Most plant breeding is based on "defect elimination" or "selection for yield". A valuable additional approach is available through the breeding of crop ideotypes, plants with model characteristics known to influence photosynthesis, growth and (in cereals) grain production. Some instances of the successful use of model characters of this kind are quoted.It is postulated that a successful crop ideotype will be a weak competitor, relative to its mass. Thus the like plants in the crop community will compete with each other to a minimum degree. This relationship of plant form to the exploitation of the enviroment may lead to two negative relationships among genotypes, namely : (a) Between the performance of cultivars at low density and at high density respectively, and (b) Between the competitive ability of cultivars against other genotypes on the one hand, and their capacity for yield in pure culture on the other.A crop ideotype will make a minimum demand on resources per unit of dry matter produced. Further, in cereals, each unit of dry matter will include such a number of florets as to ensure that the ear has sufficient capacity to accept all photosynthates either from its own green surfaces or from other parts of the plant. These criteria are to be satisfied especially at high fertility, and when the total pressure by the community on environmental resources is intensified by high density of population.A wheat ideotype is described. It has a short, strong stem; few, small, erect leaves; a large ear (this specifically means many florets per unit of dry matter of the tops); an erect ear; awns; and a single culm.The design of crop ideotypes is likely to involve concurrent modifications of the environment. The wheat ideotype here described will call for consideration of the density of planting, the fertilizer rate, the plant arrangement and weed control.Eventually most plant breeding may be based on ideotypes.

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A new technique is described for the study of the interaction between competition for light and competition for a nutrient or for water. The technique was applied in a study of the competition between two grasses for light and for nitrogen. The aggressor species showed a negative interaction in the effect of the two modes of competition; it was slightly reduced in yield by competition for either factor alone, but when competition for both factors was operative, it achieved almost the same yield as in the absence of competition. The effect of the two modes of competition on the suppressed species showed a positive interaction; the depression in yield when there was competition for both factors greatly exceeded the sum of the effects of competition for the separate factors. An analysis is made of the nature of competition for one or more factors. Competition for even a single factor involves interaction between direct and indirect effects; when competition occurs for two factors, it leads to multiple interactions between two groups of effects and thus greatly intensifies the effects of competition for either factor operating alone.