40 



GROWTH OF PLANTS 



of the same genus. In such cases, cokimn 2 shows the number of species 

 of the genus reported by Ewart and the maximum longevity for any species 

 of the genus. 



While Ewart's data on longevity in dry storage cannot be considered 

 adequate to make the claim certam, still the evidence indicates strongly 

 that the seeds reported in Table 7 have much longer life span in the soil 

 than in ordinary dry storage. The life span of grains of timothy in the 

 U.S.D.A. buried-seed tests exceeds the records in dry storage, and much 

 other similar evidence could be cited to show that some seeds live longer 

 in moist soil than in ordinary air storage. Recent work by Kjaer ^^ con- 

 firms this conclusion. In five-year tests he found that the seeds of the 

 following retained their vitality in soil better than in dry storage: Poly- 

 gonum tomentosum, 20 vs. per cent; Thlaspi arvense, 87 vs. 1 per cent; 

 Vicia Ursula, 50 vs. 5 per cent; Daucus carota, 43 vs. 10 per cent; Plantago 

 major, 30 vs. per cent; Cirsium arvense, 55 vs. per cent. He reports 

 Dorph-Petersen's results mth seeds of Sinapis arvensis: buried 10 years, 

 87 per cent germination; dry-stored, 21 per cent; buried 18 years, 17 per 

 cent; dry-stored per cent. Avery and Blakeslee ^ state that Datura seeds 

 stored in the laboratory are all dead after nine or ten years, but Datura 

 seeds buried in the soil outside (U.S.D.A. buried seeds) still show 97.5 per 

 cent germination after 39 years. 



The fact that seeds of many wild plants remain in the soil for long periods 

 in a dormant viable condition means that the soil is always well stocked 

 with seeds which are capable of germination when the soil is disturbed. 

 This assures the persistence of the species. It also makes the task of the 

 farmer and gardener in fighting weeds difficult, for when the soil is once 



