In botany, a stipule is an outgrowth borne on either side (sometimes just one side) of the base of a leafstalk (the petiole). A pair of stipules is considered part of the anatomy of the leaf of a typical flowering plant, although in many species the stipules are inconspicuous or entirely absent (and the leaf is then termed exstipulate). In some older botanical writing, the term "stipule" was used more generally to refer to any small leaves or leaf-parts, notably prophylls. The word stipule was coined by Linnaeus from Latin stipula, straw, stalk.
nodes have stipules; species with unilacunar nodes lack stipules.
According to duration
The three types of stipules according to duration are caducous, deciduous and persistent. Caducous stipules fall off before the leaf unfolds, while deciduous stipules fall off immediately after the leaf unfolds. Persistent stipules remain attached to the plant.
According to shape, size and position
Stipules can be considered free lateral, adnate, interpetiolar, intrapetiolar, ochreate, foliaceous, bud scales, tendrillar or spiny.
A stipule can be fused to the stem, or to the other stipule from the same node.
A stipule is "adnate" if it's fused together on part of the petiole length, but the anterior is still free.
A stipule is "interpetiolar" if it is located in between the petioles, as opposed to being attached to the petioles, and generally one stipule from each leaf is fused together, so it appears that there's just one stipule between each leaf.
A stipule is "intrapetiolar" if it is located in the angle that's between a stem and a petiole. In this case, the two stipules generally form together and appear to be one stipule.
A stipule is "ochreate" if a single stipule appears to be a solid tube that goes all the way around the stem.
A stipule is "foliaceous" if it is leaf-like. These are generally used to photosynthesize.
A stipule is considered a "bud scale" if it is hard or scaly and protects leaf buds as they form. These generally fall off as soon as the leaf unfolds.
A stipule is considered "tendrillar" if they are long thin tendrils, and are generally used by climbing plants.
A stipule is considered "spiny" if they are long and pointy. These are generally used to deter animals.
Purpose of stipules
Stipules have various functions. Some stipules are not well understood or may be vestigial.
It is known that foliaceous stipules are used like leaves to make energy for the plants. Sometimes stipules protect the next leaf or bud as it grows in then falls off after the leaf unfolds, as with Tulip Poplars. Stipules can be used as climbing tendrils by climbing plants. Spiny stipules can be used to help protect the plant from animals.