Tilia stem. Light micrograph (LM) of a section through a wood stem of a lime tree (Tilia sp.).Tilia is a genus of about 30 species of trees or bushes, native throughout most of the temperate Northern Hemisphere. At centre is the pith, surrounded by a layer of wood (xylem). A thin cambium separates this from the phloem. An outer layer of collenchyma is enclosed by the outer bark. Magnification: x10 when printed at 10 centimetres wide.