Definition: Arilliforms are a group of plants that have flat, irregular leaves. This term comes from Latin because it means "flat" in Greek. Definition: Arilliform plants are characterized by their flat leaf surfaces. They typically lack stipules (stalk-like structures) and produce leaves that are not pinnate (like those in the palm or thumb). These plants are also known as "ariliform," which is derived from their shape similar to an aril, a fruiting body of a plant.