Field identification of Pinus nigra and Pinus resinosa often involves applying stress to a leaf, usually by bending or tying it in a knot. During this test P. resinosa generally breaks quickly, while P. nigra typically bends without failing. In an attempt to elucidate the structural basis for this difference, measurements of deflection and load bearing capacity were collected for 1st-year and 2nd-year leaves of both species using a bending test. P. nigra displayed a considerably higher load bearing capacity than P. resinosa, as well as greater flexural rigidity. Anatomical differences were investigated through image analysis of freshly cut vibratome sections using brightfield and florescence microscopy. This analysis showed no significant difference between species in the relative amounts of cross-sectional area associated with the endodermis, transfusion tissue, and vascular bundles. However, P. nigra leaves exhibited a larger cross-sectional leaf area and a thicker lignified hypodermis. P. resinosa had more resin canals, higher resin canal and mesophyll cross-sectional area, and the resin canals were closer to the surface. Young's Modulus of P. nigra was ca. twice that of P. resinosa, as was the load bearing capacity. Scanning Electron Microscopy of broken leaves showed that P. resinosa broke cleanly at the point of failure, while P. nigra exhibited irregular tearing along the leaf axis, which was associated with the elongated fibrous cells of the extensive hypodermis. We conclude that the higher flexural rigidity and load bearing capacity of P. nigra leaves are primarily related to the fact that P. nigra has significantly more hypodermal cells reinforcing the structural integrity of the leaf.

Key words: Fibrous Hypodermis, Flexural Rigidity, Load-bearing Capacity, Resin Canals, Young's Modulus