Petiole
The petiole or petiole (from the Latin petiolus, diminutive form of pes, pedis, foot, trunk of a plant) is the stalk that joins the blade of a leaf to its leaf base or to the stem. It is missing in the leaves called sessile or seated.
The petiole can be a determining characteristic for the identification of the plant.
The petiole develops in the germinative phase of the seedling, in the embryonic tissue called the hypocotyl. From the elongation of the hypocotyl arises the cotyledon or plumule (precursor tissue of the leaf) and the terminal or apical meristem. The outer cells differentiate and mature into an epidermal layer. Some of the inner cells become collenchyma, providing support for the young petiole. Groups of elongated cells form the provascular strands. The rest of the petiole is made up of parenchymatic cells located between the epidermis and the provascular strands, forming the cortex, together with those located within these strands, which form the medulla.
The parenchyma that radiates between the precursors of the vascular bundles forms the so-called medullary rays.
In some plants, the petioles flatten and split open, and the true leaves may become reduced or disappear entirely. These are known as phyllodium, phylloides, phyllodia, or cladophylla. Thus, the phyllode comes to serve the purpose of the blade. Phyllodes are common in the genus Acacia, especially in the Australian species, at the same time in Acacia sect. Phyllodineae. Sometimes, especially in younger plants, the partially formed phyllodes also bear reduced leaves.