S U M M A R YTo gain detailed information on the structure and dynamics of the foliage within a canopy, the longevity of leaves of cacao trees (Theobroma cacao L.) growing under normal commercial planting shade in Bahia State, Brazil, was determined relative to their position within the canopy and the time of leaf emergence.In the experimental stand, the relative light intensity above the cacao canopy ranged between 30 and 100% of full daylight, and 4-10 '' o at ground level. The mean leaf area index for the canopy, and the extinction coefficient of the foliage were 3-9 and 0-61, respectively.The longevity of leaves of adult trees of cacao with closely packed crowns varied with their position within the canopy, and/or the irradiance received, and the time of emergence. In particular longevity decreased greatly with the height of the leaf from ground level. The mean longevity for the upper foliage (more than 220 cm above the ground) within the canopy was 181 d; the mean longevity of the lower leaves (0-150 cm above the ground) was about twice that of the upper leaves.x\pproximately 23 °o of emergent leaves fell within 60 d of emergence, following infection (about 14% of loss) or as a result of wind damage (about 5 % of loss). Thereafter, in mature and senescent leaves, leaf fall followed normal physiological changes.

summary The photosynthetic and respiratory activities of the leaves of a tropical tree, cacao, Theobroma cacao L., grown under shading, were determined in relation to leaf age and light conditions within the canopy, in order to gain a deeper insight into characteristics of the leaf population composing a canopy. The specific leaf weight and leaf water content varied with the height of leaves from the ground and/or the irradiance. The net photosynthetic rate also varied markedly depending on leaf age and irradiance. The leaves in full or nearly full irradiance (< 70% full daylight) attained the maximum rater. 60 d after the leaves emerged, and the rate decreased to nearly zero at a leaf age of c. 270 d. The nocturnal leaf respiratory activity was proportional to the photosynthetic activity during the day, but the proportional ratio was not constant throughout the survival period of leaves. Within the closed canopy, the upper leaves, which much higher ratios of respiratory to photosynthetic activity, under full or nearly full irradiance, tended to have much shorter mean longevities (c. 160 d) than the lower leaves under a lower irradiance (c. 310 d).