The study herein reported consists of observations on the vegetative development of maize with special emphasis on leaf production. The objective has been to obtain a better picture of the dynamics of hybrid vigor as expressed through growth processes such as rate, correlation, sequence and final size. Most of the results were obtained from two Iowa inbred strains and their reciprocal hybrids, and for that reason may not be entirely characteristic of a general population.

Specific researches on growth and heterosis of maize are so numerous as to prohibit any but general comment. Orthodox explanations of hybrid vigor or heterosis have drawn heavily on the theories of inheritance. Shull (23) was long the proponent of a hypothesis which suggested that the diversity of the parental elements brought together in some way stimulated growth and development in the offspring. Later, Jones (10, 11) developed a concept in which the increased vigor of a hybrid was attributed to the greater number of active, favorable hereditary characters it possessed, the combined contributions of both parents being greater than that possessed by either parent alone. Rasmusson (18) offered the interaction hypothesis in which the combined action of the hereditary factors, as well as the sum of their, individual actions, plays a part in any expression of heterosis. Finally, there should be considered the experiments of Sveschnikova on hybrids of Vicia (24). As opposed to the ordinary concept of hybrid vigor, this author presents instances in which decreased vigor was obtained in crosses between very similar forms of the same species.