The previous papers in this series (Jennings and de Jesus, Jr., 1968;Jennings and Herrera, 1968) concerned the magnitude and direction of interphenotypic competition in varietal mixtures and hybrid populations of contrasting plant types of rice (Oryza sativa L.). Relative yielding ability of the components of the mixtures was established from long-term yield trials of material grown in pure stand. It was concluded that competitive ability and agronomic worth were negatively associated in the materials studied under the agronomic and environmental variables provided.There are many basic implications and consequences of these findings to plant breeders, geneticists, and students of evolution. Their universality depends, in part, on a fuller understanding of the mechanism of competition and competitive ability. The literature contains many descriptions of the effect of competition on reproductive rates of organisms. Comparable attention, however, has not been given to such critical questions as: (1) What are the effects of competition on plant development? (2) At what growth stage does competition begin?(3) What are the plant characters that confer competitive ability? (4) What is the factor for which rice plants compete? Answers to these questions are required to increase understanding of the phenomenon of competition. This paper treats these aspects. MATERIALS AND METHODSTwo sets of materials were studied: the pure-line varieties BJ and MTU, described in the first paper, and separate F 6 bulks of the monogenically differentiated tall and dwarf plants reported in the second paper of this series. EVOLUTION 22:529-542. September, 1968 BJ is an exceptionally strong competitor in mixtures with MTU. The competitvely inferior MTU, however, is typically about 30 cm taller at maturity than the high'ly competitive BJ. Similar examples in other crops of the taller plant being less competitive have confused interpretation of competitive ability and was the reason for the choice of these varieties.Both sets of material were grown with the same experimental design. Each variety or F6 bulk was grown in pure stand in single-plant hills at a spacing of 30 X 20 em. Mixtures of plant types were planted adjacent to the pure stands at the same spacing as follows: BJ and MTU seedlings were transplanted alternately in rows in a checker-board fashion such that each BJ plant was surrounded by four MTU plants and each MTU plant was surrounded by four BJ plants. The tall and dwarf mixture was established in the same manner. The BJ -MTU mixture and the BJ and MTU pure stands were given 20 kg Njha. A nitrogen amendment of 40 kgjha was given the tall-dwarf mixture and pure stands.Data were recorded from the pure stands and the mixtures at weekly intervals beginning 32 days after sowing (11 days following transplanting). Ten plants of each phenotype were sampled at random each week until flowering. Data recorded included plant height, tiller number, leaf number, leaf length, area per leaf, leaf area index (LAI), dry weight of aerial portions...

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