Mimosa roemeriana, Sensitive Briar

by Bob Harms 

Mimosa roemeriana (M. quadrivalvis var. platycarpa, Schrankia roemeriana), Sensitive Briar, is a prostrate creeping perennial, that blooms in late spring and disappears soon after. Its common name appropriately describes the response by which its leaflet pairs fold together when disturbed, as seen on the right in the following photo.

This reaction is controlled by the cells of a swelling at the base of each leaflet, a pulvinus. These are clearly visible on the upper surface of the leaflet, pale yellowish–green.

Leaf upper surface with pulvini at the base of each leaflet.

The flower heads are pink (paling with age), globular clusters, very similar to those of the other Hays County mimosas.

But the pods are quite different, the basis for its scientific synonym M. quadrivalvis var. platycarpa — square-valved with a broad pod. Apart from the densely prickled surface, the replum is very broad, superficially resembling valves, thus giving it a somewhat square shape, sometimes slightly rounded — and the pod does not narrow between the seeds, even if one doesn't develop.

Cross section of pod, showing funiculi attachment from the replum.

Pod dissected showing replum and valves.	New pods.