Though largely sedentary- plants are able to move through growth or turning as an adaptive response to environmental stimuli.
What you intend the students to learn about this idea?
That plants to have adaptations allowing them to fill niche positions within particular environments. These are known as Tropisms and are related to both growth responses and rhythmic activities that are generally influenced by environmental stimuli (VCAA, 2005). These responses can be either positive (towards the stimuli) or negative (away from the stimuli).
Different types of Tropism:
Geotropism- movement or growth in response to gravity.
Chemotropism- movement or growth in response to chemicals.
Heliotropism- movement or growth in response to sunlight.
Phototropism- movement or growth in response to lights or colors of light.
Thermotropism- movement or growth in response to temperature.
Thigmotropism- movement or growth in response to touch or contact.
Students need to recognize that plants respond to external, environmental stimuli and have developed a range of adaptations enabling them to survive within specific environmental niches.
What else you know about this idea (that you do not intend students to know yet)?
Human manipulation of tropisms through genetic modification of plants in order to exploit characteristics and traits displayed by one species by placing them into another.
Agricultural and Horticultural applications of knowledge of plant tropisms.
Difficulties/Limitations connected with teaching this idea.
It is difficult establish that plants react to stimuli much in the same way that other organisms do. This is generally associated with the notion that animals can quickly and obviously respond while a plants response may not always be so immediately obvious.
Knowledge about students' thinking which influences your teaching of this idea.
This will strongly relate with the above difficulties and limitations.
Other factors that influence your teaching of this idea.
The relatively slow response rate of plants limits the activities that can be used to display plant tropisms purely because of the time available to deliver the content.
Teaching procedures (and particular reasons for using these to engage with this idea).
Visual Display: ThigmotropismExamples of thigmotropic adaptations of plants- plants that are able to move, quite quickly, in response to external stimulus.
This clip is of a Mimosa plant. Native to Central and South America, it has developed an adaptation allowing it to rapidly fold its leaves in response to physical contact or excess heat. It utilizes this as a defense mechanism against predatory insects who are unable to maintain purchase on the leaves when they fold inwards, and also to protect its leaves from sun damage during excessive periods of high temperature. This is not explained in the clip and may be a good discussion point with the students in order to see what reasoning they can come up with for the development of this adaptation.
This clip (taken from The Private Life of Plants- Attenborough, BBCDVD1235, September 2003) demonstrated the thigmotropic adaptation of the Venus Fly Trap. Attenborough thoroughly explains the significance of this and how it is an adaptive response by the plant in order for it to gain nutrients in what is an otherwise nutrient poor environment.
Practical Activity
Geotropism Aim: To observe the effects of gravity upon the growth of germinating corn seeds.
Method: # Soak corn seeds in a 5 percent solution of bleach for 30 minutes to kill any mold spores attached to the seeds.
Soak the seeds in water overnight.
Select four seeds and pin to a piece of cardboard so the point of one seed is down, one has the point to the right, one to the left, and one up (see Fig.1 below).
Label the cardboard and place in a plastic bag with two folded, moist paper towels at the bottom.
Hang the bag in a dark place to avoid any influence of light.
After germination has begun (two days), inspect the card daily.
Record your observations of the direction of growth of both the root and the stem.
Data Summary and Analysis:
For this experiment, students should sketch the orientation of roots and shoots at 2, 5, and 7 days after germination.
Fig.1- Display of experiment setup. Note the placement of the pins- to the side of the corn so as only to pierce the seeds starch storage region and not damage any of the growing areas.
Plant roots are positively geotropic while stems are negatively geotropic. Regardless of the position of the embryo (located at the pointed end of the kernel), the roots will grow down and the stems up. In this experiment light must be controlled so that phototropic effects are not present.
An advantage of this practical is that though the overall process is slow, it is easy and quick to set up and can be easily inspected every few days or whenever the students have a class in order to observe the seeds geotropic response upon germination.
Phototropism Aim: To observe and describe the effects of phototropism on germinating seedlings that are growing under differing light conditions.
Method:Plant several corn seeds in four small pots and allow to grow until the shoots are visible.
Take four light-proof boxes and make a small window at a different place in each box (top, bottom, right, and left side). Line each box with black paper or spray with black paint.
When the corn shoots have emerged, place one pot in
each of the four boxes.
Observe the plants every two days.
Record notes on the size, orientation, color and leaf shapes of the plants. (see record table below)
Data Summary and Analysis:
Details of the set up of each of the pots, factors that need to be considered and the subsequent observations can all be recorded in a results table.
This practical also can be constructed and forgotten. Alternatively, it could be setup by the teacher 10 days prior to when it is needed allowing students to make predictions about what they think they will find when they remove the boxes. The results can be written/drawn up in the table and discussed. This would allow other class work to continue and without having to revisit the practical or the topic at a later date and thus potentially interrupting other areas of work.
Specific ways of ascertaining students' understanding or confusion around this idea (include likely range of responses).
Student responses to the above activities and ability to articulate their understanding through class discussion are the key ways of ascertaining students knowledge of this area. Experiments and activities can be written up in practical reports and may be designed to incorporate additional questions for the students to answer. This could then be collected for assessment, providing a written example of both the students progress and understanding and also any areas which may require further revision by the teacher.
Phototropism Results Table:
Factors to consider
Observations
Pot 1
Pot 2
Pot 3
Pot 4
Position of light source -draw a diagram displaying the
light source in relation to each plant.
Different types of Tropism:
The above hyperlinks relate to the Biology-Online dictionary. http://www.biology-online.org/dictionary/
ThigmotropismExamples of thigmotropic adaptations of plants- plants that are able to move, quite quickly, in response to external stimulus.
This clip is of a Mimosa plant. Native to Central and South America, it has developed an adaptation allowing it to rapidly fold its leaves in response to physical contact or excess heat. It utilizes this as a defense mechanism against predatory insects who are unable to maintain purchase on the leaves when they fold inwards, and also to protect its leaves from sun damage during excessive periods of high temperature. This is not explained in the clip and may be a good discussion point with the students in order to see what reasoning they can come up with for the development of this adaptation.
This clip (taken from The Private Life of Plants- Attenborough, BBCDVD1235, September 2003) demonstrated the thigmotropic adaptation of the Venus Fly Trap. Attenborough thoroughly explains the significance of this and how it is an adaptive response by the plant in order for it to gain nutrients in what is an otherwise nutrient poor environment.
Practical Activity
GeotropismAim: To observe the effects of gravity upon the growth of germinating corn seeds.
Method:
# Soak corn seeds in a 5 percent solution of bleach for 30 minutes to kill any mold spores attached to the seeds.
Data Summary and Analysis:
For this experiment, students should sketch the orientation of roots and shoots at 2, 5, and 7 days after germination.
Fig.1- Display of experiment setup. Note the placement of the pins- to the side of the corn so as only to pierce the seeds starch storage region and not damage any of the growing areas.
Plant roots are positively geotropic while stems are negatively geotropic. Regardless of the position of the embryo (located at the pointed end of the kernel), the roots will grow down and the stems up. In this experiment light must be controlled so that phototropic effects are not present.
Phototropism
Aim: To observe and describe the effects of phototropism on germinating seedlings that are growing under differing light conditions.
Method: Plant several corn seeds in four small pots and allow to grow until the shoots are visible.
each of the four boxes.
Data Summary and Analysis:
Details of the set up of each of the pots, factors that need to be considered and the subsequent observations can all be recorded in a results table.
This practical also can be constructed and forgotten. Alternatively, it could be setup by the teacher 10 days prior to when it is needed allowing students to make predictions about what they think they will find when they remove the boxes. The results can be written/drawn up in the table and discussed. This would allow other class work to continue and without having to revisit the practical or the topic at a later date and thus potentially interrupting other areas of work.
Both practicals adapted from activities by Ashley Swonger, Agricultural Education Instructor, Ridgedale High School, Morral, OH 43337.
Available at: http://www-cms.ag.ohio-state.edu/AgSciLPs/HTML/507.html
Phototropism Results Table:
-draw a diagram displaying the
light source in relation to each plant.
- draw result after 10 days.