1.3 - Growth Curves

Classification of Plants Based on Life Cycles

• Plants are classified as annual, biennial or perennial plants.

1. Annual Plants

• Complete one life cycle in one season or one year.
• Usually die after the biological cycle from germination to flowering or seed production.
• Examples: paddy, pumpkin and watermelon plants.

2. Biennial Plants

• Take two years and two growth seasons to complete their life cycle.
• First growth season: vegetative growth of roots, leaves and stems.
• Growth stops briefly during winter.
• Second growth season: growth continues in spring and summer; plants flower, produce fruits and seeds, then die.
• Most grow in temperate regions.
• Examples: cabbage, carrot and silver cock’s comb.

3. Perennial Plants

• Live for more than two years; lifespan depends on species and conditions.
• Classified as woody perennial plants or herbaceous perennial plants.
• Most flower and bear fruits repeatedly throughout their lives.
• Have structures adapted to their surroundings and temperature changes.
• Examples: grass, hibiscus and mango plants.

Growth Curves in Plants

Growth Curve of Annual Plants

• A sigmoid curve of dry mass against time.
• Stage A — decreasing dry mass: food stored in the cotyledon is used for germination before leaves emerge and carry out photosynthesis.
• Stage B — increasing dry mass: growth rate increases rapidly because the plant carries out photosynthesis.
• Stage C — constant dry mass: growth rate is zero; the plant is matured.
• Stage D — decreasing dry mass: occurs slowly because of aging, a lower photosynthesis rate, shedding of leaves and flowers, and seed dispersal.

Growth Curve of Biennial Plants

• Two combined sigmoid curves.
• First growth season: leaves are produced, photosynthesis occurs and food is stored in tubers.
• Second growth season: stored food is used to produce flowers and seeds.

Growth Curve of Perennial Plants

• A series of small sigmoid curves; each year produces one sigmoid curve.
• Growth occurs throughout the plant’s lifespan.
• Growth rate is high in spring and summer because high light intensity increases photosynthesis rate.
• Growth rate decreases in winter.

Experiment: Growth Curve of a Corn Plant

• Aim: Investigate the growth curve of a corn plant.
• Hypothesis: The growth curve of a corn plant is sigmoid.
• Variables:
  • Manipulated: number of days after planting.
  • Responding: dry mass of corn seeds or seedlings.
  • Constant: type of corn seed.
• Method:
  1. Soak 30 corn seeds for at least one hour.
  2. Dry three randomly selected seeds in an oven at 100 °C for 5 minutes; weigh repeatedly until dry mass is constant and calculate the average for day 0.
  3. Plant the remaining seeds 30 cm apart and water daily.
  4. Every three days, collect and clean three seedlings.
  5. Dry at 100 °C, increasing oven time by 10 minutes up to 30 minutes as seedlings enlarge; weigh repeatedly until dry mass is constant.
  6. Continue until day 21 and plot average dry mass against time.