THE PRINCIPLES OF SEED-TESTING 489 



4. True Value 



It is obvious that one gets an imperfect idea of the quality 

 of a seed by considering the purity and germination apart from 

 one another. A seed may be pure, but of low germinating 

 power through age, " heating," want of ripeness, etc. Another 

 seed may be very impure, but what there is of it, true to name, 

 may germinate well. Either report alone would be misleading. 

 I had one sample of Timothy grass to test, intended for experi- 

 mental work. It was pure and looked good seed, but germinated 

 only 10 per cent. It was in consequence useless. Hence it is 

 usual to combine the purity and germination percentage to get 

 the True Value of the seed. This is expressed by the following 

 formula, where — 



P = percentage of purity, 



G = percentage of germination, and 



T V = True Value, P x G = T V. 



100 



Thus a sample of perennial rye showing 90 per cent, purity and 

 80 per cent, germination has a true value of 72 — i.e. every 100 lb. 

 contains only 72 lb. really good seed. 



In the Irish Station the percentages of purity and germination 

 are stated separately. 



Many factors affect the germination of a given seed. We 

 are not yet in a position to say what is the best or optimum 

 condition in each respect for the production of the highest 

 degree of germination in the laboratory of each particular kind 

 of seed. In nature the seed is still less placed under the 

 optimum conditions in every respect ; it is in consequence 

 usual to assume that the percentage of germination in nature 

 will be 5 per cent, less than under the more or less controllable 

 artificial conditions. Further, 5 to 8 per cent, is allowed as a 

 margin of error in cases of dispute in estimating percentage of 

 germination in the Seed Station. (Advantage is sometimes taken 

 of this by the trade to add on to a Station's report 5 to 8 per 

 cent.) The poorer and more uneven the seed the greater will be 

 the difference in two tests of it even under apparently identical 

 conditions. One is apt to lose sight of the fact that the object 

 of testing the seed should be, not to obtain the highest possible 

 degree of germination under the most perfect artificial con- 

 ditions, but to secure a good working idea of the germinating 



