The term 'plant stem tissue' refers to the specialized cells that make up the stem of a plant. PITH, which is a four-letter word, is the central core of the stem and is composed of soft, spongy tissue. It is made up of parenchyma cells, which are responsible for storing food and water, and providing support to the stem. PITH also helps to protect the stem from mechanical damage and provides insulation against extreme temperatures.