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'Pith' is a light substance that is found in vascular plants. It consists of soft, spongy parenchyma cells, and is located in the center of the stem. It is encircled by a ring of xylem (woody tissue), and outside that, a ring of phloem (bark tissue). In most plants the pith is solid, but some plants, e.g. grasses and umbellifers, the pith has a hollow centre forming a hollow tube except at the points where leaves are produced, where there is a solid plate across the stem. A few plants, e.g. walnut, have distinctive chambered pith with numerous short cavities in the pith.
The pith varies in diameter from about 0.5 mm to 6-8 mm in solid pith, and up to 150 mm or more in the stems of some plants with hollow pith, e.g. some bamboos. Freshly grown pith in young new shoots is typically white or pale brown, commonly darkening with age. In woody plants (trees, shrubs), the pith becomes surrounded by successive annual layers of wood; it may be very inconspicuous but is always present at the centre of a trunk or branch.
The cells in the peripheral parts of the pith may in some plants (e.g. ''Hedera helix'') develop to be different from cells in the rest of the pith. This layer of cells is then called ''the perimedullary region of the pith''.
The pith of the sola or other similar plants is used to make the pith helmet ^[1].
The pith of some plants, as sago, is edible to humans.

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References

1. AskOxford.com - Pith helmet