Plant Growth and Development

Seed Germination

• NCERT: Chapter 21 - Plant Growth and Development (Class 11)

Structure of a seed:

• Dicotyledonous seeds: Consists of two cotyledons (seed leaves), an embryo axis with a radicle (embryonic root), a plumule (embryonic shoot), and a seed coat.
• Monocotyledonous seeds: Has one cotyledon, an embryo axis with a radicle and plumule, and a protective layer called the coleoptile.

Factors affecting seed germination:

• Water: Essential for seed imbibition and metabolic processes.
• Oxygen: Required for respiration and energy production during germination.
• Temperature: Optimal temperature range for germination varies depending on the plant species.
• Light: Some seeds require light for germination, while others are light-insensitive.

Stages of seed germination:

• Imbibition: Water uptake by the seed, causing it to swell and soften.
• Radicle emergence: The radicle emerges from the seed coat and grows downward, becoming the primary root.
• Plumule emergence: The plumule emerges from the seed coat opposite the radicle, developing into the shoot system.
• Seedling growth: The seedling continues to grow and develop, forming leaves and roots.

Plant Hormones

• NCERT: Chapter 22 - Plant Growth and Development (Class 11)

Types of plant hormones:

• Auxins: Promote cell elongation, root initiation, and apical dominance.
• Gibberellins: Stimulate stem elongation, seed germination, and fruit growth.
• Cytokinins: Promote cell division, shoot and root development, and delay senescence.
• Abscisic acid: Involved in regulating water balance, seed dormancy, and fruit ripening.
• Ethylene: Promotes fruit ripening, senescence, and abscission of leaves and flowers.

Mechanisms of hormone action:

• Hormones bind to specific receptors on target cells, triggering signal transduction pathways.
• These pathways often involve the production of second messengers, such as calcium ions, which initiate cellular responses.

Physiological effects of plant hormones:

• Cell division: Cytokinins promote cell division in meristematic tissues.
• Cell elongation: Auxins and gibberellins stimulate cell elongation in various plant organs.
• Tropisms: Auxins and gravity influence phototropism (directional growth in response to light) and gravitropism (directional growth in response to gravity).
• Fruit development: Auxins and gibberellins promote fruit set and growth.

Phototropism and Gravitropism

• NCERT: Chapter 22 - Plant Growth and Development (Class 11)

Mechanisms of phototropism:

• Phototropism is primarily mediated by the plant hormone auxin.
• Auxin accumulates on the shaded side of the plant, promoting cell elongation and causing the plant to bend towards the light source.
• The photoreceptor responsible for detecting light is called phytochrome.

Mechanisms of gravitropism:

• Gravitropism involves the sensing and response to gravity by specialized structures called statocytes or gravity sensors.
• Statocytes contain starch-filled plastids called statoliths, which sediment in response to gravity.
• This sedimentation triggers a cascade of events that leads to the differential distribution of auxin, resulting in root growth downward and shoot growth upward.

Ecological significance of phototropism and gravitropism:

• Phototropism helps plants optimize light capture for photosynthesis.
• Gravitropism ensures proper root anchoring in the soil and shoot orientation towards sunlight.

Apical Meristems and Primary Growth

• NCERT: Chapter 22 - Plant Growth and Development (Class 11)

Structure of shoot and root apical meristems:

• Shoot apical meristem (SAM): Located at the tip of the shoot, responsible for the production of new leaves, stems, and flowers.
• Root apical meristem (RAM): Found at the tip of the root, responsible for primary root growth and the formation of lateral roots.

Cell division and differentiation in apical meristems:

• Apical meristems contain actively dividing cells that undergo mitosis.
• These cells differentiate into various tissues, including the epidermis, cortex, vascular tissues (xylem and phloem), and pith.

Formation of primary tissues:

• Protoderm: The outermost layer of cells derived from the SAM, eventually forming the epidermis.
• Ground meristem: Located between the protoderm and procambium, gives rise to the cortex and pith.
• Procambium: Central region of the SAM, differentiates into primary xylem and phloem.

Primary growth of plants:

• Primary growth refers to the growth of plants in length due to cell division and differentiation in apical meristems.
• It results in the formation of new leaves, stems, and roots.

Secondary Growth

• NCERT: Chapter 22 - Plant Growth and Development (Class 11)

Mechanism of secondary growth:

• Secondary growth occurs due to the activity of two lateral meristems, the vascular cambium, and the cork cambium.
• Vascular cambium: Produces secondary xylem (wood) towards the inside and secondary phloem towards the outside.
• Cork cambium (phellogen): Forms the protective layer of bark, consisting of cork cells.

Formation of secondary tissues:

• Secondary xylem (wood): Consists of vessels, tracheids, xylem fibers, and xylem parenchyma.
• Secondary phloem: Composed of sieve tubes, companion cells, phloem fibers, and phloem parenchyma.

Formation of bark and annual rings:

• Bark is the outermost layer of the plant, consisting of cork cells and secondary phloem.
• In woody plants, secondary growth produces annual rings, which are alternating layers of light (spring/summer wood) and dark (autumn/winter wood) xylem.

Wood anatomy:

• Vessels: Large, hollow cells responsible for water transport.
• Tracheids: Elongated cells with tapered ends, involved in water transport and support.
• Fibers: Long, thick-walled cells that provide structural support.
• Parenchyma: Thin-walled cells involved in various functions, including storage, photosynthesis, and support.

Leaf Structure and Photosynthesis

• NCERT: Chapter 13 - Photosynthesis in Higher Plants (Class 11)

External and internal structure of a leaf:

• External structure: Includes the leaf blade (flattened part), petiole (stalk), and stipules (leaf-like structures at the base of the petiole).
• Internal structure: Consists of epidermis, mesophyll (containing chloroplasts), and veins (vascular tissues).

Light-dependent reactions of photosynthesis:

• Take place in the thylakoid membranes of chloroplasts.
• Involves the conversion of light energy into ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
• The electron transport chain and chemiosmosis are key processes in ATP synthesis.

Calvin cycle (dark reactions) of photosynthesis:

• Occurs in the stroma of chloroplasts.
• Involves the fixation of carbon dioxide from the atmosphere into organic molecules (sugars) using ATP and NADPH.

Factors affecting photosynthesis:

• Light intensity: Affects the rate of photosynthesis, reaching a plateau at high light levels.
• Carbon dioxide concentration: The rate of photosynthesis increases with an increase in carbon dioxide concentration.
• Temperature: Optimal temperature range for photosynthesis varies among plant species.
• Water availability: Water is required for photosynthesis, and drought conditions can reduce its rate.

Floral Structure and Pollination

• NCERT: Chapter 2 - Sexual Reproduction in Flowering Plants (Class 12)

Structure of a flower:

• Consists of sepals (leaf-like structures outermost layer), petals (colorful leaves that attract pollinators), stamens (male reproductive part with anther and filament), and pistil (female reproductive part with stigma, style, and ovary).

Types of pollination:

• Self-pollination: Transfer of pollen from the anther to the stigma of the same flower or different flowers of the same plant.
• Cross-pollination: Transfer of pollen from the anther of one flower to the stigma of another flower on a different plant.

Adaptations for pollination:

• Flowers have evolved various adaptations to attract pollinators, such as shape, color, fragrance, and nectar production.
• Pollinators include bees, butterflies, birds, bats, and the wind.

Fertilization and Seed Development

• NCERT: Chapter 2 - Sexual Reproduction in Flowering Plants (Class 12)

Events of fertilization:

• Pollination: Transfer of pollen from the anther to the stigma.
• Pollen-pistil interaction: Pollen grains germinate on the stigma, producing a pollen tube.
• Pollen tube growth: The pollen tube grows through the style, delivering sperm cells to the ovary.
• Double fertilization: One sperm cell fuses with the egg cell, forming the zygote, while another sperm cell fuses with two polar nuclei, forming the endosperm nucleus.

Formation of endosperm and embryo:

• Endosperm: Nutritive tissue that surrounds the embryo, derived from the fusion of a sperm cell and two