Keela flower

From Memory Alpha, the free Star Trek reference

Keela flowers are a variety of flowers native to the Delta Quadrant.

They are a remarkably strong plant, their stems are flexible and impossible to break. But occasionally on the same plant there’s a bloom whose stem is not so flexible and breaks easily.

Neelix compared those rigid and inflexible keela flowers to Lieutenant Tuvok. (VOY: "Learning Curve")