Why has it been difficult to identify Malpighiales using a single set of visible characteristics?

Identifying the order Malpighiales using a single set of visible characteristics has proven difficult because the group is morphologically diverse and lacks a unique, defining suite of features that applies consistently across its 40 families.1

While the order is supported as a coherent clade through molecular analysis, it is not considered highly distinctive in its physical appearance. Historically, the families now included in Malpighiales were scattered across various other orders, and many were previously thought to be related to different groups entirely.1

Although no single diagnostic trait exists, botanists note that members often share certain common, though not universal, characteristics:
* Leaves are frequently toothed and stipulate.1
* Floral construction often follows a pattern common to basal eudicots, featuring free sepals, petals, and stamens, along with a nectary and superior ovaries with three to five fused carpels.1
* Seeds typically contain little endosperm and possess a fibrous layer in the seed coat.1

Because of this lack of a singular, easily identifiable morphology, the order remains challenging to characterize based on visible traits alone.1