David C. Zlesak
The core or pith of plant stems are typically either hollow or contain loosely packed, spongy parenchyma cells. The pith of this butternut or white walnut (Juglans cinerea) is distinctive in that it is chambered with dark bands of sclerenchyma plates separating hollow zones. Black walnut (Juglans nigra), a close relative, also has chambered pith and has bands that are typically a bit lighter in color. Parenchyma cells are relatively large, of variable shape, and have thin cell walls. As developing stems elongate, parenchyma can tear and disintegrate in many plant species. Another cell type found in plants is sclerenchyma. Sclerenchyma cells help provide support to plant tissue and have thickened, secondary cell walls containing cellulose and are often impregnated with lignin. As stems of white and black walnuts grow, parenchyma cells eventually collapse leaving mainly sclerenchyma cells in distinctive plates.