Pith's a substance that's found in vascular plants. It consists of soft, spongy parenchyma cells, and's located in the center of the stem in eudicots (both herbaceous and woody) and in the center of the roots in monocts. It's encircled by a ring of xylem (woody tissue), and outside that, a ring of phloem (bark tissue). In some plants the pith's solid, but for most it's soft. A few plants, for example walnut, have distinctive chambered pith with numerous short cavities. The word comes from the… (More on Pith)