Field botanists often have clever ways to identify species that are difficult to distinguish from each other without a microscope or reference. Usually the scientific basis or mechanism of such a “test” is unknown, but the test is reliable enough to be widely used. The amount and pattern of cell wall lignification (blue-white fluorescence) in the internal tissues of these two superficially similar looking pine needles has been determined to be the causal basis for the “leaf bending test,” often used to discriminate these two species in the field. Careful biophysical and anatomical analyses revealed that Pinus nigra (top) appears flexible when bent due to a relative lack of internal lignification. Its structural integration causes the leaf structure to fail in compression via a succession of crimping on the flat adaxial face of the leaf. In contrast, Pinus resinosa (bottom) “snaps” at the rounded vertex of the leaf when bent as a result of resistance to tensile forces transmitted through the highly integrated internal leaf tissue, which is provided by the relative abundance of lignification in the walls of the mesophyll cells and the thick lignified periclinal walls of the endodermal cells (Meicenheimer et al., 2008).
Image courtesy of Eric M. Chapman.