Some Examples of Unusual Orientation of Vascular Strands in Dorsiventral Leaves

It is a well known fact that in dorsiventral leaves of monocotyledons and dicotyledons the xylem strand in transverse sections is situated immediately above the phloem strand. Or, in other words, the xylem strand in such leaves is situated on the side of the palisade parenchyma, and the phloem strand underneath, on the side of the spongy parenchyma. Both descriptions are fit for use because of the fact that the palisade parenchyma is on the upper side of such leaves and the spongy parenchyma on the lower side. This relative position of the two main parts of the vascular bundles in dorsiventral leaves is brought about by the radial symmetry of the vascular bundles and the association of the xylem and the phloem within the bundles, the xylem being next the centrum and the phloem towards the outside of the stem (for which facts no interpretation can be given). Without any tendency to torsion the vascular bundles from the blade enter the petiole or the leaf sheath and hence go downwards into the stem.