auxin – a plant hormone that regulates growth

(Pronounced ox-in)

To remember the meaning of auxin, use the following mnemonic:

The ox ingested (auxin) too much growth hormone. 

Auxin - Stimulates in the Shoots and

               Restricts in the Roots

The start letters S and R help you remember which is which.

Example

In a shoot

The shaded side contains more auxin and grows longer, causing the shoot to bend towards the light.

In a root

If a root is placed horizontally, the bottom contains more auxin than the top side. This makes the bottom side of the root grow less than the top side, causing the root to bend in the direction of gravity. This is the opposite of auxin in the shoot.

Note: Auxins change the rates of elongation in plant cells and control growth in stems and roots. Stems and roots respond differently to high concentrations of auxins; cells in stems grow more, cells in roots grow less. 

Auxin is a crucial plant hormone that controls various aspects of plant growth and development, with its most important function being the regulation of cell elongation and division. It is primarily produced in the growing tips of shoots and young leaves, then transported to other parts of the plant. Auxin promotes cell elongation by causing cell walls to become more flexible, allowing cells to expand and grow longer, which is why it's particularly important for stem growth and the development of plant height. 

One of auxin's most well-known roles is in tropisms, directional growth responses to environmental stimuli. In phototropism (response to light), auxin accumulates on the shaded side of the plant stem, causing those cells to elongate more than cells on the light side, which makes the plant bend towards the light source. Similarly, in gravitropism (response to gravity) auxin helps roots grow downwards and shoots grow upwards.