266 



GENETICS AND PLANT BREEDING 



cultivation. Tlie fact that a plant with 

 such a long history of use could be so 

 greatly improved in the short space of 

 a few decades is pertinently suggestive 

 for our handling of other plants and 

 animals with much shorter histories of 

 domestication. 



Despite much research and a great 

 deal of speculation, we still have no 

 certain knowledge as to the ancestry 

 of the modem cultivated type of com. 

 It is clear, however, that com early be- 

 came distributed over a fairly wide 

 range. It was grown by most of the 

 American Indians. As a basic Indian 

 food crop, the distribution of com was 

 over a number of areas more or less 

 sharply isolated from one another. In 

 each of these there appeared much 

 variation, some of which has long since 

 disappeared. Some of it has been pre- 

 served in the germ plasm of com. Be- 

 cause the corn-growing areas were 

 more or less isolated from one another 

 and represented different environ- 

 mental conditions, different sets of 

 variants have been preserved in the 

 different areas. It is an ancient observa- 

 tion that com tends to gain in vigor as 

 diverse kinds are combined. Both the 

 Indians and the early white settlers fre- 

 quently made a practice of planting 

 blue grains and red or yellow grains 

 and white grains together, or making 

 other combinations which tended to 

 result in a mixing of kinds, because the 

 yields in such mixed plantings were 

 higher than those obtained when a 

 single t}pe of com was planted by 

 itself. 



With the advent of scientific plant 

 breeding came efforts to produce true 

 breeding stocks of plants. It was early 

 discovered that when corn is self-fer- 

 tilized there results a segregation of 

 lines, often with quite different charac- 

 teristics. Repeated self-fertilization of 

 these lines ultimatelv produces stable 

 or nearly stable stocks, although gen- 

 erally a great number of the lines are 



lost during the inbreeding operations. 

 It is a universal observation that in- 

 breeding in corn to produce pure lines 

 is attended by marked degeneration. 



When different surviving inbred 

 lines of com are crossed they most 

 frequently produce hybrid progeny 

 which not only exceed their parents in 

 vigor, but generally are more vigorous 

 than the stocks from which the inbred 

 lines were originally derived. 



Hybrid corn is valued first of all 

 for its relative vigor, which not only 

 makes for greater yield, but which also, 

 apparentlv, provides better adaptabil- 

 ity to environmental conditions. Too, 

 being a genetically uniform stock, hy- 

 brid com grows and develops much 

 more uniformly than do randomly pol- 

 linated stocks. 



Recently the techniques of hybridi- 

 zation have been extended to produce 

 what are known as double hvbrids, or 

 sometimes "double-crossed" com. In- 

 bred A is crossed with inbred B to pro- 

 duce the hybrid AB, Inbred C is 

 crossed with inbred D to produce the 

 hybrid CD. The two inbreds are then 

 crossed to produce the double hybrid 

 ABCD. In many instances such dou- 

 ble hybrids have certain added advan- 

 tages. Occasionally they are more vig- 

 orous than their immediate hybrid 

 parents, but more usually their advan- 

 tages are found in somewhat greater 

 uniformity of growth and development 

 and a little wider range of adaptability, 

 probablv brought about by the com- 

 bination of four rather than two se- 

 lected germ plasms. 



The hvbridization of com has now 

 reached practically an assembly-line 

 stage. Tlie production of hybrid com 

 seed is a big business, with competitive 

 aspects which will keep alive attempts 

 to produce new and better h\brids. 

 Like any strain of plants, the efficiency 

 of a hybrid is dependent upon a proper 

 balance of inherited characteristics, the 

 soil, climate, and other factors which 



