TABLE III.

Frequency Distribution of Height of Plant of Leaming Corn and Four Inbred Strains Derived from It.

Height of Plant in Inches.

58.5 63.5 68.5 73.5 78.5 83.5 88.5 93.5 98.5 103.5 108.5 113.5 118.5 123.5 128.5 133.5 138.5 143.5 N

A

C. V.

TABLE IV.

Frequency Distribution of Length of Ear of Leaming Corn and Four Inbred Strains Derived from It.

In ordinary fields of corn they are generally kept out of sight by continual crossing with normal types which are dominant. Plants carrying such factors for abnormality, when self-fertilized, produce them in approximately one-fourth of their progeny. Some of the normal plants in the same progeny carry the abnormality and some do not. Sooner or later, progenitors are used which do not carry any of these striking abnormalities, after which they cease to appear.

The rate at which reduction in growth takes place and the final size and productiveness of the several lines, after the reduction comes to an end, vary in different lines. Of the four Leaming strains the D line has regularly been taller and larger and has yielded more than the others. The rate of reduction has been nearly alike in all of the four lines although A was reduced in yield somewhat more quickly than any of the others. The attainment

Figure 20.

Comparative production of a variety of Leaming corn, two inbred strains derived from this variety, and their first generation hybrid. Grown in adjoining rows, they yielded 96, 32, 20 and 115 bushels per acre respectively.

of uniformity may also proceed at a different rate, depending upon the degree of heterozygosity of the plant chosen as progenitor. Some strains remain variable for many generations while others become uniform in nearly every feature after a few generations of self-fertilization.

From the foregoing facts it is obvious that inbreeding is a process of sorting out. From a mixture of many genetically different individuals all varying in hereditary composition and in heterozygosity any number of homozygous lines can be ultimately obtained, each differing to a greater or less degree from every other. A naturally cross-fertilized species is thus changed into an artifically self-fertilized species. In uniformity and constancy these artificially inbred plants are quite comparable to naturally self-fertilized species, with the important difference that in corn they are markedly reduced in size and vigor.

RESULT OF CROSSING.

The vigor which is lost by inbreeding is at once restored when two self-fertilized lines descending from different plants at the start

are crossed. This is shown in figure 20. Here the ears produced by the original non-inbred variety are shown in comparison with the ears produced by two lines self-fertilized 12 generations and the

first generation hybrid between these two lines. An equal number of plants of the four lots were grown in adjoining rows and yielded 96, 32, 20 and 115 bushels per acre respectively. A comparison of a large number of first generation crosses between inbred strains derived from the same variety showed that the yield of the hybrids was increased 180 per cent., height of plant 27, length of ear 29, number of nodes 6, and rows of grain on the ear 5 per cent. above the average of their inbred parents.* From this it is seen that size characters such as height of plant and length of ear are affected more noticeably by hybrid vigor than the number of parts, such as nodes and rows of grain on the ear, while yield, which sums up

Figure 22. Representative ears of three inbred strains of dent corn and two first generation hybrids resulting from the crossing of the two adjoining types, harvested at the same time to show the difference in maturity.

the entire growing capacity of the plant, is increased more than anything else. In other words hybrid vigor has much the same effect as favorable environmental factors. Fertile soil, good season and careful cultivation influence the growth of the corn plant. Under these conditions corn grows taller, the ears are larger and the production of grain is much greater than under the less favorable conditions, while the number of nodes or the rows of grain on the ear are not so much changed.

*"The effects of inbreeding and crossbreeding upon development." Connecticut Agric. Exper. Station Bull. 207.