The rigid structure found in plants and trees is called xylem.

Here's why:

* Xylem is a type of vascular tissue that transports water and dissolved minerals upwards from the roots to the rest of the plant.

* It's composed of specialized cells that are dead at maturity, leaving behind thick, rigid cell walls that provide structural support.

* This rigidity is essential for the plant to stand upright and grow tall, especially in trees.

While other structures like the cell wall also provide rigidity, it's the xylem that is specifically responsible for the overall structural support of the plant.