Two-way traffic in the vascular tissue of a conifer needle

Conifer needles transport water and sugar through systems of essentially parallel tubes of tracheids (water) and sieve elements (sugar). This comparatively simple architecture is contrasted by the complex tissue surrounding them, and extending out to the bundle sheath, a cylindrical wall of cells that protects the vascular bundle against intrusion of air. This tissue consists largely of two cell types: transfusion tracheids carrying the outgoing water driven by transpiration and transfusion parenchyma carrying sugars produced in the mesophyll outside of the vascular bundle into the sieve elements. We have studied this tissue – unique to gymnosperms – by X-ray tomography on intact conifer needles and by TEM, revealing a surprising structure, reminiscent of an anisotropic “swiss cheese”, where the water moves out through the continuum, and the sugar moves in through the holes forming percolating clusters. We shall discuss main structural features of this tissue and how it can perform the complex task of conducting water and sugar in opposite directions.