Final answer:
Collenchyma cells are elongated, stringy cells with unevenly thickened walls found under the epidermis of plant stems that provide structural support, notably seen in the 'strings' of celery. Sclerenchyma cells also offer support with lignin-thickened walls but are usually dead at maturity.
Step-by-step explanation:
The elongated, stringy cells found in the partially peeled stem of a plant are known as collenchyma cells. These cells are characterized by their elongated shape with unevenly thickened walls, and they are crucial for providing structural support to the stem and leaves of the plant. Collenchyma cells are alive even at maturity and are generally located just below the epidermis. A common example of collenchyma cells can be observed in the 'strings' found in a celery stalk. Unlike collenchyma, sclerenchyma cells, which also provide support, are often dead at maturity and have walls thickened with lignin. There are two main types of sclerenchyma cells: fibers, which are long and slender, and sclereids, which are smaller and contribute to the gritty texture in pears.
In addition to support cells, plant stems contain vascular tissues including xylem and phloem, which transport water, minerals, and sugars throughout the plant. The xylem cells, such as tracheids and vessel elements, are also elongated and thick-walled, but they serve primarily in water conduction.