SUMMARY From foliar emergence until senescence, the abaxial diffusive resistance of primary foliage, and of regrowth foliage produced after defoliation, was measured at several irradiances with a ventilated diffusion porometer. Concurrent measurements were made of stomatal dimensions and frequency, including scanning electron‐microscopy, and of leaf area. The diffusive resistance of newly emerged primary leaves was about 15 s cm−1 in maple and 20 s cm−1 in oak at all irradiances. The stomata became sensitive to light within 2 weeks of emergence, oak being slightly earlier than maple. Thereafter, the resistance at an irradiance of 0.5 cal cm−2 min−1 fell to a seasonal minimum of 3 s cm−1 in oak and 5–7 s cm−1 somewhat later in maple. With the onset of senescence, and earlier in maple than in oak, the resistance of primary leaves in bright light increased and the stomata became less sensitive to light. As leaf colour changed from green to red or brown, the resistance increased to about 60 to 80 s cm−1 in both oak and maple. The resistance of regrowth foliage that emerged in July was about 5–7 s cm−1 in maple and 12–15 s cm−1 in oak, thus lower than that in newly emerged primary foliage. Within a week of emergence, 3 weeks sooner than with primary foliage, the resistance at 0.5 cal cm−2 min−1 fell to about 3 s cm−1 in both species. During July and August, the resistance of regrowth foliage of maple was about one‐third that of the primary foliage, but about the same in both regrowth and primary foliage of oak. At the time of regrowth leaf emergence (July), the mean temperature was 10° C higher than that at primary leaf emergence (May), but in both species the initial rates of leaf expansion were similar. Although the areas of individual primary leaves were about twice those of regrowth leaves, the stomatal frequencies and dimensions were similar. Maximum stomatal length was achieved before the leaves were fully expanded and before the stomata became responsive to light. The development of light sensitivity was apparently independent of leaf expansion and the attainment of final stomatal size, but strongly dependent on environmental regulation of stomatal metabolism.

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