Pith  - PITH is a tissue in the center of trunks, branches, and twigs, made up of large cells.  When a twig begins to grow from a bud, the greatest volume of tissue is the pith.  The central pith cells are like large balloons.  A sheath of thicker-walled cells surrounds the large pith cells.  The pith cells often contain chlorophyll, and the chlorophyll may remain green and active for several years, even after several layers of wood surround the pith.  As the pith cells lose their contents, the cells appear like empty balloons or bubbles.  Where a branch meets the joining stem, the pith of the joining stem and the pith of the branch are often separated by a mass of thicker-walled cells.  This mass of cells appears as a plug at the base of the branch.  It is called the pith protection zone.  It is a zone made up of thicker-walled cells that contain a high amount of phenols in hardwoods and sterpenes in conifers.  If a pathogen infects the newly forming branch, the pith protection zone will resist spread of the pathogen into the joining stem.  A similar pith protection zone often forms at nodes between the young developing central leaders. The cells that form the protection zone can be seen in the dome tissue in the bud.  The pith protection zone is the first protection boundary in the young developing tree.  There is no pith in roots.  They have a stele.  This is one way to tell where trunk ends and root begins.  Most oak species stems have a star-shaped pith.  Oaks are not the only ones.  Some trees may also have a square shaped pith in the stem.  Walnuts are chambered.  Pith in branches is most of the time on the up side.  The stele of woody roots is most of the time on the lower side.

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