Final answer:
Sclerenchyma cells are specialized plant cells with thick, lignified secondary walls that provide support to the non-growing parts of plants. These cells are mostly dead at maturity and are essential for plant strength and structure.
Step-by-step explanation:
The specialized plant cells that have thick, rigid non-stretchable walls and support the non-growing parts of plants are sclerenchyma cells. These cells are characterized by their secondary cell walls that are thickened with deposits of lignin, making them crucial for providing strength and structure to plants. There are two types of sclerenchyma cells: fibers, which are long and slender, and sclereids, which are smaller in size and contribute to the gritty texture in pears. Sclerenchyma cells are generally dead at maturity, enabling them to offer support to the non-growing parts of the plant through their rigid structure.