The petiole is the stalk that supports a leaf in a plant and attaches it to the stem. Many people often call it a stem, which is incorrect. A stem is the part of the plant that serves as the main source of support and produces nodes and roots, and that’s not what we observe in petioles.
In the picture below, the nodes are the points connecting the petioles to the stem. We call the leaves that are connected to the stem through simple petiole petiolates. In some cases, leaves are attached to the stem without a petiole, so we call them sessile. Petioles themselves can also have differences. For example, a cross-section of a petiole can have different shapes, and they can either be rigid or somewhat spongy to the touch. These differences can help plant scientists determine the species of a plant. Petioles, also, come in different sizes. For example, the picture below shows thin petioles, but a celery stick is actually also the petiole part of the celery plant.
Since leaves are responsible for fueling plants through photosynthesis, the petiole serves to transport the energy made in the leaf to the rest of the plant. It also serves to transport nutrients and water that are absorbed by the roots and passed up through the xylem, to the leaf.