316 ESSENTIALS OF BOTANY 
time of planting one seed its descendants might be made 
to cover more than 5,000,000 acres of wheat-fields. 
Wheat-breeding is still making such rapid progress that 
it is not now possible to say how much the quality and 
quantity of our wheat crop may yet be improved by the 
introduction of better varieties. The total number of 
acres in the United States differs considerably from year 
to year. It seems likely, as a rule, to exceed 45,000,000 
acres. The average yield ranges between ten and fifteen 
bushels per acre, although it is possible with the most 
improved seed on the best soils to raise more than forty 
bushels per acre. Choice of the best seed would undoubt- 
edly increase the average yield to from thirteen to eighteen 
bushels. It is easy to see how important a gain this would 
be, even if the price of wheat were no more than seventy 
cents per bushel. 
395. Hybridizing. — Hybridizing, as the term is now 
generally used, means the production of seed by the action 
of pollen of one variety or species on the pistil of another 
variety or species. Nearly always both species must at 
least belong to the same genus in order to produce seed 
that will grow, and often different species of the same 
genus cannot be made to hybridize so as to secure good 
seed. The offspring produced by hybridization are known 
as hybrids. 
It has long been known that hybrid plants are often: 
extraordinarily variable, but the law which governs their 
characteristics (in many though not in nearly all cases) 
was not discovered until 1865.1 
Recently much use has been made of hybridizing in 
order to set plants to varying, and the most desirable 
1 See Bailey’s Plant Breeding, Chapter IV, The Macmillan Company. 
