<P> Secondary xylem is formed during secondary growth from vascular cambium . Although secondary xylem is also found in members of the gymnosperm groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta, the two main groups in which secondary xylem can be found are: </P> <Ol> <Li> conifers (Coniferae): there are some six hundred species of conifers . All species have secondary xylem, which is relatively uniform in structure throughout this group . Many conifers become tall trees: the secondary xylem of such trees is used and marketed as softwood . </Li> <Li> angiosperms (Angiospermae): there are some quarter of a million to four hundred thousand species of angiosperms . Within this group secondary xylem is rare in the monocots . Many non-monocot angiosperms become trees, and the secondary xylem of these is used and marketed as hardwood . </Li> </Ol> <Li> conifers (Coniferae): there are some six hundred species of conifers . All species have secondary xylem, which is relatively uniform in structure throughout this group . Many conifers become tall trees: the secondary xylem of such trees is used and marketed as softwood . </Li> <Li> angiosperms (Angiospermae): there are some quarter of a million to four hundred thousand species of angiosperms . Within this group secondary xylem is rare in the monocots . Many non-monocot angiosperms become trees, and the secondary xylem of these is used and marketed as hardwood . </Li>

What are groups or layers of cells with similar origin and common function called