Primary and Secondary Xylem
Primary xylem is the xylem that is formed during primary growth from procambium. It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. Metaxylem has wider vessels and tracheids than protoxylem.
Secondary xylem is the xylem that 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:
- 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.
- 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.
Read more about this topic: Xylem
Famous quotes containing the words primary and/or secondary:
“The primary function of myth is to validate an existing social order. Myth enshrines conservative social values, raising tradition on a pedestal. It expresses and confirms, rather than explains or questions, the sources of cultural attitudes and values.... Because myth anchors the present in the past it is a sociological charter for a future society which is an exact replica of the present one.”
—Ann Oakley (b. 1944)
“Scientific reason, with its strict conscience, its lack of prejudice, and its determination to question every result again the moment it might lead to the least intellectual advantage, does in an area of secondary interest what we ought to be doing with the basic questions of life.”
—Robert Musil (1880–1942)