Figure 33.7 Ground Tissue Cell Types (A) Parenchyma cells in the petiole of Coleus. Note the thin, uniform cell walls. (B) Collenchyma cells make up the outer cell layers of this spinach leaf vein. Their walls are thick at the corners of the cells and thin elsewhere. (C) The two types of sclerenchyma cells. Left: Fibers in a sunflower stem (Helianthus). The thick secondary walls are stained red. Right: Sclereids. The extremely thick secondary walls of sclereids are laid down in layers. They provide support and a hard texture to structures such as nuts and seeds.