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.

 

 

   

  The effect of auxins is seen in the plant's reaction to external stimulation.   

    Phototropism is the movement of the plant body toward light.    

    Geotropism is the growth of roots toward the ground and leaves away from the ground, away from the pull of gravity.   

    Photoperiodism is the system that plants use to determine the season: more daylight hours means it is the growing season.

              Plant growth occurs in areas called meristems. This is the area in which a cells function is determined. There are two types of meristems, one is lateral and the other is apical.

              These are the sites for primary growth in a plant that are found at the shoot and root tips where the cells function is determined and it becomes a permanent part of the plant.

               These are the parts of the plant that are responsible for the thickening of the plant. So as the plant continues to increase in height its girth and its root sections are increased to support the new growth. So year by year the plants girth is increased layer by layer hence, in trees, its age can be determined by the rings formed in its trunk.