Vascular Anatomy of Angiospermous Leaves, with Special Consideration of the Maize Leaf

Abstract In this brief review an attempt has been made to discuss some of the important features of the vascular anatomy of angiospermous leaves, especially those related to assimilate transport. Accordingly, emphasis has been placed on the small or minor veins, which are closely related spatially to the mesophyll, and which play a major role in the uptake and subsequent transport of photosynthates from the leaf. The small veins are enclosed by bundle sheaths that intervene between the mesophyll and vascular tissues and greatly increase the area for contact with mesophyll cells. In the minor veins of dicotyledonous leaves, parenchymatic cells having organelle‐rich protoplasts and numerous cytoplasmic connections with sieve elements dominate quantitatively. It is these so‐called intermediary cells that apparently are directly involved with the loading of assimilates into the sieve elements. In the maize leaf the small and intermediate bundles have two types of sieve tubes, relatively thin‐walled ones that have numerous cytoplasmic connections with companion cells, and thick‐walled ones that lack companion cells but have numerous connections with vascular parenchyma cells. The companion cell‐sieve tube complexes are virtually isolated symplastically from other cells of the vascular bundle and from the bundle sheath. Thick‐walled sieve tubes similar to those in the maize leaf have been recorded in the leaves of other grasses.