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Pith, ormedulla, is atissue in thestems ofvascular plants. Pith is composed of soft, spongyparenchyma cells, which in some cases can storestarch. Ineudicotyledons, pith is located in the center of the stem. Inmonocotyledons, it extends only into roots.[1] The pith is encircled by a ring ofxylem; the xylem, in turn, is encircled by a ring ofphloem.
While new pith growth is usually white or pale in color, as the tissue ages it commonly darkens to a deeper brown color. Intrees pith is generally present in young growth, but in the trunk and older branches the pith often gets replaced – in great part – by xylem. In some plants, the pith in the middle of the stem may dry out and disintegrate, resulting in a hollow stem. A few plants, such aswalnuts, have distinctive chambered pith with numerous short cavities (see image at middle right). The cells in the peripheral parts of the pith may, in some plants, develop to be different from cells in the rest of the pith. This layer of cells is then calledthe perimedullary region of the pithamus. An example of this can be observed inHedera helix, a species ofivy.
The term pith is also used to refer to the pale, spongy inner layer of the rind, more properly calledmesocarp or albedo, ofcitrus fruits (such as oranges) and otherhesperidia. The word comes from theOld English wordpiþa, meaning substance, akin toMiddle Dutchpitte (modern Dutchpit), meaning the pit of a fruit.[2]
The pith of thesago palm, although highly toxic to animals in its raw form, is an important human food source inMelanesia andMicronesia by virtue of its starch content and its availability. There is a simple process of starch extraction from sago pith that leaches away a sufficient amount of the toxins and thus only the starch component is consumed. Current processes for starch extraction are generally only about 50% efficient, however, with the other half remaining in residual pith waste.[3] The form of the starch after processing is similar totapioca.
Other foods sometimes mistakenly called piths includeheart of palm (actually the core of the bud) andbanana piths (actually the rolled up young leaves).
The spongy wood of thepith wood plant or other similar species, often mistakenly called pith, was once used to makepith helmets.[4][5]
Pith wood is acleaning tool used inwatchmaking to clean watch parts[6]: 144 and tools. It is used to remove oil from the tips of tools to prevent the contamination of watch movements. A pith wood consists of a piece of pith (such aselder[6]: 281 or mullein[7]).
Dried pith (which is actually the center of the leaf) of certainrush plants soaked in fat or grease, held using arushlight, was used as home lighting. Beginning in the 17th century, it would continue to be used in this method until the mid-20th century. It saw a brief revival during World War 2.
