When excised epicotyl segments of Pisum are incubated for periods of up to 24 hours in 0.01 M potassium maleate they exhibit a considerable increase in their ability to absorb and esterify phosphate. In comparison, hypocotyl segments of Gossypium and Helianthus subjected to similar treatments show no change in their ability to absorb and esterify phosphate. The difference in response is attributed to differences between the species themselves rather than to their morphological nature. It is possible to reduce the magnitude of the increase in the rate of phosphate uptake occurring in segments of Pisum epicotyls by increasing the phosphate concentration of the outside medium. This reaction closely resembles that which occurs when slices of potato tuber are similarly treated. In contrast to potato slices, the rate of respiration of Pisum epicotyl segments decreased during the incubation period. The increase in the rate of phosphate uptake is accompanied by an increase in the rate of sulphate absorption and protein synthesis. Other results showed that the rate of phosphate absorption by fresh and incubated tissue is limited by different systems. It would appear that the rate of uptake of phosphate by fresh segments is limited by its rate of diffusion into the tissue, whilst that absorbed by incubated segments is limited by the metabolic activity of the segment.