e.g. potato.
* Woody: Hard textured stems with secondary xylem.
* Sapwood: The outer layer of secondary xylem which is still active in fluid transport, and contains live parenchyma cells.
* Heartwood: Secondary xylem which is no longer active in fluid transport and contains no living cells, serving only as structural support. It is darker in color due to the deposition and oxidation of sexondary metabolites, such as polyphenolic compounds.

Stem structure