General
3: Responding to the environment: Plants
This online synchronous meeting will take place on Date | Time. Attendance is mandatory.
Introduction
Plants respond to the environment by either growing towards or away from a stimulus. Various plant growth substances (hormones) that are produced in the plant control this growth movement. This type of growth movement is called tropic movement. The word tropism means ‘to turn’. A tropism is the response to an external stimulus, causing a plant to grow towards (positive tropism) or away (negative tropism) from the stimulus. Only phototropism, which is a response to light, and geotropism, which is a response to gravity, needs to be covered in the DBE gr.12 syllabus.
Auxins are growth hormones found in plants. Auxins stimulate or inhibit (prevent) growth in areas. A high concentration of auxins in an area will stimulate cell elongation and cell differentiation, especially in stem tips (growth tips). This results in apical dominance, when the growth point at the tip of the stem grows upwards. This action inhibits the development of axillary buds on the lateral branches below so they don’t grow because all the plant’s energy is used to grow upward. If the tip is removed the axillary buds develop into lateral branches causing the plant to grow thicker on the sides (like when cutting a hedge).
Only about three days are allocated on the ATP for the teaching of plant responses to the environment. It is important to use diagrams for the teaching and learning of this topic. You will also notice that we have included terminology lists as these are crucial for good performance. Please ensure that your learners do regular terminology activities and tests.
A clinostat can be used to demonstrate plant growth responses to an external stimulus.Module overview
This module deals with the responses of plants to light and gravity as an external stimulus. There is a detailed terminology list, followed by the general functions of the auxins, gibberellins and abscisic acid. The use of plant hormones in controlling weeds is described as well as the role of auxins in geotropism and phototropism.
Learning outcomes
By the end of this module, you will be able to:
- Describe
the general functions of auxins, gibberellins and abscisic acid
- Briefly
describe the use of plant hormones in controlling weeds
- Briefly
describe the role of auxins in geotropism and phototropism
- Interpret
and conduct investigations based on the role of plant hormones
- Describe
the role of chemicals and thorns in plant defence mechanisms.
- Describe
the general functions of auxins, gibberellins and abscisic acid
Study materials
Module structureUnit 1: The general functions of auxins, gibberellins and abscisic acid
- Click on the link below to review additional information on this topic
- Complete the online activity 3.1 below.
Unit 2: The control of weeds using plant hormonesUse of auxins as weed killers
- Hormone weed killers are auxin-based selective herbicides
- The auxins used in these weed killers are made by chemical synthesis
- The main factor that controls the weed growth in these herbicides are the auxins
- These herbicides can only kill weeds.
Advantages of hormone weed killers:- They are non-toxic to animals and humans
- There is no longer the need for weeding the garden.
Watch the short video below.
- Hormone weed killers are auxin-based selective herbicides
Unit 3: The role of auxins in geotropism and phototropism- Click on the link below to review additional information on this topic
- Using a NEW Word document, complete activity 3.2 on pages 9 - 13 of the module guide
- Save the document in the following format: Full name_Module 3
- Use the submission tool below to submit your document.
Unit 4: The role of chemicals and thorns in plant defense mechanisms- Click on the link below to review additional information on this topic
Subject methodologyEnrichment resources
Review the links below which will take you to some useful websites which we encourage you to explore.
Technology supportReview the video below: this topic could lead to an interesting discussion amongst your learners. What is South Africa's position on using pesticides on crops / plants?
Module summaryThis module covers all the requirements for the DBE NSC exams w.r.t. the topic: plant responses to the environment. It is important to use diagrams to explain observations made w.r.t. the response of a plant on an external stimulus.