Class 12 Biology Notes Chapter 13 (Chapter 13) – Lab Manual (English) Book

Detailed Notes with MCQs of Chapter 13 from your Biology Lab Manual, which deals with studying ecological adaptations. Understanding these adaptations is crucial, not just for your practical exams, but also because questions related to organismal adaptations to their environment frequently appear in various government exams.

This chapter primarily involves observing and identifying morphological features that help plants and animals survive in specific habitats, mainly aquatic (hydrophytic) and dry (xerophytic) conditions.

Chapter 13: Study of Plant and Animal Morphological Adaptations

Core Concept: Adaptation is any attribute of an organism (morphological, physiological, behavioural) that enables it to survive and reproduce in its habitat. These adaptations arise over evolutionary time through natural selection. We will focus on morphological adaptations – those related to the physical structure of the organism.

A. Study of Morphological Adaptations of Plants

1. Plants in Xerophytic (Dry/Desert) Conditions

• Aim: To study the morphological adaptations of plants like Opuntia (Prickly Pear) and Acacia for survival in water-scarce environments.
• Principle: Xerophytes face water scarcity and high temperatures. Their adaptations aim to minimize water loss (transpiration), store water, and absorb maximum available water.
• Key Adaptations Observed:
  • Reduced Leaves/Spines: In Opuntia, leaves are modified into spines. This drastically reduces the surface area for transpiration. Spines also offer protection against herbivores. In Acacia, leaves are often small (microphyllous) or bipinnately compound with small leaflets, reducing the surface area exposed to sunlight and wind. Some Acacia species have phyllodes (flattened petioles) that perform photosynthesis and reduce water loss.
  • Thick Cuticle: A thick, waxy layer (cuticle) on the epidermis of stems and leaves (if present) prevents water loss through evaporation.
  • Sunken Stomata: Stomata are often located in pits or grooves, sometimes covered with hairs. This creates a humid microenvironment around the stomata, reducing the water potential gradient and thus lowering the rate of transpiration.
  • Succulence (Stem/Leaves): Opuntia has a flattened, fleshy green stem (called phylloclade) that stores large amounts of water and mucilage. This stem also takes over the function of photosynthesis. Some other xerophytes might have succulent leaves.
  • Extensive Root System: Roots are typically very long and spread wide, penetrating deep into the soil to absorb maximum available water. Root hairs are abundant.
  • CAM Photosynthesis (Physiological adaptation often linked to morphology): Many succulents like Opuntia use Crassulacean Acid Metabolism (CAM) where stomata open primarily at night to take in CO2 (minimizing water loss during the day) and fix it into organic acids. CO2 is released from these acids for photosynthesis during the day when stomata are closed.

2. Plants in Aquatic (Hydrophytic) Conditions

• Aim: To study the morphological adaptations of plants like Hydrilla (submerged) and Eichhornia (Water Hyacinth - free-floating) for survival in aquatic environments.
• Principle: Hydrophytes live partially or fully submerged in water. They face challenges like low oxygen availability (in submerged parts), low light penetration, water currents, and the need for buoyancy. Their adaptations address these issues.
• Key Adaptations Observed:
  • Reduced Root System: Roots are often poorly developed, mainly for anchorage rather than absorption (e.g., Hydrilla). In free-floating plants like Eichhornia, roots may be fibrous and feathery, primarily for balancing. Water and mineral absorption occurs through the general body surface.
  • Poorly Developed Vascular Tissues: Xylem and phloem are less developed as water transport is not a major issue; surrounding water provides support. Mechanical tissues (like sclerenchyma) are also reduced or absent.
  • Aerenchyma (Air Chambers): Large air spaces (aerenchyma) are present in stems, leaves, and roots. These provide buoyancy to the plant (especially floating ones like Eichhornia) and facilitate gaseous exchange, allowing oxygen to diffuse to submerged parts. The spongy petiole of Eichhornia is a classic example.
  • Finely Dissected Leaves (Submerged Plants): Submerged plants like Hydrilla often have highly dissected leaves. This increases the surface area for absorption of nutrients and dissolved gases directly from the water and minimizes resistance to water currents.
  • Broad Floating Leaves with Stomata on Upper Surface: Floating plants like Nymphaea (Water Lily - though Eichhornia leaves also float) have broad leaves that float on the surface. Stomata are present mostly on the upper epidermis for gaseous exchange with the atmosphere. A waxy cuticle on the upper surface prevents wetting and clogging of stomata.
  • Absence of Cuticle or Thin Cuticle: Submerged parts generally lack a cuticle or have a very thin one, as water loss is not an issue; instead, absorption through the surface is facilitated.

B. Study of Morphological Adaptations of Animals

(Note: These are often studied theoretically or using models/charts in the lab)

1. Animals in Xerophytic Conditions (e.g., Camel)

• Aim: To understand the morphological adaptations of animals living in deserts.
• Principle: Desert animals face extreme heat and water scarcity. Adaptations focus on conserving water, tolerating heat, and efficient locomotion on sand.
• Key Adaptations:
  • Water Conservation: Concentrated urine and dry faeces; ability to tolerate significant dehydration.
  • Fat Storage: Hump stores fat (not water), which can be metabolized to produce metabolic water and energy when needed. Fat localized in the hump also minimizes its insulating effect over the rest of the body, aiding heat loss.
  • Protection from Sand/Sun: Long eyelashes and closable nostrils keep sand out. Thick fur provides insulation against daytime heat and nighttime cold. Leathery pads on feet for walking on hot sand.
  • Physiological: Tolerance to wide fluctuations in body temperature.

2. Animals in Aquatic Conditions (e.g., Fish)

• Aim: To understand the morphological adaptations of animals living in water.
• Principle: Aquatic animals need adaptations for buoyancy, locomotion (swimming), respiration (extracting dissolved oxygen), and osmoregulation in water.
• Key Adaptations:
  • Streamlined Body: Reduces friction/drag, allowing efficient movement through water.
  • Fins: Provide propulsion (caudal fin), steering (pectoral, pelvic fins), and stability (dorsal, anal fins).
  • Gills: Specialized respiratory organs covered by an operculum (in bony fish) for efficient extraction of dissolved oxygen from water.
  • Scales/Mucus: Provide protection and reduce friction. Mucus coating also aids in osmoregulation and protection against pathogens.
  • Lateral Line System: Sensory organs running along the sides of the body detect vibrations and pressure changes in the water.
  • Swim Bladder (in bony fish): Gas-filled sac that helps regulate buoyancy.

(Amphibians like Frogs show adaptations for both aquatic and terrestrial life: webbed feet for swimming, moist skin for cutaneous respiration in water, lungs for breathing on land, powerful hind legs for jumping.)

Inference from all studies: The specific morphological features observed in each organism are directly correlated with the environmental challenges of its habitat (xerophytic or aquatic) and represent evolutionary solutions for survival and reproduction in that environment.

Multiple Choice Questions (MCQs) for Practice:

1. The modification of leaves into spines in Opuntia is primarily an adaptation to:
   a) Enhance photosynthesis
   b) Reduce transpiration and deter herbivores
   c) Store water
   d) Attract pollinators

2. Aerenchyma tissue is characteristically found in:
   a) Xerophytes
   b) Halophytes
   c) Hydrophytes
   d) Mesophytes

3. Which of the following is NOT a typical adaptation of xerophytic plants?
   a) Sunken stomata
   b) Thick cuticle
   c) Poorly developed root system
   d) Succulent stems or leaves

4. In free-floating hydrophytes like Eichhornia, the roots are mainly adapted for:
   a) Absorption of water and minerals
   b) Anchorage to the soil
   c) Balancing the plant
   d) Storage of food

5. The flattened, green, photosynthetic stem of Opuntia is called a:
   a) Phyllode
   b) Cladode
   c) Phylloclade
   d) Rhizome

6. Stomata in floating leaves of aquatic plants (like water lily) are predominantly found on the:
   a) Upper epidermis
   b) Lower epidermis
   c) Equally on both surfaces
   d) Submerged parts only

7. The streamlined body shape in fish is an adaptation for:
   a) Regulating buoyancy
   b) Detecting water currents
   c) Reducing friction during swimming
   d) Respiration

8. Which adaptation helps camels conserve water in deserts?
   a) Storing water in the hump
   b) Producing highly concentrated urine
   c) Having large, broad feet
   d) Possessing thick fur

9. Finely dissected leaves in submerged hydrophytes like Hydrilla help in:
   a) Reducing water loss by transpiration
   b) Increasing surface area for absorption and reducing water resistance
   c) Storing air for buoyancy
   d) Anchoring the plant firmly

10. The lateral line system in fish is primarily involved in:
    a) Respiration
    b) Osmoregulation
    c) Sensing vibrations and pressure changes
    d) Reproduction

Answer Key:

1. b
2. c
3. c
4. c
5. c
6. a
7. c
8. b
9. b
10. c

Study these adaptations carefully, focusing on the structure and its functional significance in relation to the specific environment. Good luck with your preparation!