Tropism – a plant’s growth response toward or away from a stimulus. Positive tropism – when a plant grows toward the stimulus Negative tropism – when a plant grows away from the stimulus

 Plants show growth responses, or tropisms, to: Touch Light Gravity tch?v=IIsvRwNRoVw&NR=1

 Thigmotropism (thigmo = touch) Vines often show positive thigmotropism – they coil around anything they touch. ex., grapes, morning glories LpQNyI6g&NR=1 aG4tOaAU&feature=related uZ_1cQnv4&feature=related atch?v=m2xKjA69jNM watch?v=ktIGVtKdgwo&N R=1

 Phototropism Leaves, stems, and flowers exhibit positive phototropism – they grow toward the light. Why ? antmotion/movements/tropism/tropisms. htmlhttp://plantsinmotion.bio.indiana.ed u/plantmotion/movements/tropism/trop isms.htmlhttp://plantsinmotion.bio.indiana.ed u/plantmotion/movements/tropism/trop isms.html NV7cvW8k

 Gravitropism Roots show positive gravitropism – they grow downward. What about stems? 9r6MAVU

 Hormones and tropisms Plants can respond to these stimuli because they produce hormones. A plant hormone is a chemical that effects how the plant grows and develops. Auxin – speeds up the rate at which a plant’s cells grow. It controls the response to light. It builds up on the shaded side of the stem, so those cells grow faster. The stem bends toward the light.

 Seasonal changes Responses to these changes include … Photoperiodism – a plant’s response to seasonal changes in length of night and day. The amount of darkness determines the time of flowering in many plants.

Short-day plants – flower when nights are longer than a critical length. Long-day plants – flower when nights are shorter than a critical length. Critical night length – the number of hours of darkness that determines whether or not a plant will flower. So… if a short-day plant has a critical night length of 11 hours, does it require more or less than 11 hours of night for flowering? More.

 When do short-day plants bloom? Fall or winter. ex., Chrysanthemums, poinsettias Long day plants? Spring or summer. There are also day-neutral plants. ex., rice, tomatoes

 Dormancy -- - a period when an organism’s growth or activity stops. - helps plants survive freezing temps and a lack of liquid water. Cooler weather, shorter days … leaves stop making chlorophyll. We can then see the other pigments in leaves. New red pigments are produced. Then what’s left of the water and sugar goes out of the leaves. Time to rake.