Why do some legume fruits not split open when ripe?

While the typical fruit of the Fabaceae family is a legume (or pod) that splits open—or dehisces—along its sutures to release seeds, evolution has led to various modifications in many species.

Some legumes have become indehiscent, meaning they do not split open when ripe. These modifications include:

• Retention of basic form: Some species, such as Robinia (locusts) and Cercis (redbud), maintain the general shape of a standard pod but remain closed at maturity.12
• Reduction to nutlets: In plants like Melilotus (sweet clover), the fruit is reduced to a single-seeded structure that resembles a tiny nutlet.12
• Segmented pods: Some fruits are divided transversely into single-seeded segments that break apart rather than splitting along the sutures, as seen in Desmodium.12
• Fleshy modifications: In some tropical species like Andira inermis (angelin tree), the fruit coat becomes fleshy and plumlike.12

All these variations are considered derivative modifications traceable back to the ancestral, dehiscent pod type.12