Foliar sclereids of Olea europaea may function as optical fibres

Adaxial epidermises that contained an intact "subepidermal" "T"-shaped sclereid network, were isolated from olive tree leaves using a new method of enzymatic maceration. In the leaf, the sclereid network was anchored on the adaxial epidermis. The basal parts of the T-shaped sclereids penetrate the palisade cell layers into the underlying spongy parenchyma cells. In paradermal sections, the basal parts of the sclereids appeared as light-emitting sources or spots among the dark-green background of the palisade layers. The light spots almost disappeared when the intercellular air spaces were filled with immersion oil that had a similar refractive index to that of cell walls. It is plausible that light is propagated through multiple reflections between the thick cell wall of sclereids and the air filling the intercellular spaces, the former having a significantly higher refractive index. Sclereids act like synthetic optical fibres and, besides other functions, they may contribute to the improvement of the light microenvironment within the mesophyll of the thick and compact sclerophyllous leaves of Olea. Key words: foliar sclereids, optical fibres, light guiding, Olea europaea.