70 FORAGE PLANTS AND THEIR CULTURE 



tion that seeds grown locally produce superior crops to 

 those brought from a distance. Theoretically this has 

 been ascribed to adaptation or acclimatization, usually 

 with the idea that superiority is attained by the elimina- 

 tion more or less gradually of the inferior individuals. 

 Much more experimental data are needed on this subject, 

 which indeed has been but little investigated in the United 

 States. 



62. Standards of purity and germination. Attempts 

 have been made to establish standards of purity and 

 germination for all farm seeds. There is, however, a con- 

 siderable variation in the purity of many forage crop 

 seeds, depending on the place grown, the season and the 

 care exercised. Furthermore, it is not so much the amount 

 of the impurity as the character of it which is most im- 

 portant. Thus, alfalfa seed containing 1 per cent of 

 dodder seed is less desirable than that containing no 

 dodder but a larger percentage of other weed seeds. A 

 few seeds of dangerous weeds like Canada thistle are far 

 more serious than many seeds of ordinary weeds. 



The viability of seed varies not only with its age, but 

 with the care in handling, and with the season, especially 

 at harvest time. In some seeds, indeed, the viability 

 is less when fresh than when one year old. On the whole 

 there is little choice between 100 pounds of forage seed 

 germinating 90 per cent and 120 pounds germinating 75 

 per cent. 



While fixed standards of purity and germination are 

 scarcely practicable, it is well to know what degrees of 

 purity and germination are found in high grade com- 

 mercial seed. The average purity and germination found 

 in the trade is of less consequence, as this is influenced by 

 the efficiency of legislation and inspection : 



