Sun and shade leaves of 21 deciduous tree species were measured and tissue development compared. Species considered tolerant of shade had normally thinner sun leaves and less reduction in thickness in shade than did species considered intolerant. Sun—leaf/shade leaf ratios in the palisade layer were 1.22, 1.32, and 2.36 for the tolerant, intermediate, and intolerant classes respectively, while for the mesophyll the equivalent ratios were 1.9, 1.27, and 1.51. Thickness of both palisade and spongy tissue was highly correlated with blade thickness in both sun— and shade—grown leaves. Druse and prismatic crystals were prevalent in the mesophyll and vascular tissues of 14 species.