Hybrid intensification of plant height in cotton and the relationship of node number and internodal length to the phenomenon

Intensification of plant height in the F1 generation was very marked. Hybrid intensity was transmitted to both sesqui-hybrids and the F2 generation, although the vigor waned somewhat from the conjugate generation to the back cross progeny and from the back cross generations to the F2 population. The reversed swing in the backcross and the perjugate offspring may be attributed to Galtonian regression toward the average of the races, to a weakening of the height growth due to an upset in the normal function of some of the plants caused by the crossing of species, and probably to the segregating tendencies of height genes. However, a splitting for height would hardly be expected in offspring of parental plants so nearly equal in axis length. The difference between the node numbers of the two parental strains was greater than the divergence in height. The curves for node number indicate distinct distributions which is not true for height of plant. The distributions for both parental strains lay within the same limits for height. The contrast of height allelomorphs was not sufficient for the detection of dominance activity. Many of the Upright plants were just as tall as the Pima plants and vice versa. One group could not overshadow the other. On the other hand, the contrast of the allelomorphs for node number was wide enough to get a measure of dominance. Node number in the F1 generation expressed full dominance and no intensification. The sesqui-hybrids and the F2 generation gave more indication of segregation for node number than they did for plant height. The paths of hereditary transmission of plant height, on the one hand, and that of node number, on the other, followed different courses. The axis length was much intensified in the F1 generation and then tended to regress, with an increase in fluctuation, to the average of the parental races. The node number assumed a position of full dominance in the F1 generation, diverged into the two respective groups in the sesqui-hybrids, and formed a unimodal distribution in the F2 generation. The adjustments of plant height and node number, although the two characters were following different routes of inheritance, were made by the flexibility of the internodal lengths of the plants. Hybrid intensity was due to an increase in the lengths of internodes and not to additions of joints or nodes plus internodes.