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Crops grow and use water because they intercept radiation from the sun, the .sky, and the atmosphere. Diurnal changes of solar radiation dictate the diurnal course of photosynthesis and transpiration, and the vertical gradient of radiant flux in a canopy is a measure of the absorption of energy by foliage at different heights. Without exaggeration, the distribution of radiation within a plant community is the most important single element of microclimate.
Early ecological studies of radiation climate were mainly descriptive and were limited in scope by rather primitive instrumentation. A new quantitative approach to the subject was initiated by Monsi and Saeki (1953) and by Kasanaga and Monsi (1954) whose models of light distribution in plant canopies were a basis for many subsequent studies, both experimental and theoretical. About half the literature published in the last 15 years is concerned with the development of more elaborate models- an indication that it is easier to investigate light distribution at the desk than in the field I About a quarter of the literature describes new measurements, and the balance consists of review articles. Reviews have been so thorough and frequent (Saeki, 1963; Anderson, 1964; Reifsnyder and Lull, 1965; Loomis, Williams and Duncan, 1966) that my contribution to this symposium may appear premature, but I shall try to justify the exercise by being deliberately provocative. As an opening shot, crop ecologists are not concerned with the distribution of radiation per se but with rates of photosynthesis and with yield. The literature reveals a curious reluctance to test models of light penetration in crops by comparing predicted rates of dry matter accumulation with measurements in the field. We have scarcely begun to exploit models for the solution of agronomic problems.