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Field botanists often have clever ways to identify species that are difficult to distinguish from each other without a microscope or reference to herbarium specimens. 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 because relative lack of internal lignification and 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 that 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. The discovery of the anatomical basis underlying the contrasting biophysical properties of the needles of these two species of pine may stimulate ecological investigations and could have applications as far reaching as the engineering of structures such as manufactured cables or sandwich composite materials.

For further detail, see: Meicenheimer et al.—Anatomical basis for biophysical differences between Pinus nigra and P. resinosa (Pinaceae) leaves, Volume 95, Issue 10, pages 1191–1198. Photo credit: Eric M. Chapman.