Chapter 7 — Internal Organisation of Plants
Key Concepts Quick Recap
| Term | Definition |
|---|
| Meristem | Actively dividing tissue; undifferentiated |
| Apical meristem | At root and shoot tips; primary growth (length) |
| Lateral meristem | Vascular cambium and cork cambium; secondary growth (girth) |
| Intercalary meristem | At internodes; growth of internodes (grass) |
| Parenchyma | Simple permanent tissue; thin-walled; storage/photosynthesis |
| Collenchyma | Simple permanent tissue; unevenly thickened; flexible support |
| Sclerenchyma | Simple permanent tissue; lignified walls; support |
| Xylem | Complex tissue; water conduction; tracheids + vessels + xylem fibres + xylem parenchyma |
| Phloem | Complex tissue; food conduction; sieve tubes + companion cells + phloem fibres + phloem parenchyma |
| Epidermis | Outermost protective layer |
| Endodermis | Innermost layer of cortex; Casparian strips control water entry into vascular bundle |
I. Choose the Best Answer (MCQs)
1. Which meristem is responsible for increase in girth of a plant? (a) Apical meristem (b) Intercalary meristem (c) Lateral meristem (d) Wound meristem
Answer(c) Lateral meristem Lateral meristems (vascular cambium and cork cambium) are responsible for secondary growth — increase in girth (width) of dicot stems and roots. Apical meristems increase length (primary growth); intercalary meristems are at the base of internodes in grasses.
2. Casparian strips are found in: (a) Epidermis (b) Cortex (c) Endodermis (d) Pericycle
Answer(c) Endodermis Casparian strips are suberin-impregnated bands in the radial and transverse walls of endodermal cells. They prevent apoplastic movement of water — all water must pass through the endodermal cell (symplastic pathway), allowing selective absorption.
3. The water-conducting tissue in plants is: (a) Phloem (b) Xylem (c) Parenchyma (d) Collenchyma
Answer(b) Xylem Xylem conducts water and dissolved minerals from roots to leaves (upward). It is composed of tracheids, vessels (most efficient), xylem fibres, and xylem parenchyma. Vessels are found in angiosperms; tracheids in both gymnosperms and angiosperms.
4. Sieve tubes in phloem are associated with: (a) Tracheids (b) Companion cells (c) Xylem fibres (d) Sclerenchyma
Answer(b) Companion cells Sieve tube elements (enucleate, perforated sieve plates) are metabolically supported by companion cells — nucleated cells connected to sieve tubes by plasmodesmata. Companion cells control sieve tube activity and loading/unloading of sugars.
5. In a dicot stem, the arrangement of vascular bundles is: (a) Scattered throughout (b) In a ring (c) In two rings (d) Only in the cortex
Answer(b) In a ring In dicot stems, vascular bundles are arranged in a ring (eustele). Endarch xylem (protoxylem towards centre). In monocot stems, vascular bundles are scattered throughout the ground tissue (atactostele). No cambium in monocot vascular bundles (closed bundles).
6. The layer that gives rise to lateral roots is: (a) Endodermis (b) Pericycle (c) Cortex (d) Epidermis
Answer(b) Pericycle Lateral (branch) roots originate endogenously from the pericycle — the outermost layer of the stele. They push through cortex and epidermis to emerge. This is unlike lateral shoots which originate exogenously (from axillary buds).
7. What is the heartwood of a tree? (a) Young functional xylem (b) Old, non-functional xylem at the centre, filled with resins/tannins (c) Phloem (d) Secondary cortex
Answer(b) Old, non-functional xylem Heartwood (duramen) = old, non-conducting secondary xylem at the tree's core; dark-coloured due to deposits of resins, tannins, oils. Sapwood (alburnum) = outer, functional, light-coloured secondary xylem that conducts water.
II. Short Answer Questions
Q1. Distinguish between parenchyma, collenchyma, and sclerenchyma.
Answer
| Feature | Parenchyma | Collenchyma | Sclerenchyma |
|---|
| Cell wall | Thin (cellulose) | Unevenly thickened at corners (pectin + cellulose) | Uniformly thick, lignified |
| Living/Dead | Living | Living | Dead at maturity |
| Function | Storage, photosynthesis (chlorenchyma), gas exchange (aerenchyma) | Flexible mechanical support | Rigid mechanical support |
| Location | Everywhere | Hypodermal in dicot stem; petiole | Fibres in xylem, phloem; stone cells (sclereids) in fruits |
| Example | Pith cells | Sunflower stem | Coconut husk fibres, nut shell |
Q2. Explain the internal structure of a dicot root.
Answer
From outside to inside:
1. Epiblema (Root epidermis): Outermost; single layer; no cuticle; root hairs absorb water. 2. Cortex: Wide zone of parenchyma cells; stores starch; large intercellular spaces. 3. Endodermis: Innermost cortex layer; barrel-shaped cells with Casparian strips (suberin); controls water passage. 4. Pericycle: One layer inside endodermis; gives rise to lateral roots and vascular cambium. 5. Vascular bundles: Radial arrangement (xylem and phloem alternate on different radii); 2–4 xylem strands (diarch to tetrarch). 6. Pith: Small or absent in dicot root; parenchyma.
Key diagnostic feature: Radial vascular bundles (xylem and phloem alternate) — distinguishes root from stem.
Q3. What is secondary growth? How does it occur in dicot stems?
Answer
Secondary growth is the increase in girth (width) of the plant body due to activity of lateral meristems (cambium). It occurs in dicots and gymnosperms; absent in most monocots.
Vascular cambium activity:
- Fascicular cambium (between xylem and phloem within bundle) + Interfascicular cambium (between bundles) → form a complete cambium ring.
- Cambium divides: inner cells → secondary xylem (wood); outer cells → secondary phloem.
- Secondary xylem accumulates more than phloem → increases girth.
Annual rings:
- Spring wood (early wood): wide vessels, light.
- Autumn wood (late wood): narrow vessels, dense.
- One spring wood + one autumn wood = 1 annual ring → used to determine tree age (dendrochronology).
Cork cambium (Phellogen):
- Arises in cortex.
- Produces cork (phellem) outward and phelloderm inward → bark.
- Old bark periodically shed.
Common Mistakes
- Confusing xylem (water conduction) and phloem (food conduction).
- Saying monocot stems have a ring of vascular bundles — they are scattered (atactostele).
- Forgetting Casparian strips are in endodermis (not epidermis).
- Thinking lateral roots originate from cortex — they originate from pericycle (endogenous origin).
Exam Checklist
- Meristem types: apical (length), lateral (girth), intercalary (internodes).
- Simple tissues: parenchyma (thin wall), collenchyma (corners thickened), sclerenchyma (lignified).
- Casparian strips: endodermis; suberin; control apoplastic water movement.
- Dicot root: radial vascular bundles (xylem-phloem alternate).
- Dicot stem: vascular bundles in a ring; open (with cambium).
- Secondary growth: vascular cambium → secondary xylem + secondary phloem; annual rings.
