Pith is a substance that is found in vascular plants. It consists of soft, spongy cells, and is located either in the center of the stem or the center of the roots in flowering plants. It is encircled by a ring of xylem (woody tissue), and outside that, a ring of phloem (bark tissue). In some plants the pith is solid, but for most it is soft. A few plants, such as walnuts, have distinctive chambered pith with numerous short cavities.
The word comes from the Old English word piþa, meaning substance, akin to Middle Dutch pit, meaning the pit of a fruit. The modern word pithy (concise and forcefully expressive) derives from it. The inner rind of citrus fruits and other hesperidium is also called pith. The pith and the peel are where about three quarters of the nutrients of an orange are. The pith itself is bitter and is usually added to marmalade or otherwise prepared to be eaten.
The Daily Omnivore 
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