P LANT R ESPONSES How plants move and communicate

T ROPISMS Tropisms are the growth of a plant toward or away from a stimulus, including: Phototropism: in response to light Gravitropism (geotropism): in response to gravity Thigmotropism: in response to touch Auxins are plant growth hormones that promote elongation of cells

T ROPISMS : CELL ELONGATION In general, tropisms involve cell elongation or suppression of cell elongation on one side of a plant, causing the plant to grow in a particular direction.

P HOTOTROPISM Look at the sprouts in the bottom picture and the explanatory diagram at the top.

G RAVITROPISM In this plant, shoots grow upwards and roots grow downwards in response to gravity. On which side of the shoot and root do you think auxins are more concentrated?

G RAVITROPISM IN ROOTS In roots, however, auxin concentration on the lower side of the root suppresses cell elongation. The upper side of the root continues to grow, causing the roots to bend downward.

T HIGMOTROPISM In some plants, vining stems or tendrils will grow in response to touch.