Final answer:
Collenchyma cells are specialized plant cells with irregular shapes and thicker, uneven cell walls that provide structural support to stems and leaves.
Step-by-step explanation:
The specialized plant cell that is irregular in shape with thicker cell walls is the collenchyma cell. Collenchyma cells are known for their elongated shape and unevenly thickened walls which provide structural support mainly to the stem and leaves. These cells are alive at maturity and are typically found beneath the epidermis. An everyday example of collenchyma cells can be seen in the "strings" of a celery stalk. Unlike sclerenchyma cells, which also provide support but are usually dead at maturity, collenchyma cells are alive and flexible enough to support growing tissues.