THE PRINCIPLES OF SEED-TESTING 487 



2. Purity 



By practical purity is meant that the seed is not only 

 true to name, but that it contains nothing else in measurable 

 quantity. The two chief sources of impurity are inert matter, 

 such as stones, particles of soil, broken seeds, stalks, chaff — 

 all dead weight — and the seeds of other plants, chiefly weeds. 

 Great improvement has taken place in this branch of the seed 

 trade. The better houses have often elaborate machinery by 

 which the impurities are removed. Unfortunately, however, 

 these very impurities find a ready market, and the supply within 

 the trade is not equal to the demand ! 



Many of the weed seeds are highly objectionable or injurious. 

 The dodder is very generally present in some clovers, and 

 may do a great deal of harm as a parasitic pest. The dodder 

 seed is in size and colour so like a particle of soil as to be 

 indistinguishable to the farmer. Fortunately for Ireland, the 

 dodder does not thrive well, and there are few cases on record 

 of injury to crops caused by it. I had one case before me in 

 1900 of the destruction of a flax field by the flax dodder, and 

 the correspondent mentioned cases of great damage in some 

 fields in earlier years. 



In England the dodder has, I understand, done much harm 

 locally, from year to year, but does not often ripen its seeds. 

 In some of the Continental and American clovers, however, ripe 

 dodder seeds are plentiful. In England an ounce of dodder 

 in a tori of clover is the limit of impurity considered permissible. 

 On the Continent the limit varies — from absolute freedom to from 

 five to ten dodder seeds in one kilogramme of clover. Most 

 foreign clovers contain dodder, and may require several sievings 

 before becoming free from it. At the recent International 

 Conference in Hamburg it was decided to invite from all over 

 the world information as to the prevalence of dodder in the 

 flora, etc., of each country. 



The procedure followed in testing the purity of a seed is 

 quite simple. A definite weight, varying according to the kind 

 of seed, is taken from an average sample of the bulk. The 

 impurities of the two kinds already mentioned are separated 

 out, weighed, and expressed as a percentage. A difficulty arises 

 in some cases where the seeds are blind or deaf, i.e. have all the 

 external characters of the true seed, but lack the kernel. This 



