Exercises
Q1. Draw illustrations to bring out the anatomical difference between
(a) Monocot root and Dicot root
(b) Monocot stem and Dicot stem
Ans. (Diagrams are required; key structural differences are described below.)
(a) Monocot root vs Dicot root:
Dicot root: Has 2–6 xylem bundles (tetrarch is common); pith is absent or very small; cortex is well developed; endodermis, pericycle are present; xylem vessels are large. Example: Sunflower, Bean root cross section.
Monocot root: Has many (more than 6) xylem bundles (polyarch); pith is large and well developed; cortex is relatively less developed; endodermis with Casparian strips; pericycle is present; xylem vessels are smaller. Example: Grass, Wheat root.
(b) Monocot stem vs Dicot stem:
Dicot stem: Vascular bundles are arranged in a ring (along a cylinder); they are open (have cambium between xylem and phloem) and conjoint and collateral; pith is present in the centre; ground tissue is differentiated into cortex and pith; epidermis may have trichomes.
Monocot stem: Vascular bundles are scattered throughout the ground tissue (no ring arrangement); they are closed (no cambium) and conjoint and collateral; pith cannot be differentiated from cortex (called ground tissue); each vascular bundle is surrounded by a bundle sheath of sclerenchyma. Example: Maize stem.
Cut a transverse section of young stem of a plant from your school garden and observe it under the microscope. How would you ascertain whether it is a monocot stem or a dicot stem? Give reasons.
Answer
To ascertain whether it is a monocot or dicot stem:
(i) Arrangement of vascular bundles: If vascular bundles are arranged in a ring (circular arrangement)
⇒ dicot stem. If vascular bundles are scattered throughout the ground tissue (no defined ring)
⇒ monocot stem.
(ii) Presence of cambium: If vascular bundles are open (cambium present between xylem and phloem)
⇒ dicot stem. If vascular bundles are closed (no cambium)
⇒ monocot stem.
(iii) Bundle sheath: If each vascular bundle is surrounded by a sclerenchymatous bundle sheath
⇒ monocot stem.
(iv) Differentiation of ground tissue: If ground tissue is clearly differentiated into cortex (outer) and pith (inner, large)
⇒ dicot stem. If ground tissue is undifferentiated (no cortex/pith distinction)
⇒ monocot stem.
(v) Hypodermis: Sclerenchymatous in monocots, collenchymatous in dicots.
The transverse section of a plant material shows the following anatomical features – (a) the vascular bundles are conjoint, scattered and surrounded by a sclerenchymatous bundle sheath. (b) phloem parenchyma is absent. What will you identify it as?
Answer
The plant material is a monocot stem. The features described – scattered vascular bundles, bundle sheath of sclerenchyma around each vascular bundle, and absence of phloem parenchyma – are characteristic of a monocot stem (e.g., a cross section of maize stem). In dicot stems, vascular bundles are arranged in a ring and phloem parenchyma is present. In monocot stems, vascular bundles are scattered and lack phloem parenchyma.
What is stomatal apparatus? Explain the structure of stomata with a labelled diagram.
Answer
The stomatal apparatus consists of a stomatal pore (aperture), the two guard cells surrounding it, and the subsidiary cells (accessory cells) present around the guard cells.
Structure of stomata
(i) Stomatal pore: An opening (aperture) in the epidermis through which gaseous exchange and transpiration occur.
(ii) Guard cells: Two specialised, bean-shaped (kidney-shaped in dicots) or dumbbell-shaped (in monocots/grasses) cells that border the stomatal pore. They contain chloroplasts and can synthesise food. The cell wall is unevenly thickened – the inner wall (facing the pore) is thick and inelastic; the outer wall is thin. This uneven thickening causes the guard cells to bulge outward when turgid, opening the pore, and to become flaccid and close it.
(iii) Subsidiary (accessory) cells: Epidermal cells immediately surrounding the guard cells that differ from ordinary epidermal cells in structure.
Guard cells open stomata when they accumulate water (become turgid) and close them when they lose water (become flaccid). Stomata are generally present in large numbers on the lower epidermis of leaves.
Name the three basic tissue systems in the flowering plants. Give the tissue names under each system.
Answer
(i) Epidermal tissue system: Forms the outermost covering of the plant body. Tissues: epidermis (epidermal cells), stomata (guard cells + subsidiary cells), epidermal appendages (trichomes and root hairs).
(ii) Ground tissue system (Fundamental tissue system): All tissues other than the vascular tissue and the epidermis. Tissues: parenchyma (cortex, pith, mesophyll), collenchyma (provides flexibility), sclerenchyma (provides rigidity and support). It includes cortex, medullary rays, and pith in stems; mesophyll in leaves; cortex and pith in roots.
(iii) Vascular tissue system (Conducting tissue system): The conducting tissues that transport water, minerals, and food. Tissues: xylem (tracheids, vessels, xylem parenchyma, xylem fibres) and phloem (sieve tubes, companion cells, phloem parenchyma, phloem fibres). Together they form vascular bundles.
How is the study of plant anatomy useful to us?
Answer
Study of plant anatomy is useful in many ways:
(i) Identification and classification: Anatomical characters help in identifying and classifying plants, especially when external features are insufficient.
(ii) Wood technology: Understanding wood anatomy helps in selecting appropriate wood for specific uses – timber, paper, furniture, construction.
(iii) Medicine: Anatomical study helps in identifying plant tissues used as drugs and in detecting adulteration.
(iv) Agriculture: Knowledge of stem anatomy helps understand grafting success (cambium alignment needed), and root anatomy helps in understanding nutrient and water uptake.
(v) Evolutionary studies: Anatomical features provide evidence for evolutionary relationships.
(vi) Forensic botany: Plant anatomy can be used to identify plant parts found at crime scenes.
Describe the internal structure of a dorsiventral leaf with the help of labelled diagrams.
Answer
A dorsiventral leaf (typical of dicots) has distinct upper (adaxial) and lower (abaxial) surfaces. Its internal structure consists of three main parts:
(i) Epidermis:
• Upper epidermis (adaxial): Single layer of compactly arranged cells; covered by a thick cuticle; stomata are absent or few.
• Lower epidermis (abaxial): Single layer; covered by thinner cuticle; numerous stomata (more than upper surface); guard cells present.
(ii) Mesophyll: The ground tissue between upper and lower epidermis containing chloroplasts.
• Palisade parenchyma: Located just below the upper epidermis; cells are elongated, column-shaped, tightly packed, and contain many chloroplasts. Main site of photosynthesis.
• Spongy parenchyma: Located below the palisade layer, above the lower epidermis; cells are irregularly shaped, loosely arranged, with large intercellular spaces that facilitate gaseous exchange. Fewer chloroplasts than palisade cells.
(iii) Vascular bundles: Found in the veins of the leaf. Each vascular bundle is surrounded by a layer of compactly arranged bundle sheath cells (parenchymatous in dicots). Xylem is present on the upper side (adaxial) and phloem on the lower side (abaxial) of each vascular bundle.
Important Extra Questions
What is the difference between open and closed vascular bundles?
Answer
In open vascular bundles, a strip of cambium is present between the xylem and phloem. Cambium is a meristematic tissue that can produce secondary xylem and phloem, allowing secondary growth. Open vascular bundles are found in dicot stems. In closed vascular bundles, cambium is absent between xylem and phloem, so secondary growth is not possible. These are found in monocot stems.
What are Casparian strips? What is their significance?
Answer
Casparian strips are bands of suberin (a waterproof fatty substance) deposited on the radial and transverse walls of endodermal cells in roots. They prevent apoplastic (intercellular) movement of water and solutes across the endodermis, forcing everything to pass through the living protoplast (symplastic pathway) of endodermal cells. This allows the plant to regulate selective absorption of minerals into the vascular tissue.
What is secondary growth? In which plants does it occur?
Answer
Secondary growth is the increase in girth of the stem or root due to the activity of lateral meristems (vascular cambium and cork cambium). Vascular cambium produces secondary xylem (wood) on the inner side and secondary phloem on the outer side. Cork cambium (phellogen) produces cork cells outwardly. Secondary growth occurs in dicotyledonous plants and gymnosperms. Monocots generally do not undergo secondary growth.
Describe the structure and functions of xylem.
Answer
Xylem is the water-conducting vascular tissue. Its components are: (i) Tracheids – elongated, dead cells with lignified cell walls and pits for water movement laterally. (ii) Vessels (tracheae) – longitudinal chains of cells with perforated end walls; more efficient water conductors than tracheids. (iii) Xylem fibres – thick-walled dead cells for mechanical support. (iv) Xylem parenchyma – living cells for storage of food and lateral conduction. Functions: upward transport of water and dissolved minerals from roots to leaves; provides mechanical support to the plant body.
What is a bundle sheath? How does it differ between monocots and dicots?
Answer
A bundle sheath is a layer of cells surrounding each vascular bundle in a leaf. In monocots (and C4 plants like maize), the bundle sheath cells are large, have numerous chloroplasts, and form a compact ring around the vascular bundle – this forms the structural basis of Kranz anatomy important in C4 photosynthesis. In dicots (C3 plants), the bundle sheath is parenchymatous with fewer chloroplasts and is less prominent in function.