What are stipules and how do they occur in Gentianales plants?

Stipules are small, typically leaflike appendages found at the base of a plant's petiole (leaf stalk). In the order Gentianales, these structures are common, though their presence and form vary across the different families.1

In many members of the order, leaves are accompanied by these appendages, which may sometimes be reduced to a simple ridge on the stem located between adjacent leaf stalks. While stipules are a characteristic feature for many species within the order—such as those in the Rubiaceae family, which are noted for having opposite, stipulate leaves—they are notably absent in almost all genera of the Gentianaceae family.234

Generally, in plants where they are present, stipules often develop before the rest of the leaf to provide protection to the young blade, frequently falling off once the leaf reaches maturity. Some species in the order may also secrete mucilage from glandular hairs, known as colleters, located on these stipules or at the base of the leaf stalk.52