Light micrograph of a radial longitudinal section of a young Pine stem (Pinus). Tracheids in the xylem show bordered pits in face view. Rays are cut in longitudinal section. Most conifers are profusely branched trees. In advanced forms the leaves become reduced to simple needles or scales. Secondary xylem is mainly tracheids with bordered pits. Male cones have pollen sacs on microsporophylls. The much larger female cones have ovuliferous scales in the axils of bracts. Conifers first appeared in the Upper Carboniferous. The cosmopolitan pines spread during the Mesozoic and today some 600 species form extensive forests. Many produce timber of great economic importance, while their resins and ethereal oils are used in medicine and elsewhere.