Unit 9: Plants                                                                1   2   3   4   5   6

5. Interaction in plants

Interaction in plants is not easy to observe, as their reactions are often very slow and subtle. Despite this, we can see that plants react, in a coordinated way to stimuli such as light, humidity or temperature.

When a stimulus reaches a plant, some of its cells sense it and react or send a signal to other cells (for example, they produce a substance which is distributed around the organism) leading to a joint, coordinated response of part, or all of the plant.

The most frequent reactions of plants are tropisms, nastic movements and changes in their vital processes.

a) Tropisms

     Tropisms are plant responses which consist of directing their growth towards,

     or away from stimulus.

Animation: Tropisms

b) Nastic Movements

     Nastic movements are plant responses which consist of rapid movements

     of some parts of the plant. They are usually reversible and temporary.

     For example:

-Photonasty is a response to light. Examples are the way leaves or flowers turn, following the sun; or the opening and closing of some flowers depending on whether it is day or night.

-Thermonasty is a response to temperature. For example, some flower close when temperature drops down too much, or some leaves fold when the temperature is too high.

-Thigmonasty is a response to contact. For example, the mimosa shrub folds its leaves when touched; the flytrap, a carnivorous plant, snaps its leaves shut when an insect brushes against it.

c) Changes in a plant’s vital processes

     Some plants react to certain stimuli by modifying some of their vital processes.

     The most  characteristic reactions of this type are the seasonal changes

     of many plants, like responses to temperature, the light or the length

     of the day and night.

     For example, flowering in spring, shedding leaves in autumn, fruits maturing

     in summer.

5.1. What is the main difference between nastic movements and tropisms?

5.2. Vine tendrils (runners) wrap around other stems and nearby objects.

5.3. Answer the questions:

a. What type of plants response is shown in photographs, tropisms or nastic

    movements? Why?

b. Identify the type and stimulus which act in each case.