The relationships of foliage assimilation capacity per unit area (PPmax) with leaf dry mass per unit area (LMA) and nitrogen content per unit area (NP) differ between species and within species grown in different habitats. To gain a more mechanistic insight into the dependencies of PPmax on LMA and NP, this literature study based on 597 species from a wide range of earth biomes with woody vegetation examines the relations between leaf photosynthetic capacity and the components of LMA (leaf density (D, dry mass per volume) and thickness (T)), and also the correlations of D and T with leaf nitrogen content and fractional leaf volumes in different tissues. Across all species, PPmax varied 12-fold and photosynthetic capacity per unit dry mass (Pmmax) 16-fold, NP 12-fold, and nitrogen per unit dry mass (Nm) 13-fold, LMA 46-fold, D 13-fold, and T 35-fold, indicating that foliar morphology was more plastic than foliar chemistry and assimilation rates. Although there were strong positive correlations between PPmax and NP, and between Pmmax and Nm, leaf structure was a more important determinant of leaf assimilation capacities. PPmax increased with increasing LMA and T, but was independent of D. By contrast, Pmmax scaled negatively with LMA because of a negative correlation between Pmmax and D, and was poorly related to T. Analysis of leaf nitrogen and tissue composition data indicated that the negative relationship between D and Pmmax resulted from negative correlations between D and Nm, D and volumetric fraction of leaf internal air space, and D and symplasmic leaf fraction. Thus, increases in leaf density bring about (1) decreases in assimilative leaf compounds, and (2) extensive modifications in leaf anatomy that may result in increases in intercellular transfer resistance to CO2. Collectively, (1) and (2) lead to decreased Pmmax, and also modify PPmax versus LMA relationships.