Pinus: a conifer. TS leaf (i.e. pine needle). Two vascular strands, endodermis, cortex with resin canals, and epidermis with sunken stomata. 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 period. The cosmopolitan pines spread during the Mesozoic period and today some of 600 species form extensive forest. Many produce timber of great economic importance while their resins and ethereal oils are used in medicine and elsewhere.