Improvements in apparatus for control of environmental factors in plant growth are described, together with methods of estimating the total illumination effective from sun and electric lamps. Results are given of 9 series of wheat cultures in water, sand, and soil. Each series was subjected to 2 planes of temp., but with illumination and atmospheric humidity uniform. Atmospheric humidity exercises little influence on the protein content of wheat during seed development. With illumination probably deficient in intensity, lower temp. favored the production of dry matter, as if conserving limited supplies of photosyn-thetic products. Under these conditions the lower temp. generally favored both the % and yield of protein in the tissues. The failure of seed formation in plants thus closely confined is chiefly due to unfavorable conditions for pollination. With illumination probably adequate in intensity, young wheat plants contained greater % of sucrose and protein when grown at lower temp. Mature plants contained increased % of carbohydrates and gave markedly increased yields, especially of grain, at the lower temp. The higher sugar content at lower temp. appears to favor protein synthesis, but at higher temp. deficiency of nitrate supply seems to operate as a limiting factor. Although the amount of protein synthesized is less at the nigher temp., its % in the tissue is increased, apparently because of greater respiratory loss of carbohydrates than at the lower temp. To explain these effects, the conception is advanced that there is a critical balance between the temp. responses of photosyntheses and respiration.