1) Control And Coordination

• . Plants can respond to changes in the external environment.
• · The responses are usually slower than animal responses and include:

  roots and shoots growing towards or away from a particular stimulus

  plants flowering at a particular time

  • ripening of fruits.
• . When part of a plant responds by growing in a particular direction, it is called a tropism.
• · There are different types of tropism: Gravity and Light
• Gravity can cause the growth of shoots to be negaively gravitropic and also posiviley gravitopic
• negaively gravitropic = away
• positively gravitopic = towards
• Light can cause the growth of shoots to be negaively phototropic and also posiviley phototropic
• Tropisms are controlled by a group of plant hormones called auxins.
• Experiments have shown the steps that are involved in the response to light:

  1. More light reaches one side of the shoot
  2. More auxin is sent down the shaded size of the shoot
  3. This results in cells on the shaded side elgongating
  4. The shot, therfore, geows towards the light
•  In roots, auxin moves to the bottom of the root and causes less elongation, so the root grows downwards.
• Gibberellins are another group of plant hormones and are important in initiating seed germination.
• Ethene is a gas and also a plant hormone. It controls cell division and ripening of fruits.