2. Reciprocal crosses between inbred strains may show small differences in amount of growth in early stages after germination due to differences in embryo size or seed condition. These differences usually disappear by the time the plants flower. In crosses of a Rice pop inbred with very small seeds and a yellow dent inbred with large seeds marked differences were obtained in the reciprocals. Three weeks after planting the dent parent was nearly twice as tall as the pop parent and proportionally larger in overall size dimensions. At this stage the dent × pop F1 is taller than the dent parent while the pop × dent F1 occupies an intermediate position between the two parents. The hybrids and parents tassel and silk in the same order as their initial embryo weights: (1) dent × pop, (2) dent parent, (3) pop × dent, (4) pop parent. At the end of the season the two reciprocal crosses are equal in production of grain and in height and both are taller and more productive than either parent. Production of grain of the hybrid is about 15 times that of the pop parent and nearly twice as much as the dent. Both reciprocals reach full maturity at about the same time but the one that is smaller at the start continues rapid growth longer to reach eventually the same height and production of grain in approximately the same length of time. Since one of the hybrids starts smaller after germination and ends up larger in the amount of material produced than the larger parent, in the same period of growth, one is growing at a faster rate than the other.
The parents and reciprocal crosses also differ in the number of tillers. The dent inbred averages .03, dent × pop 2.06, pop × dent 1.24, and pop inbred 2.83 tillers per plant. The larger number of tillers is shown by the hybrid with the non-tillering seed parent. In these reciprocal crosses having the same genic constitution, tillering is an expression of initial vigor large enough to overcome any differences in maternal effect. Differences that may exist in the cytoplasm of these two widely diverse reciprocal crosses have no effect on the final reaction product between the external environment and the nuclear construction of the hybrids.
D. F. Jones