BL UE-GRA SSES 163 



The purity of commercial seed is commonly 70-80 and 

 rarely 85 per cent. The viability may reach 80-90, 

 but usually is only 65-80 per cent. In some cases very 

 fresh seeds refuse to germinate. The seeds begin to 

 sprout under favorable conditions in 9 or 10 days, but 

 many require a longer time, up to 28 days. Light has no 

 effect on germination, but rapid alternation of tempera- 

 tures is necessary for the best results. 



The legal weight of a bushel is 14 pounds, but the 

 weight varies from 14 to 28 pounds. One pound contains, 

 according to different authorities, 2,400,000 seeds (Ste- 

 bler) ; 1,860,000 (Hunter) ; 2,185,000 (Hunt) ; 3,888,000 

 (Lawson). 



Among the objectionable weed seeds that may occur 

 in Kentucky blue-grass are buckhorn, yellow dock, and if 

 adulterated with Canada blue-grass, it may contain 

 Canada thistle. 



152. Hybrids. Natural hybrids of Poa pratensis with 

 P. trivialis and with P. compressa have been described in 

 Europe, but both are very rare. Their parentage has not 

 been proven by breeding, but is surmised from structural 

 characters and the association with their supposed parents. 



G. W. Oliver of the U. S. Department of Agriculture 

 has successfully hybridized Poa pratensis with P. arachni- 

 fera, Texas blue-grass, using the pistillate plants of the 

 latter. The hybrids show much diversity and produce 

 but little seed due to defective stamens. 



153. Canada blue-grass (Poa compressa). Botany. 

 Canada blue-grass, also known as Virginia blue-grass, 

 flatstem bluegrass and wire-grass, is native to the Old 

 World, ranging throughout temperate Europe and Asia 

 Minor. It was found near Quebec as early as 1792 by 

 Michaux and in 1823 by Richardson on the upper Saskatch- 



