma Githago, and Trifolium re-pens. Either these seeds do not live 

 five years in the soil or the germination conditions used were not suitable 

 for overcoming the dormancy. In the case of Trifolium repens the latter 

 may be the explanation, for after 20 years of burial in the U.S.D.A. 

 experiments, some hard viable seeds were recovered. Another possible 

 explanation is that Beal may have buried mainly soft seeds. Considering 

 all the seeds that lived in the soil in Deal's experiment for 40, 50, or 60 

 years, the following families are represented: Onagraceae, Portulacaceae, 

 Plantaginaceae, Amaranthaceae, Cruciferae, Compositae, and Scrophula- 

 riaceae. All these families are represented in the U.S.D.A. list of seeds 

 that lived 20 years, except Amaranthaceae. Every genus that showed live 

 seeds in Beal's list after 40 or 50 years' burial also showed live seeds in 

 the U.S.D.A. list after 20 years' burial, with the exception of Amaranthus, 

 which did not survive in the latter's record, and Lepidium which was not 

 represented. Also the long life span species, so far as they appear in both 

 records, agree closely. 



The size of the seed is evidently of no significance in determining life 

 span in the soil. Nelumho seeds, which have the longest record for life 

 duration in the soil, are rather large, as are Rohinia seeds. On the other 

 hand, many of the seeds that survive well in the soil are small: tobacco, 

 celery, timothy, evening primrose, mullein, and others. Depth of burial, 

 at least within the range used by the U.S.D.A. experiments, also has no 

 effect on life span. 



Leguminous seeds and others that will not absorb water are ideally 

 adapted for long life in the cool soil; the hard coats assure dormancy and 

 the low water content prevents waste of stored foods by respiration. 

 Many of the seeds with long life span in the soil are not hard; these are 

 maintained in the dormant condition in the soil by lack of light, as in the 

 case of celery and tobacco seeds, or by seed coat or embryo characters. It 

 is harder to explain, however, why the stored foods are not soon exhausted 

 by respiration. This is especially true in the light of measurements of the 

 initial respiration shown by dormant seeds that absorb considerable water. 



