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Premise: Previous studies have suggested a trade-off between trichome density (D t ) and stomatal density (D s ) due to shared cell precursors. We clarified how, when, and why this developmental trade-off may be overcome across species., Methods: We derived equations to determine the developmental basis for D t and D s in trichome and stomatal indices (i t and i s ) and the sizes of epidermal pavement cells (e), trichome bases (t), and stomata (s) and quantified the importance of these determinants of D t and D s for 78 California species. We compiled 17 previous studies of D t -D s relationships to determine the commonness of D t -D s associations. We modeled the consequences of different D t -D s associations for plant carbon balance., Results: Our analyses showed that higher D t was determined by higher i t and lower e, and higher D s by higher i s and lower e. Across California species, positive D t -D s coordination arose due to i t -i s coordination and impacts of the variation in e. A D t -D s trade-off was found in only 30% of studies. Heuristic modeling showed that species sets would have the highest carbon balance with a positive or negative relationship or decoupling of D t and D s , depending on environmental conditions., Conclusions: Shared precursor cells of trichomes and stomata do not limit higher numbers of both cell types or drive a general D t -D s trade-off across species. This developmental flexibility across diverse species enables different D t -D s associations according to environmental pressures. Developmental trait analysis can clarify how contrasting trait associations would arise within and across species., (© 2024 The Authors. American Journal of Botany published by Wiley Periodicals LLC on behalf of Botanical Society of America.)