As with animals, some plants grow large in size, trees for example, while others are very small, such as mosses. Plants also use a variety of hormones to control their growth and development just as animals do.
Auxins are the plant growth hormones. They work by making cells grow longer and divide. Auxins are confined to the soft growing zones known as meristems, unspecialized cells that form differing parts of a plant.
Auxins affect the growth of the stems, leaves and roots by stimulating the plant cells.