Light micrograph of a transverse section of a mature Pine stem (Pinus). Three rings of secondary xylem, and trace to side branch. 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.