Light micrograph of a transverse section of a young Pine stem (Pinus). Shows pith, one year of secondary xylem, trace to side branch, cortex with numerous resin canals, epidermis. 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.