6.3 Support, Growth and Stability in Plants

Importance and Main Systems of Support

Support helps a plant to:
- stand upright to obtain sunlight;
- expose its leaves to sunlight for photosynthesis;
- support its own weight and the weight of its leaves, flowers and fruits;
- withstand wind.
The main support structures are the stem and roots.
The stem supports itself, the leaves, flowers and fruits.
Roots support the plant by gripping the soil; some plants also have roots above the ground for additional support.

Support Systems of Terrestrial and Aquatic Plants

Terrestrial Plants

Plant type	Main support system
Woody plant	Hard, strong and tough woody tissue built from lignin, a tough and hard complex substance; examples: rambutan and angsana trees
Herbaceous plant (non-woody)	Water stored in stem cells produces cell turgidity; soft stems wilt when the plant lacks water

Additional Support in Terrestrial Plants

Buttress roots: support large, tall trees such as durian and angsana trees.
Prop roots: grow from the stem or branches into the soil; found in banyan, pandan and fig trees.
Stilt roots: support mangrove trees living in swamps.
Tendrils: fine, coiled structures that wrap around another plant or object; found in cucumber, bitter gourd and pumpkin plants.
Clasping roots: grip another plant or structure; found in orchids and money plants.
Hollow stems: provide additional support to non-woody plants.

Aquatic Plants

Do not have woody supporting tissue like terrestrial plants.
Obtain their main support from the buoyancy force of water.
Stems and leaves contain aerenchyma tissue:
- made of thin-walled cells;
- forms air spaces;
- air spaces increase buoyancy.
Some aquatic plants have swollen, large, hollow stems to increase buoyancy; examples: water hyacinth and lotus.

Determining the Age of Woody Plants

The common method uses the growth rings in a woody stem.
One growth ring represents one year.

Method 1: Cutting Down the Tree

Cut down the selected tree.
Cut across the trunk.
Count the visible growth rings.
The number of rings gives the age of the tree in years.

Method 2: Boring the Tree

Do not cut down the tree; bore into it with a special drill.
Drill to 75% of the drill-bit length or half the circumference of the tree.
Remove the drill bit and count the growth rings in the extracted sample.
This method prevents the tree from being cut down.

Support System and Stability in Plants

A plant is stable if it does not topple easily.
Stability is affected by the centre of gravity and base area.
The lower the centre of gravity, the more stable the plant.
Tall woody trees such as chengal trees increase stability through:
- buttress roots that spread widely over the ground surface;
- a tap root that penetrates deep into the ground;
- a trunk with a large circumference.
Support systems that maintain stability include:
- turgid cells in herbaceous plants;
- woody tissue and supporting roots in woody plants;
- water buoyancy and aerenchyma in aquatic plants.

Experiment 6.2: Plant Growth Pattern

Aim: Study the growth pattern of green bean seedlings.
Problem statement: What is the growth pattern of green bean seedlings?
Hypothesis: The growth pattern of green bean seedlings is sigmoid-shaped.

Variables:

Manipulated: Time
Responding: Height of seedlings
Constant: Type of seedlings

Materials: Green bean seeds, cotton wool, water
Apparatus: Petri dishes, forceps, ruler

Soak three green bean seeds in a Petri dish filled with water until they swell.
Keep the apparatus in a dark place overnight.
Transfer the germinated seedlings to another Petri dish containing moist cotton wool.
Measure the length of each seedling with a ruler.
Repeat the measurement every day for seven days and record each height.
Calculate and record the average seedling height for each day.
Plot average height of green bean seedlings (mm) against time (day).

Expected graph: A sigmoid-shaped growth curve.
Conclusion: Green bean seedlings show a sigmoid growth pattern; the hypothesis is accepted.