Vascular tissue in the stem and roots of woody plants can conduct light.

The role of vascular tissue in conducting light was analysed in 21 species of woody plants. Vessels, fibres (both xylem and phloem fibres) and tracheids in woody plants are shown to conduct light efficiently along the axial direction of both stems and roots, via their lumina (vessels) or cell walls (fibres and tracheids). Other components, such as sieve tubes and parenchyma cells, are not efficient axial light conductors. Investigation of the spectral properties of the conducted light indicated that far-red light was conducted most efficiently by vascular tissue. Light gradients in the axial direction were also investigated and revealed that conducted light leaked out of the light-conducting structures to the surrounding living tissues. These properties of the conducted light suggest a close relationship with metabolic activities mediated by phytochromes. The results therefore indicate not only that signals from the external light environment can enter the interior of stems above ground and are conducted by vascular tissue towards roots under ground, but also that the light conducted probably contributes directly to photomorphogenic activities within them.