WEED IMPURITIES IN AGRICULTURAL SEEDS 39 



of 1902 and 1903 indicated that stratification in sand and 

 freezing were favorable to germination. 



Per cent of germination 

 Weed Before stratification After stratification 



Milkweed (A. syriaca) o 12 



Western ragweed o 18 



Lamb's quarter o 88 



Cocklebur o 25 



Mr. H. S. Fawcett made a study of 52 species of weed seeds. 



The samples were gathered after maturity in Septem- 

 ber, October and November of 1904. The seeds were 

 threshed out, and placed in paper envelopes. Fifty seeds 

 of each kind were placed in November in sand in boxes 

 kept during the winter in the greenhouse under uniform 

 conditions. Similar plantings were made each month 

 until May. The sand in all these germination trays was 

 kept moist. 



In addition a large number of weed samples was placed 

 out of doors for the purpose of subjecting them to freez- 

 ing and thawing. The seeds were placed in sacks in a 

 wooden box and covered with a thin layer of sand ; the 

 box was then deposited about one foot below the surface, 

 where it remained till paring". The general effect of 

 freezing and thawing was to increase the percentage of 

 germination and lessen the dormant period, especially in 

 case of seeds with hard coats ; in all cases the dormant 

 period of hard-coated seeds seems to be greater than that 

 of thin-coated seeds. The following examples illustrate 

 these differences: 



The dormant period of common pigweed (Aniarantus 

 retro flexus) was nine and one-third days when kept in 

 packages in a dry room, and six and one-third days after 

 having wintered out of doors ; its germination was in- 

 creased from 40 to 50 per cent. In common pigeon grass 

 the average dormant period was lessened from eleven to 

 seven and one-quarter days ; the percentage of germina- 



