Pronunciation: ak-TIN-o-steel.

Etymology: This word is formed from two Greek roots that mean a ray of light (aktina- ακτίνα) and an upright monument (stele- στήλη). The reference is to a structure of xylem and phloem in a stem which is radially-symmetrical in cross section.

An actinostele is a type of protostele in which the xylem forms a star in x-section. Typically, the phloem lies in the “valleys” between the points of the star (see the photo from Biodidac below).

This photo of a Ranunculus root cross section shows the classic actinostele structure. Xylem (stained red) forms a star pattern in the center. Phloem (stained green) lies in the arms of the xylem.

By Jack R. Holt. Last revised: 01/05/2009