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Pith  [ Botany ]
Adjective: Pithy
  Medulla (Adj: Medullar) 

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  The pith or medulla is the spongy tissue forming the central cylinder of the stem of most flowering plant, especially those of the dicotyledonous. It consists of cellular tissue composed mainly of parenchyma.  

Cross section of a stem of Astrophytum myriostigma
The pith (Medulla) is the whitish spongy tissues encircled by the ring of vascular
bundles in the centre

Pith is a light substance originating from the ground meristem that is found in vascular plants, it is largely composed of spongy parenchyma tissue modified for storage, that has very little structural strength and is located in the centre of the stem of the Dicotyledons and in the non-woody roots of monocotyledon to the inside of the stele.  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 of woody plants, up to 30 mm in succulent stem of cacti or even 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 rings of wood -- it may be very inconspicuous but is always present at the centre of a trunk or branch.   Compare with: Cortex





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