Topic 2: Growth and Development
FULL NOTES PDF1. Introduction to Growth and Development
Qualitative and quantitative changes in plant life cycles.
Growth and development are fundamental characteristics of life, representing the dynamic transition of an organism from a simple zygote to a complex multicellular adult. While often used interchangeably, they represent two distinct physiological phenomena.
Growth: An irreversible, permanent increase in the dry mass of living material or protoplasm, primarily due to the synthesis of complex organic molecules like proteins. It is strictly quantitative.
Development: The qualitative increase in complexity of an organism, involving the differentiation of cells into specialized tissues and organs, leading to improved functional capacity and morphological changes.
Theoretical Foundations: Quantity vs Quality
Biologically, growth is considered the “increase in size, volume, or mass,” whereas development is the “maturation and specialization.” For instance, as a plant seedling grows taller (growth), it also begins to produce flowers (development). Development involves gene expression changes where specific genes are switched on or off to allow a cell to become a nerve cell, a muscle cell, or a xylem vessel.
The Cellular Basis of Qualitative Change
Cellular differentiation is the hallmark of development. In the early stages of life, all cells (blastomeres) are identical. However, through the process of Differentiation, cells acquire distinct structural and functional characteristics. This is driven by differential gene expression, positional information, and chemical signaling within the embryo.
2. The Concept of Dry Mass
In biological research, Dry Mass is considered the most accurate parameter for measuring growth. While other metrics like fresh mass, length, or height are easier to measure, they are often subject to fluctuations that do not reflect true biological growth.
Why is Dry Mass the Gold Standard?
Parameters such as “Fresh Mass” can be misleading because water content in cells varies significantly based on environmental conditions. For instance, a plant cell may increase in size simply by taking in water via osmosis during a rainstorm—a process that is entirely reversible. Similarly, an animal may lose weight through dehydration without actually losing cellular protoplasm.
Critique of Other Definitions
- Increase in size: Flawed because swelling due to water uptake is not “true” growth.
- Increase in cell number: During embryonic cleavage, cells divide rapidly but do not increase in size; thus, the total mass remains constant or even decreases slightly due to respiration.
- Increase in Height: Only accounts for one dimension and ignores the overall accumulation of biomass.
Experimental Determination of Dry Mass
To determine dry mass, an organism must be killed and dried in an oven (usually at $70^\circ C$ to $100^\circ C$) until a constant mass is achieved. This ensures all volatile water is removed. While accurate, this method has the distinct disadvantage of being destructive, as the organism cannot be measured again at a later stage. Consequently, researchers must use large populations to sample growth over time.
3. Factors Influencing Growth
Light is a critical external factor for photosynthesis and growth regulation.
Growth is a highly regulated process controlled by a complex interplay of environmental (external) and physiological (internal) factors. These factors work synergistically to determine the rate and extent of growth.
| External (Environmental) Factors | Internal (Physiological) Factors |
|---|---|
| Nutrients: Essential elements ($N, P, K, Mg, Fe$) serve as building blocks for proteins, chlorophyll, and DNA. | Genes: The hereditary material provides the “blueprint” and sets the limit for the maximum potential size. |
| Temperature: Most metabolic reactions are enzyme-mediated; growth typically increases with temperature up to an optimum ($25^\circ C – 35^\circ C$). | Hormones: Growth regulators like Auxins, Gibberellins, Cytokinins, and Abscisic Acid in plants; GH and Thyroxine in animals. |
| Light: Drives photosynthesis in plants and regulates photoperiodism and circadian rhythms in animals. | Enzymes: Biological catalysts that lower activation energy for anabolism and catabolism. |
| Oxygen & CO2: Oxygen is vital for aerobic respiration (ATP production), while CO2 is the carbon source for plants. | Metabolic Status: The availability of ATP determines whether biosynthesis can proceed at high rates. |
A Deep Dive into Hormonal Regulation
In plants, growth is strictly regulated by phytohormones. Auxins promote cell elongation and apical dominance, while Gibberellins stimulate internode elongation and seed germination. Cytokinins promote cell division (cytokinesis) and delay leaf senescence. In contrast, Abscisic Acid (ABA) acts as a growth inhibitor, promoting dormancy and stomatal closure during stress.
4. Patterns of Growth
Different species exhibit diverse growth trajectories based on their evolutionary adaptations, ecological niches, and life cycles. Understanding these patterns allows biologists to predict development and resource needs.
Positive Growth: Occurs when anabolism (building up) is faster than catabolism (breaking down). This leads to an increase in biomass.
Negative Growth: Occurs when catabolism exceeds anabolism. This is common in germinating seeds before photosynthesis begins, as stored fats and starches are respired to provide energy, leading to a loss in dry mass.
Allometric Growth: Different body parts grow at different rates. For example, a human baby’s head is large relative to its body, but during growth, the limbs grow faster to reach adult proportions.
Isometric Growth: All body parts grow at the same rate, meaning the organism maintains its shape throughout its life cycle (e.g., many species of fish).
Limited (Determinate): Growth stops once the organism reaches a specific size or reproductive maturity (e.g., humans, annual plants).
Unlimited (Indeterminate): Growth continues throughout the life of the organism, often seen in perennial trees and some marine invertebrates.
Mathematical Representation of Allometry
Allometric growth can be mathematically expressed by the equation: $y = bx^a$, where $y$ is the size of the organ, $x$ is the total size of the organism, $b$ is a constant, and $a$ is the allometric coefficient. If $a > 1$, the organ is growing faster than the body (positive allometry).
5. The Sigmoid Growth Curve
The standard S-shaped curve representing the phases of biological growth.
The “S-shaped” or Sigmoid Curve is the mathematical representation of the grand period of growth. It illustrates how growth rates change as an organism matures and resources become limited.
The Four Detailed Phases of the Sigmoid Curve
- Lag Phase: The initial period where cells are adapting to the environment or mobilizing reserves. In seeds, dry mass decreases as respiration consumes stored food.
- Log (Exponential) Phase: The period of maximum growth rate. Cells divide rapidly, and the organism increases in size at a constant percentage rate. Resources are abundant.
- Stationary Phase: The rate of new cell production equals the rate of cell death. The organism has reached its adult size or the “carrying capacity” of its environment.
- Decelerating (Decline) Phase: Senescence (aging) begins. In populations, this may be due to toxic waste accumulation or resource exhaustion, leading to a net decrease in living mass.
Ecological Implications of the Growth Curve
In population biology, the Sigmoid curve is known as the Logistic Growth model. It suggests that no population can grow indefinitely. The limit at the stationary phase is called the Carrying Capacity ($K$). Factors that slow growth as the population approaches $K$ include competition for food, space, predation, and disease.
6. Growth in Arthropods: Intermittent Growth
Arthropods (insects, crustaceans, spiders) face a unique challenge: they are encased in a rigid, chitinous exoskeleton. This armor provides protection and support but prevents continuous expansion of the body. Consequently, arthropods exhibit Intermittent Growth.
The Process of Ecdysis (Moulting)
Growth can only occur when the old, restrictive exoskeleton is shed. The process begins with the secretion of moulting fluid, which contains enzymes that digest the inner layers of the old cuticle. Simultaneously, a new, soft, folded cuticle is secreted by the epidermis.
Steps of Ecdysis
- The arthropod stops feeding and becomes inactive.
- The old cuticle splits (usually along the back).
- The animal crawls out, leaving the exuvia (shed skin).
- The animal takes in air or water to expand the soft new cuticle before it hardens (tanning).
Discontinuous Growth Graph
If one plots the length of an insect over time, the graph looks like a staircase rather than a smooth curve. Each horizontal “step” represents an instar (period between moults), and the vertical “jump” represents the actual growth during ecdysis.
7. Metamorphosis Dynamics
Complete metamorphosis involves a pupal stage where radical tissue reorganization occurs.
Metamorphosis is the complex transition from a juvenile larval form to a sexually mature adult. This process allows different stages of the life cycle to occupy different niches, reducing competition between parents and offspring.
Stages: Egg → Larva → Pupa → Adult. The larva (e.g., caterpillar) is a feeding machine, while the pupa is a non-feeding stage where the body is rebuilt using imaginal discs. Examples: Butterflies, Bees, Beetles.
Stages: Egg → Nymph → Adult. The nymph resembles the adult but is smaller and lacks wings. With each moult, it gradually looks more like the adult. Examples: Cockroaches, Locusts.
The Endocrine Control of Metamorphosis
Metamorphosis is orchestrated by three key hormones. The Brain Hormone (PTTH) stimulates the prothoracic gland to release Ecdysone. However, the outcome of the moult depends on the level of Juvenile Hormone (JH).
- High JH + Ecdysone: Larva moults into another Larva.
- Low JH + Ecdysone: Larva moults into a Pupa.
- Zero JH + Ecdysone: Pupa moults into an Adult.
8. Cellular Basis of Growth: The Cell Cycle
At the microscopic level, growth is achieved through three integrated processes: Cell Division (increasing number), Cell Expansion (increasing size), and Cell Differentiation (becoming specialized).
The Interphase: Preparing for Division
Interphase is not a “resting” phase; it is a period of intense biochemical activity.
- G1 (Gap 1): The cell grows in size and synthesizes mRNA and proteins required for DNA replication.
- S (Synthesis): DNA replication occurs. Each chromosome now consists of two sister chromatids.
- G2 (Gap 2): Synthesis of microtubules (for the spindle) and further organelle duplication.
Mitosis: Nuclear Division
| Phase | Microscopic Events |
|---|---|
| Prophase | Chromatin condenses into visible chromosomes. Nuclear envelope breaks down. Spindle fibers begin to form. |
| Metaphase | Chromosomes align at the metaphase plate (equator). Spindle fibers attach to the kinetochore of centromeres. |
| Anaphase | Centromeres split. Sister chromatids are pulled to opposite poles by shortening microtubules. |
| Telophase | Chromosomes reach poles and uncoil. Two new nuclear envelopes reform. |
Cytokinesis: Dividing the Cytoplasm
In animals, a cleavage furrow forms as a contractile ring of actin filaments “pinches” the cell in two. In plants, because of the rigid cell wall, a cell plate forms at the equator and grows outward until it fuses with the parent cell wall.
10. Plant Meristems and Primary Growth
Growth in plants is localized in meristematic tissues at the tips.
Unlike animals, whose growth occurs throughout the body, plant growth is Localized. These specialized regions are called Meristems.
Located at the very tips of roots and shoots. They produce new cells that increase the length of the plant. This is the only type of growth in many monocots and annual herbs.
Found in woody plants. They include the Vascular Cambium (produces wood) and Cork Cambium (produces bark). They increase the girth/diameter of the plant.
Anatomy of the Root Tip
- Root Cap: Protects the meristem as it pushes through soil. Secretes mucilage.
- Zone of Division: Contains the Quiescent Centre where mitosis is slow but steady.
- Zone of Elongation: Cells take in water by osmosis into vacuoles, stretching the cell wall. This is where the most visible growth occurs.
- Zone of Maturation: Cells differentiate into epidermis, cortex, xylem, and phloem. Root hairs develop here.
11. Secondary Thickening (Wood Production)
Annual rings in a cross-section of a tree trunk, showing secondary xylem layers.
Secondary growth is the characteristic increase in thickness of gymnosperms and woody dicots. It is driven by the Vascular Cambium, a ring of meristematic cells between the primary xylem and phloem.
The Dynamics of the Vascular Cambium
The cambium produces Secondary Xylem towards the center and Secondary Phloem towards the periphery. Over time, the massive accumulation of secondary xylem—lignified and rigid—forms the **wood**. The phloem remains as a thin layer in the bark.
Heartwood and Sapwood
- Heartwood: The older, central portion of wood. The vessels are blocked by tyloses and deposits of resins and tannins. It is dark, hard, and purely for support.
- Sapwood: The younger, outer layers. It is lighter in color and actively conducts water and mineral salts.
Dendrochronology: Reading the Rings
In seasonal climates, the cambium is more active in spring (producing large, thin-walled vessels) than in summer/autumn (producing small, thick-walled cells). This alternation creates visible Annual Rings. By studying these rings, scientists can determine the age of a tree and reconstruct historical climate data (e.g., wide rings indicate wet, favorable years).
12. Seed Dormancy and Viability
Dormancy is a physiological state where a viable seed fails to germinate even under ideal environmental conditions. It is an evolutionary “strategy” to ensure the seed germinates only when survival is most likely.
- Physical: Hard, impermeable seed coat (testa) prevents water/oxygen entry.
- Physiological: Presence of growth inhibitors like Abscisic Acid (ABA).
- Morphological: The embryo is immature and needs time for after-ripening.
- Scarification: Mechanical scratching or acid treatment of the seed coat.
- Stratification: Exposure to a period of cold (chilling) to degrade inhibitors.
- Light: Some seeds (photoblastic) require specific light triggers to activate gibberellins.
Seed Viability and Longevity
Viability is the period during which a seed remains capable of germinating. It is preserved by keeping seeds in a dry, cool, and well-aerated environment. While some seeds (like Willow) lose viability in days, others (like the Lotus) can remain viable for over a millennium!