Final answer:
Sclerenchyma cells about as long as they are wide are called sclereids, which contribute to the hardness of structures like nut shells and stone fruit pits due to their lignin-thickened cell walls.
Step-by-step explanation:
Sclerenchyma cells that are about as long as they are wide are called sclereids. These cells are known for the hardness they impart to plant structures such as nut shells and stone fruit pits. Unlike collenchyma cells, many sclerenchyma cells are dead at maturity. There are two types of sclerenchyma cells: fibers, which are long and slender, and sclereids, which are smaller and provide the gritty texture found in pears. Both types of cells have secondary cell walls that are thickened with deposits of lignin, an organic compound that is a key component of wood. Sclerenchyma fibers are utilized by humans to make products such as linen and rope.