Pith is a light substance that's 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, for example grasses and umbellifers, the pith has a hollow centre forming a hollow tube except at the points where leaves are produced, where there's a solid plate across the stem. A few plants, for e… (More on Pith)