When looking at a transverse section there is an interesting difference between the vascular bundles that line the sloping sides of the adaxial groove with those lining the rest of the periphery. Vascular bundles along the periphery of the adaxial groove have their protoxylem oriented towards the outside in contrast to the other peripheral vascular bundles that have their protoxylem oriented towards the inside (Arnold & Daugherty, 1963, p. 212).
Images of vascular bundles at 200x taken with a Dino-Lite AD7013 MT 5.0 MP and resized in Adobe Photoshop CS6. The first image shows vascular bundles along the periphery with protoxylem oriented towards the inside of the petiole. The second image shows vascular bundles along the adaxial groove with their protoxylem oriented towards the outside of the petiole.