An instrument used in measuring plants growth is

An auxanometer is a specialised instrument designed to measure the rate of growth in plants, particularly elongation growth. It works by magnifying small increments of growth using a lever, pulley, or rotating drum system connected to the tip of a growing plant. As the plant grows, even tiny increases in height are amplified and recorded, making it possible to measure growth that would be too small to detect by direct observation alone.

The other instruments listed measure entirely different environmental quantities:

• A hygrometer measures the relative humidity (moisture content) of the air.
• A barometer measures atmospheric pressure.
• A thermometer measures temperature.

None of these three instruments is designed to detect or quantify changes in plant size or growth rate.

Exam tip: The prefix auxano- comes from the Greek word for "growth" (the same root as the plant hormone auxin). Linking "auxanometer" to "auxin" is a reliable way to remember that both relate to plant growth.