Growth and Development

Fundamental Concepts of Growth and Development

  • Growth

    • Growth is defined as the irreversible increase in the size and dry mass of an organism.
    • Growth is brought about through two primary cellular processes:
      1. Cell Division: Cells divide by mitotic cell division. This process leads to an increase in the number of cells.
      2. Cell Enlargement: Cells assimilate food substances and synthesize new materials. This process leads to an increase in the size of cells.

  • Development

    • Development is characterized as the irreversible change in the structure and the increase in the complexity of an organism.
    • Differentiation of cells results in the formation of different kinds of specialized cells, which carry out specific functions. This process increases the complexity and specialized nature of individual cells.
    • Similar types of specialized cells group together to form tissues, organs, and systems. This collective organization increases the complexity of the body organization.
    • Summary: Development increases the complexity of cells and the complexity of body organization.

Plant Growth and Development: Seed Germination

  • Seed Dormancy

    • Seeds stay in a dormant state under adverse or unfavorable conditions.

  • The Germination Process

    • During germination, a seed grows and develops into a seedling.
    • Necessary conditions for seed germination include water, oxygen, and warmth.

  • Timeline and Stages of Germination (Example: Mung Bean)

    • Day 2: Water is absorbed through the micropyle. The seed coat breaks, and the radicle emerges. Insoluble food is converted to a soluble form by enzymes to facilitate growth and development.
    • Day 3: Root hairs develop on the radicle to increase the surface area for water and mineral absorption.
    • Day 4: The hypocotyl grows in a curved shape. The hypocotyl elongates and brings the cotyledons up above the soil. The cotyledons protect the delicate plumule tip.
    • Day 5: The hypocotyl straightens. The cotyledons turn green and begin to carry out photosynthesis.
    • Day 6: Young leaves develop to make food by photosynthesis. The cotyledons, having exhausted their food reserves, fall off.

  • Detailed Function of Germination Conditions

    • Water:
      • Softens the seed coat and makes the cotyledons swell to break the seed coat, allowing the radicle to emerge.
      • Activates enzymes in the seed, which help convert insoluble food reserves into simpler and soluble forms.
      • Acts as a medium (solvent) for the transport of soluble food.
    • Warmth:
      • A warm environment allows the enzymes in the seed to work efficiently.
    • Oxygen:
      • Oxygen is required for respiration, which provides the energy necessary for the growth of the embryo.

Regions of Growth in Plants

  • Primary Growth

    • Growth of plants occurs primarily at the tips of the roots and shoots, as well as the periphery of roots and stems.
    • These regions contain meristems, which are groups of undifferentiated cells able to divide by mitotic cell division throughout the plant's life.
    • Growth in apical meristems (at root and shoot tips) leads to primary growth, which is an increase in the length of the plant.

  • Regions of the Root and Shoot Tip

    • Region A: Region of Cell Division
      • Apical meristem produces new cells by mitotic cell division.
    • Region B: Region of Elongation
      • Cells enlarge by taking in water to form many small vacuoles.
    • Region C: Region of Differentiation
      • Small vacuoles in the cells fuse to form a large central vacuole.
      • The cell walls become thicker and more rigid due to the deposit of newly made cellulose fibres.
      • The cells differentiate to perform specific functions.

  • Secondary Growth

    • Growth at the periphery of roots and stems occurs in the lateral meristem (cambium) located between the xylem and phloem in woody plants.
    • Growth in these regions leads to secondary growth, which is an increase in the thickness or girth of the plant.
    • The newly formed cells differentiate into secondary xylem and secondary phloem.
    • Secondary growth leads to the formation of annual rings in woody stems.
    • For successful competition for light, stems must grow to a great height, which requires an increase in thickness to support the structure.

Formation of Annual Rings in Woody Stems

  • The growth of secondary xylem is not uniform throughout the year.
  • Spring Wood: In spring, xylem vessels have larger lumens because the water supply is abundant. Higher water content in newly formed xylem cells stretches the cell wall to a greater extent, resulting in larger cells with thinner cell walls. These appear lighter in color.
  • Autumn Wood: In autumn, the soil is frequently dry. Lower water content in the cells results in smaller cells and thicker cell walls, as the walls are less stretched than in spring. These appear darker in color.

Measurement of Growth Parameters

  • Size (Length, Height, Surface Area, and Volume)

    • Advantages: Allows continuous measurement because the organism remains alive; easy and convenient.
    • Disadvantages: Only growth in one dimension is taken into account.
    • Common Applications: Root of a seedling (length), leaf (surface area), fruit (volume).

  • Fresh Mass (Fresh Weight)

    • This is the mass with water retained.
    • Disadvantages: Shows great variation because it is affected by the water content of an organism.
    • Common Applications: The whole body of animals and plants.

  • Dry Mass (Dry Weight)

    • This is the mass with water removed.
    • Advantages: Highly reliable.
    • Disadvantages: Continuous measurement is impossible because the organism must be killed; a large number of specimens are required; it is time-consuming.
    • Common Applications: Microorganisms like yeast and bacteria, plant seeds, and seedlings.

Growth Curves and Patterns

  • Annual Plants

    • An annual plant lives for only one growing season. The growth curve typically consists of three stages:
      1. Stage 1: Dry mass decreases: Stored food is broken down to provide energy for growth during the early stage of germination.
      2. Stage 2: Dry mass increases: The rate of photosynthesis is faster than the rate of respiration, leading to a net production of organic food used for building plant materials.
      3. Stage 3: Dry mass decreases: Seeds and fruits are dispersed, and the plant undergoes senescence.

  • Mung Bean Growth Comparison (Daylight vs. Darkness)

    • Fresh Weight: Increases in both daylight and darkness initially due to water absorption. In daylight, it continues to increase due to water and food accumulation from photosynthesis. In darkness, it eventually decreases as stored food is exhausted.
    • Dry Mass: Decreases initially (Day 0 to 10) in both conditions as stored food is oxidized for energy. From Day 10 to 20, dry mass increases in daylight due to photosynthesis, while it continues to decrease in darkness due to lack of photosynthesis and depletion of food reserves.

  • Human Growth

    • Growth occurs in different parts of the body at different rates.
    • Timeline:
      • Infancy: Rapid growth.
      • Childhood: Growth slows down.
      • Puberty: Growth becomes rapid again.
      • Adulthood: Person reaches maximum size.
      • Old Age: Growth becomes negative.
    • Body Parts:
      • The brain grows rapidly during infancy and early childhood.
      • Reproductive organs grow rapidly at puberty.
      • The relative size of the head decreases with age.
      • The relative size of the limbs increases with age.
      • The relative size of the trunk remains nearly the same.
    • Sex Differences: Puberty occurs earlier in girls (typically age 11 to 14). After age 14, the growth rate of boys usually becomes faster than that of girls.

Questions & Discussion

  • MCQ 1: A plumule bends upward after some time. This is accounted for by mitosis, cell enlargement, and the uneven distribution of auxins.

  • MCQ 2: In a young root, xylem and phloem are found in the region of differentiation (labelled as region 1 in the diagram).

  • MCQ 3: An investigation with two tubes kept at 30∘C30^\circ\text{C} where seeds in Tube A (with water) germinated and Tube B (dry) did not, is testing whether water is necessary for germination.

  • MCQ 4: Cells showing active mitosis (as seen in a photomicrograph) are taken from the region of cell division (Region Q).

  • MCQ 5: In a germinating bean seedling, the food supporting growth up to the 7-day stage mostly comes from food stored in the cotyledons (Part R).

  • MCQ 6: Comparison of dry mass curves: Curve X (whole seedling) shows an initial dip then rise; Curve Y (cotyledons) shows a steady decrease as food is used.

  • MCQ 7: Photosynthetic rate is higher than respiration rate when the total dry mass of the seedling begins to increase (e.g., from Day 7 to Day 10).

  • MCQ 8: The standard growth curve for the total dry mass of germinating seeds in a favorable environment shows an initial decrease followed by an increase.

  • MCQ 9: In humans, the curve for the head (I) reaches maximum size early, the whole body (II) follows an S-shape, and the reproductive system (III) stays low until puberty.

  • MCQ 10: Carbon dioxide (CO2CO_2) contributes most to the increase in biomass of a plant through photosynthesis.

  • MCQ 11: The best parameter for recording the growth of a potted germinating seedling over time is the total surface area of the leaves (since dry mass requires killing the plant).

  • MCQ 12: Root hairs are found exclusively on the region of differentiation of the root.

  • Past Paper Descriptive Questions:

    • Germination Structures: Structures like the plumule develop into shoots (stems and leaves).
    • Amylase Activity: Amylase breaks down stored starch into maltose/glucose. This provides the respiratory substrate for energy production and the building blocks for new cell materials.
    • Dry Mass Changes: The initial decrease in dry mass is due to the respiration of stored food. The subsequent increase occurs because the young leaves carry out photosynthesis, and the rate of food production exceeds the rate of food consumption.
    • Daylight vs. Darkness Experiment: Between Day 0 and Day 12, both samples decrease in dry mass as they rely on stored food. From Day 12 to Day 18, daylight seedlings increase in mass due to photosynthesis, whereas darkness seedlings continue to lose mass as they cannot photosynthesize and continue to consume stored energy.