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SUMMARY

In nature, fluence rate and the red.far-red ratio are reduced in shadelight beneath vegetation; both show similar exponential decays with increasing canopy density. An assessment of the degree to which each of these factors controls development in natural shade was made by comparing Chenopodium album plants grown under vegetational shade with those grown under the controlled environment light treatments of low fluence rate, and combined low fluence rate and low red: far-red ratio. The observations of stem extension and specific leaf area from vegetational shade were entirely consistent with the effects of the controlled environment light treatments. The increase of specific leaf area in natural shade was probably due to the low fluence rate; the initial rapid stem extension in natural shade was probably due to the reduced red: far-red ratio. There is good evidence to suggest that the latter is a phytochrome-controlled phenomenon.