Anatomy of Flowering Plants
Welcome to HSLC Guru! This English-medium study guide on Class 11 Biology Chapter 6 — Anatomy of Flowering Plants follows the ASSEB (Assam State School Education Board) syllabus. Anatomy is the study of internal structure of organisms. The internal organisation of plants reveals a great variety of cells and tissues working together. In this chapter we will study the different types of plant tissues, the tissue systems, the internal anatomy of dicot and monocot root, stem and leaf, and the process of secondary growth that takes place in older parts of woody plants. Mastering this chapter is essential for board examinations and lays a strong foundation for further study in plant biology.
Summary
Plant Tissues: A tissue is a group of cells having a common origin and usually performing a common function. Plant tissues are broadly classified into two types — meristematic tissues and permanent tissues. Meristematic tissues are made up of actively dividing cells which are responsible for plant growth. Based on their location, meristems are of three kinds — apical meristem (found at the tips of roots and shoots, responsible for primary growth and increase in length), intercalary meristem (found at the base of leaves or internodes, especially in grasses, helping in regeneration after grazing) and lateral meristem (found along the sides of stem and root, e.g., vascular cambium and cork cambium, responsible for secondary growth and increase in girth). Permanent tissues arise from meristematic tissues by losing the capacity to divide. They are of two types — simple and complex.
Simple and Complex Permanent Tissues: Simple tissues consist of only one type of cell. Parenchyma is composed of thin-walled, isodiametric living cells with intercellular spaces; it performs photosynthesis, storage and secretion. Collenchyma consists of cells thickened at the corners due to deposition of cellulose, hemicellulose and pectin; it provides mechanical support and is found below the epidermis of dicot stems and in petioles. Sclerenchyma consists of long, narrow, dead cells with thick lignified walls; it is of two types — fibres and sclereids — and provides mechanical strength. Complex tissues consist of more than one type of cell. Xylem conducts water and minerals from roots to leaves and is composed of tracheids, vessels, xylem fibres and xylem parenchyma. Phloem conducts food materials from leaves to other parts of the plant and is composed of sieve tube elements, companion cells, phloem fibres and phloem parenchyma.
Tissue Systems and Anatomy of Dicot vs Monocot: On the basis of their structure and location, the tissues are organised into three systems — the epidermal tissue system (forms the outermost covering, includes epidermis, stomata, trichomes and root hairs), the ground tissue system (consists of all tissues except epidermal and vascular tissue, mainly parenchyma with collenchyma and sclerenchyma) and the vascular tissue system (consists of xylem and phloem arranged in vascular bundles). In the dicot root, the vascular bundles are radial with 2–4 xylem and phloem patches; pith is small or absent. In the monocot root, vascular bundles are radial with many (more than six) xylem and phloem patches and a large pith. In the dicot stem, vascular bundles are conjoint, collateral, open and arranged in a ring. In the monocot stem, vascular bundles are conjoint, collateral, closed and scattered in the ground tissue. The dorsiventral (dicot) leaf shows differentiated mesophyll into palisade and spongy parenchyma, while the isobilateral (monocot) leaf has undifferentiated mesophyll and bulliform cells in the upper epidermis.
Secondary Growth: Secondary growth is the increase in girth of stem and root by the activity of lateral meristems — vascular cambium and cork cambium. The vascular cambium forms a complete ring by joining of fascicular cambium, interfascicular cambium and medullary ray cells. It cuts off cells towards the inside, forming secondary xylem, and towards the outside, forming secondary phloem. In temperate regions, the activity of cambium is seasonal, producing distinct annual rings (springwood and autumnwood) which can be used to determine the age of a tree. In an old tree trunk, the central darker, harder, non-conducting wood is called heartwood while the outer lighter, conducting wood is the sapwood. The cork cambium (phellogen) arises in the cortex and produces phellem (cork) on the outside and phelloderm on the inside; together phellogen, phellem and phelloderm constitute the periderm. Lenticels develop in the periderm for gaseous exchange.
1-Mark Questions
Q1. What is a tissue?
Answer: A tissue is a group of cells having a common origin and usually performing a common function.
Q2. Name the meristem responsible for the increase in length of plants.
Answer: Apical meristem is responsible for the increase in length of plants.
Q3. Where is intercalary meristem found?
Answer: Intercalary meristem is found at the base of leaves or above the nodes (internodes), especially in grasses.
Q4. Name the tissue that provides mechanical support to young, growing parts of plants.
Answer: Collenchyma provides mechanical support to young, growing parts of plants.
Q5. Which tissue is responsible for the conduction of water and minerals in plants?
Answer: Xylem is responsible for the conduction of water and minerals in plants.
Q6. What is the function of phloem?
Answer: Phloem translocates organic food materials (mainly sucrose) from leaves to other parts of the plant.
Q7. What are bulliform cells?
Answer: Bulliform cells are large, empty, colourless cells found in the upper epidermis of monocot leaves; they help in rolling and unrolling of leaves to reduce water loss.
Q8. What is heartwood?
Answer: Heartwood is the central, darker, harder, non-conducting region of secondary xylem in old tree trunks; it provides mechanical support.
Q9. Name the lateral meristem that produces cork.
Answer: Cork cambium (phellogen) is the lateral meristem that produces cork.
Q10. What are lenticels?
Answer: Lenticels are lens-shaped openings in the periderm of woody stems that allow gaseous exchange between the internal tissues and the atmosphere.
2–3 Marks Questions
Q1. Differentiate between meristematic and permanent tissues.
Answer: Meristematic tissues consist of actively dividing cells that are small, isodiametric, with thin walls, dense cytoplasm and prominent nuclei; intercellular spaces are absent and they are responsible for growth. Permanent tissues consist of cells that have lost the power of division; they are of various shapes and sizes, may be living or dead, and perform specific functions such as conduction, storage and support.
Q2. Distinguish between parenchyma, collenchyma and sclerenchyma.
Answer: Parenchyma consists of living, thin-walled, isodiametric cells with intercellular spaces; it carries out photosynthesis, storage and secretion. Collenchyma consists of living cells with corner thickenings of cellulose and pectin; it provides mechanical support and may carry out photosynthesis when chloroplasts are present. Sclerenchyma consists of dead cells with thick lignified walls and no intercellular spaces; it provides mechanical strength and is of two types — fibres and sclereids.
Q3. Mention the components of xylem and their functions.
Answer: Xylem is composed of four elements — tracheids are elongated dead cells with lignified walls that conduct water; vessels are long tube-like dead structures formed by end-to-end fusion of cells, the most efficient water-conducting elements in angiosperms; xylem fibres are dead sclerenchymatous cells that provide mechanical support; xylem parenchyma is the only living component of xylem and stores food materials and helps in radial conduction of water.
Q4. Describe the components of phloem.
Answer: Phloem is composed of four elements — sieve tube elements are long, tube-like living cells joined end-to-end through perforated sieve plates and conduct food; companion cells are specialised parenchymatous cells closely associated with sieve tubes and help to maintain pressure gradient; phloem fibres (bast fibres) are sclerenchymatous cells that provide mechanical support; phloem parenchyma stores food materials and is generally absent in monocots.
Q5. What are the three types of tissue systems? Mention briefly.
Answer: The three tissue systems are: (i) Epidermal tissue system — outermost covering of plant body, includes epidermis, stomata, trichomes and root hairs; protects against water loss and mechanical injury. (ii) Ground tissue system — consists of all tissues except epidermis and vascular tissues, mainly parenchyma with collenchyma and sclerenchyma; performs photosynthesis, storage and support. (iii) Vascular tissue system — consists of xylem and phloem arranged in bundles; performs conduction.
Q6. Differentiate between heartwood and sapwood.
Answer: Heartwood is the central, dark-coloured, dead and non-conducting part of secondary xylem; it is filled with tannins, resins, oils and gums, becoming hard and durable, and provides mechanical support. Sapwood is the peripheral, light-coloured, living and functional part of secondary xylem that conducts water and minerals from roots to the leaves. Heartwood is commercially valuable timber while sapwood is less durable.
5–7 Marks Questions
Q1. Describe the internal structure (anatomy) of a typical dicot root.
Answer: A transverse section of a typical dicot root (e.g., gram) shows the following regions from outside to inside: (i) Epiblema — the outermost single-layered tissue without cuticle and stomata, bearing unicellular root hairs that absorb water and minerals. (ii) Cortex — multilayered, made up of thin-walled parenchyma cells with intercellular spaces; stores food. (iii) Endodermis — innermost layer of cortex consisting of barrel-shaped cells with Casparian strips of suberin and lignin on the radial and tangential walls. (iv) Pericycle — single layer of thick-walled parenchymatous cells just inside the endodermis; gives rise to lateral roots and part of vascular cambium. (v) Vascular bundles — radial type, with 2–4 (usually four) xylem and phloem patches arranged alternately. The xylem is exarch (protoxylem towards the outside, metaxylem towards the centre). (vi) Pith — small or absent. (vii) Conjunctive tissue — parenchymatous tissue between xylem and phloem patches.
Q2. Distinguish between dicot stem and monocot stem with the help of suitable diagrams (description).
Answer: The internal structure of dicot stem and monocot stem differs in many features. In a dicot stem (e.g., sunflower), the epidermis is single-layered with cuticle and multicellular trichomes; the hypodermis is collenchymatous; the cortex is differentiated into outer collenchyma, middle parenchyma and inner endodermis (starch sheath); the pericycle is sclerenchymatous; vascular bundles are conjoint, collateral, open (with cambium) and arranged in a ring; pith is well-developed and parenchymatous with medullary rays. In a monocot stem (e.g., maize), the epidermis is single-layered with thick cuticle but without trichomes; the hypodermis is sclerenchymatous; the ground tissue is undifferentiated parenchyma; vascular bundles are conjoint, collateral, closed (without cambium), scattered throughout the ground tissue and surrounded by sclerenchymatous bundle sheaths; pith is not differentiated. Each vascular bundle in monocots has a typical Y-shaped xylem with a water-containing lysigenous cavity (protoxylem lacuna).
Q3. Compare the anatomy of a dorsiventral (dicot) leaf and an isobilateral (monocot) leaf.
Answer: A dorsiventral leaf (e.g., mango) is held horizontally; only the lower surface receives less light. The upper epidermis has a thick cuticle and few or no stomata; the lower epidermis has many stomata. The mesophyll is differentiated into upper palisade parenchyma (elongated cells with abundant chloroplasts, performing maximum photosynthesis) and lower spongy parenchyma (irregular cells with intercellular spaces). Vascular bundles are surrounded by bundle sheaths. An isobilateral leaf (e.g., maize) is held vertically and both surfaces receive almost equal light. Both upper and lower epidermis have equal cuticle and an equal number of stomata. The mesophyll is undifferentiated, consisting of similar parenchyma cells throughout. The upper epidermis has large, empty bulliform cells that help in rolling and unrolling of leaves under water-stressed conditions. The vascular bundles are of similar size and arranged in parallel.
Q4. Describe the process of secondary growth in a dicot stem.
Answer: Secondary growth refers to the increase in thickness or girth of stem and root by the activity of lateral meristems — vascular cambium and cork cambium. (i) Formation of cambium ring: In dicot stem, the cambium present in vascular bundles is called fascicular cambium (intra-fascicular). The medullary ray cells lying between two vascular bundles become meristematic and form interfascicular cambium. The fascicular and interfascicular cambia together form a complete ring called the cambium ring. (ii) Activity of cambium: The cambium ring divides actively. Cells cut off on the inner side differentiate into secondary xylem, while cells cut off on the outer side differentiate into secondary phloem. Since more secondary xylem is produced than secondary phloem, the wood (xylem) accumulates as the central bulk. (iii) Formation of annual rings: In temperate climates, the cambium produces two types of secondary xylem — large, thin-walled, lighter spring wood in spring and narrow, thick-walled, darker autumn wood in autumn. Together they form an annual ring. The number of annual rings is used to estimate the age of a tree (dendrochronology). (iv) Cork cambium and periderm: Outside the vascular cambium, cells of cortex become meristematic and form the cork cambium or phellogen. It produces phellem (cork) outside and phelloderm (secondary cortex) inside. Together phellogen, phellem and phelloderm constitute the periderm. Cork cells are dead with suberised walls; they are impermeable to water and gases, but lenticels in the periderm allow exchange of gases.
Q5. Explain the structure and functions of the three types of tissue systems with examples.
Answer: Sachs (1875) classified plant tissues into three systems based on their structure and location. (i) Epidermal tissue system: It forms the outermost covering of the whole plant body. It consists of epidermal cells, stomata and epidermal appendages (trichomes and root hairs). Epidermal cells are usually single-layered, parenchymatous and covered with a cuticle on the aerial parts. Stomata are minute openings bordered by guard cells that regulate transpiration and gaseous exchange. Trichomes are multicellular hairs that prevent water loss; root hairs are unicellular extensions that absorb water and minerals. (ii) Ground tissue system: It includes all tissues except the epidermis and vascular tissue. The major bulk is parenchyma, while collenchyma and sclerenchyma may also occur. In leaves, the ground tissue consists of mesophyll which performs photosynthesis. In stems and roots, it forms cortex, pericycle, pith and medullary rays — taking part in storage, support and conduction of materials. (iii) Vascular tissue system: It consists of complex tissues — xylem and phloem — together making vascular bundles. Xylem conducts water and minerals upward; phloem translocates food. Vascular bundles may be radial (root), conjoint collateral open (dicot stem) or conjoint collateral closed (monocot stem).
Multiple Choice Questions (MCQs)
Q1. The meristem responsible for primary growth of a plant is —
(a) Lateral meristem (b) Apical meristem (c) Vascular cambium (d) Cork cambium
Answer: (b) Apical meristem.
Q2. Which of the following is a dead simple permanent tissue?
(a) Parenchyma (b) Collenchyma (c) Sclerenchyma (d) Chlorenchyma
Answer: (c) Sclerenchyma.
Q3. Casparian strips are present in —
(a) Epidermis (b) Cortex (c) Endodermis (d) Pericycle
Answer: (c) Endodermis.
Q4. The vascular bundles of a monocot stem are —
(a) Conjoint, collateral, open (b) Conjoint, collateral, closed (c) Radial (d) Bicollateral
Answer: (b) Conjoint, collateral, closed.
Q5. The water-conducting elements present in the xylem of angiosperms but absent in gymnosperms are —
(a) Tracheids (b) Vessels (c) Fibres (d) Parenchyma
Answer: (b) Vessels.
Q6. Bulliform cells are characteristic feature of —
(a) Dicot leaf (b) Monocot leaf (c) Dicot stem (d) Monocot root
Answer: (b) Monocot leaf.
Q7. Heartwood differs from sapwood because —
(a) It is functional in conduction (b) It contains living cells (c) It is dark coloured and non-conducting (d) It is soft and light
Answer: (c) It is dark coloured and non-conducting.
Q8. The tissue that produces cork is —
(a) Phellogen (b) Phellem (c) Phelloderm (d) Pericycle
Answer: (a) Phellogen.
Q9. Number of xylem bundles in a dicot root is —
(a) 2 to 4 (b) 6 to 8 (c) more than 8 (d) numerous
Answer: (a) 2 to 4.
Q10. Annual rings are formed due to —
(a) Continuous activity of vascular cambium (b) Seasonal variation in cambial activity (c) Activity of cork cambium (d) Activity of apical meristem
Answer: (b) Seasonal variation in cambial activity.
Fill in the Blanks
Q1. The lateral meristem responsible for secondary growth in stems is the ___________.
Answer: vascular cambium
Q2. The mesophyll of a dicot leaf is differentiated into ___________ and spongy parenchyma.
Answer: palisade parenchyma
Q3. The radial walls of endodermal cells contain bands of suberin called ___________.
Answer: Casparian strips
Q4. The cells produced by phellogen on the outside form ___________ and on the inside form phelloderm.
Answer: phellem (cork)
Q5. ___________ are openings in the periderm that permit exchange of gases.
Answer: Lenticels
True or False
Q1. Sclerenchyma cells are living and have thin walls.
Answer: False. Sclerenchyma cells are dead and have thick lignified walls.
Q2. Vascular bundles in dicot stems are conjoint, collateral and open.
Answer: True.
Q3. Companion cells are absent in phloem.
Answer: False. Companion cells are present in the phloem of angiosperms and are closely associated with sieve tube elements.
Q4. Heartwood is functionally active in the conduction of water.
Answer: False. Heartwood is non-conducting; sapwood is the functional water-conducting region.
Q5. Bulliform cells are present in the upper epidermis of monocot leaves.
Answer: True.
Glossary
| Term | Meaning |
|---|---|
| Meristem | Tissue of actively dividing cells responsible for growth. |
| Apical Meristem | Meristem at root and shoot tips; brings about increase in length. |
| Intercalary Meristem | Meristem at the base of leaves or internodes; common in grasses. |
| Lateral Meristem | Meristem along the sides of stem and root; brings about increase in girth. |
| Parenchyma | Living, thin-walled, isodiametric simple tissue performing storage and photosynthesis. |
| Collenchyma | Living tissue with corner thickenings; provides flexible support. |
| Sclerenchyma | Dead tissue with lignified walls; provides mechanical strength. |
| Xylem | Complex tissue conducting water and minerals; consists of tracheids, vessels, fibres and parenchyma. |
| Phloem | Complex tissue translocating food; consists of sieve tubes, companion cells, fibres and parenchyma. |
| Tracheid | Elongated dead xylem element with lignified walls. |
| Vessel | Tube-like dead xylem element formed by fusion of cells. |
| Sieve Tube | Tube-like phloem cell with sieve plates conducting food. |
| Companion Cell | Specialised parenchyma cell associated with sieve tube. |
| Epidermal Tissue System | Outermost covering, includes epidermis, stomata and trichomes. |
| Ground Tissue System | All tissues except epidermal and vascular; mainly parenchyma. |
| Vascular Tissue System | Xylem and phloem arranged as vascular bundles. |
| Casparian Strip | Suberin band on radial walls of endodermal cells. |
| Bulliform Cells | Large empty cells in upper epidermis of monocot leaves; help in rolling. |
| Vascular Cambium | Lateral meristem producing secondary xylem and secondary phloem. |
| Cork Cambium (Phellogen) | Lateral meristem in cortex producing cork outside and phelloderm inside. |
| Periderm | Phellogen + phellem + phelloderm; forms outer protective layer. |
| Annual Ring | Combined ring of springwood and autumnwood produced in one year. |
| Heartwood | Central, dark, non-conducting secondary xylem. |
| Sapwood | Outer, light, functional water-conducting secondary xylem. |
| Lenticel | Opening in the periderm for gaseous exchange. |