Secondary xylem is formed by a vascular cambium. The two main groups in which secondary xylem can be found are:

1. 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 marketed as softwood.
2. angiosperms (Angiospermae): there are some quarter of a million to four hundred thousand species of angiosperms. Within this group secondary xylem has not been found in the monocots. In the remainder of the angiosperms this secondary xylem may or may not be present, this may vary even within a species, depending on growing circumstances. In view of the size of this group it will be no surprise that no absolutes apply to the structure of secondary xylem within the angiosperms. Many non-monocot angiosperms become trees, and the secondary xylem of these is marketed as hardwood.

Secondary xylem is also found in members of the "gymnosperm" groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta.

Online pictures of secondary xylem are presented by Mauseth and by Curtis, Lersten & Nowak

plant anatomy | wood