304 



Doubleness in Stocks 



Double stocks are completely sterile, forming neither pollen nor 

 ovules, and consequently they are always obtained from seed set by 

 singles. 



Among the singles certain strains breed true to singleness, producing 

 only singles in successive generations, whether self-fertilised or inter- 

 bred ; these are referred to as no-d-strains. Other strains of singles, 

 indistinguishable to the eye from those of the previous class, yield 

 a mixed offspring of singles and doubles when self-fertilised or inter- 

 bred, the doubles being mostly (? invariably) in excess of the singles — 

 referred to as d-strains^. 



The behaviour of these two types of singles may be graphically 

 contrasted thus : 



no-d-single 



d-single 



singles 



singles 



singles 



and so on indefinitely. 



doubles (sterile) 



singles 



doubles (sterile) 



singles doubles (sterile) 



and so on indefinitely. 



A strain composed entirely of c?-singles would thus be " ever- 

 sporting." 



Further progress in the elucidation of this peculiar type of inheritance 

 was made when it was shown that the eversporting character results 

 from a difference in distribution of the factors concerned, among the 

 ovules and the pollen grains respectively. In a single belonging to an 

 eversporting strain the pollen grains all appear to behave alike and all 

 carry doubleness, whereas the ovules are evidently heterogeneous, rather 

 more than half carrying the double, and the remainder the single 

 character. These conclusions were arrived at through the different 

 results obtained in reciprocal unions between pure-breeding and ever- 

 sporting individuals. For while no-d-single $ x c^-single (^ gives Fi 

 plants all throwing doubles on self-fertilisation, the reciprocal cross 

 rf-single $ X no-d-single j/* gives Fi individuals of two kinds, viz. 

 those which, when self-fertilised, throw doubles, and those which 

 breed true to singleness. The composition of the resulting generations 

 in the two cases is compared below. 



'■ Crosabreds are not here in question. 



