- 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.
Click here for some pictures.
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Keslick & Son Modern Arboriculture
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