The midrib is the major stiffening element of maize leaves. Its anatomical study reveals a complex cross-sectional shape and a composite constitutive material. A mechanical model is proposed, based on composite beam theory, to account for the bending mechanical behaviour of a beam element with an heterogeneous cross-section of any transverse shape. The inputs into the model are the outlines of the various material surfaces within the cross-section, and their corresponding Young’s moduli. The predicted outputs are tested against experimental data from an in vivo flexural test. The model prediction gives a good first order approximation of the experimental data. The maize midrib can thus be considered as a sandwich beam. The model can also be applied more generally to any plant axis, once given the outlines and the rheologies of its constitutive material.