Secondary Wood

It is generally accepted that secondary xylem  has undergone a long evolutionary history. The main trends can be seen because the various stages are often related to other 'marker' characters in flowers and fruits of the plants concerned. There are instances where habitat has seemingly reversed some of these trends in various species, but overall, their 'direction' can be fairly safely defined.

The Vitis (grapevine) stem cross section shown here, was made from a three year old vine. This section shows the development of a narrow band of secondary phloem (above) and much wider band of  secondary xylem (below) a narrow zone of thin-walled cells called the vascular cambium. It is the vascular cambium that is responsible for the development of secondary wood.