Biology V: Plant Form and Function
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Plant Structure
- Tissues: meristematic (growth); permanent tissues: parenchyma (storage, photosynthesis), collenchyma (flexible support), sclerenchyma (rigid support), xylem (water and minerals; tracheids and vessels; lignified), phloem (sugars; sieve tubes and companion cells), and epidermis.
- Root: root hairs, cortex, endodermis (with the Casparian strip), pericycle, vascular cylinder.
- Stem: vascular bundles arranged in a ring (dicots) or scattered (monocots); secondary growth from the vascular and cork cambia.
- Leaf: epidermis with stomata and guard cells, palisade and spongy mesophyll, veins. Adaptations of xerophytes and hydrophytes.
Water and Mineral Transport
- Water enters root hairs by osmosis and travels by the apoplast and symplast routes; the Casparian strip forces water through the symplast.
- Transpiration drives the upward movement of water. The cohesion-tension theory: evaporation from leaves creates tension, and cohesion between water molecules pulls a continuous column up the xylem. Factors: light, temperature, humidity, wind. Guard cells control stomata through K⁺ movement and turgor.
- Translocation of sugars in the phloem follows the pressure-flow hypothesis: sugars are loaded at sources (leaves) and unloaded at sinks (roots, fruits), with water following by osmosis.
Mineral Nutrition
Macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl). Deficiency symptoms: nitrogen (yellowing of old leaves), magnesium (interveinal chlorosis), iron (chlorosis in young leaves), phosphorus (purple colouring), potassium (leaf-edge scorching). Nitrogen is obtained from nitrates and ammonium; nitrogen-fixing bacteria (Rhizobium in legume nodules) and mycorrhizal fungi improve nutrition. Carnivorous plants supplement nitrogen from insects (Taiz et al., 2015).
Plant Growth Regulators
| Hormone | Main roles |
|---|---|
| Auxin | Cell elongation, phototropism and gravitropism, apical dominance, rooting of cuttings |
| Gibberellin | Stem elongation, seed germination, fruit development |
| Cytokinin | Cell division, delays senescence, shoot formation |
| Abscisic acid | Stomatal closure, seed dormancy, stress response |
| Ethylene | Fruit ripening, leaf and fruit fall |
Tropisms are directional growth responses; photoperiodism (short-day, long-day, and day-neutral plants) is controlled by phytochrome; vernalization and dormancy.
Reproduction and Development in Flowering Plants
- Flower structure: sepals, petals, stamens (anther and filament), carpel (stigma, style, ovary).
- Pollination (wind, insects, birds) and double fertilization: one sperm nucleus fuses with the egg (zygote), the other with the polar nuclei (triploid endosperm).
- Seed and fruit development; germination (hypogeal and epigeal); requirements: water, oxygen, suitable temperature; dormancy.
- Asexual reproduction: vegetative propagation, tissue culture (see Chapter 22).
Common Mistakes
- Confusing xylem and phloem functions.
- Saying that water is "pushed up" only by root pressure.
- Forgetting that double fertilization is a feature of angiosperms only.
CHAPTER 8