NCERT Solutions for Class 11 Biology Chapter 6: Anatomy of Flowering Plants

Concept. The internal structure is studied in transverse sections (T.S.) of the mature zones of the organs. Monocot and dicot roots have the same basic layers — epidermis (epiblema), cortex, endodermis, pericycle, vascular bundles and pith — but differ in the number of xylem bundles, the size of the pith and secondary growth; monocot and dicot stems differ in the arrangement and type of vascular bundles and in the nature of the hypodermis and ground tissue.

(a) T.S. of dicot root (primary) and monocot root — Fig. 6.3

(b) T.S. of dicot stem and monocot stem — Fig. 6.4

Extra depth. All tissues inside the endodermis — pericycle, vascular bundles and pith — make up the stele; in the dicot root initiation of lateral roots and of the vascular cambium (in secondary growth) takes place in the pericycle.

Conclusion: a dicot root has two to four xylem bundles, a small pith and secondary growth whereas a monocot root has more than six xylem bundles, a large pith and no secondary growth; a dicot stem has bundles in a ring which are conjoint and open with a differentiated cortex, while a monocot stem has scattered conjoint closed bundles with a sclerenchymatous sheath in an undifferentiated ground tissue.

Concept — procedure. (1) Take a young, fresh stem and cut very thin transverse sections with a sharp blade (blade moistened with water). (2) Select the thinnest, complete section and place it in a watch glass with water. (3) Stain it with safranin (lignified tissue turns red/pink) and, if available, a counter-stain, wash with water. (4) Mount in a drop of glycerine on a slide, cover with a coverslip. (5) Observe first under low power to see the general arrangement and then under high power for details of the tissues.

How to decide. Look at the arrangement and type of the vascular bundles and the nature of the ground tissue. The stem is a dicot stem if the following are seen, and a monocot stem if the contrasting features are seen:

Reasoning. Since monocots do not form secondary tissues, their bundles lack cambium and are closed and scattered; dicots, which show secondary growth in the stem, have cambium in open bundles arranged in a ring with medullary rays between. Therefore the position of the vascular bundles (ring vs scattered) and the presence or absence of cambium are the most decisive criteria. (See Fig. 6.4 in Question 1.)

Conclusion: a stem showing vascular bundles in a ring, conjoint and open with cambium, a collenchymatous hypodermis, endodermis, pericycle and medullary rays is a dicot stem, whereas scattered, conjoint, closed bundles with sclerenchymatous sheaths in an undifferentiated ground tissue indicate a monocot stem.

Answer: it is a monocotyledonous stem.

Reasoning (point by point):

1. Conjoint bundles. Xylem and phloem lie together along the same radius — the common arrangement in stems and leaves (roots have radial bundles). So the material is a stem (or leaf), not a root.
2. Scattered bundles. In a dicot stem the bundles lie in a ring; scattered vascular bundles in the ground tissue are the characteristic feature of the monocot stem.
3. Sclerenchymatous bundle sheath around each bundle. NCERT: the monocot stem has “a large number of scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath”; a dicot stem has no such distinct sheath around each bundle.
4. Phloem parenchyma absent. The monocot stem lacks phloem parenchyma (and has water-containing cavities in the bundles); dicot stems have phloem parenchyma.
5. Closed bundles (inference). Monocot bundles are conjoint and closed (no cambium), so there is no secondary growth.

Extra depth. Such a structure is seen, for example, in the stems of maize, sugarcane and other grasses and cereals.

Conclusion: conjoint, scattered vascular bundles each surrounded by a sclerenchymatous sheath, with no phloem parenchyma, identify the material as a monocotyledonous stem.

Concept. Stomata are minute pores present in the epidermis of leaves (and young stems) that regulate transpiration and gaseous exchange between the plant and the atmosphere. Around each pore specialised cells are present.

Stomatal apparatus. The stomatal aperture (pore), the guard cells and the surrounding subsidiary cells together constitute the stomatal apparatus.

Structure of a stoma.

1. Guard cells. Each stoma is composed of two bean-shaped (kidney-shaped) guard cells which enclose the stomatal pore. In grasses the guard cells are dumb-bell shaped.
2. Unequal wall thickness. The outer walls of the guard cells (away from the stomatal pore) are thin, while the inner walls (towards the pore) are highly thickened. This makes the guard cells bend outwards when turgid, opening the pore, and straighten when flaccid, closing it.
3. Chloroplasts. The guard cells possess chloroplasts (unlike other epidermal cells) and regulate the opening and closing of the stomata.
4. Subsidiary cells. A few epidermal cells in the vicinity of the guard cells become specialised in shape and size and are called subsidiary cells.
5. Distribution. Stomata are present in the epidermis of leaves; in a dorsiventral leaf there are generally more stomata on the lower (abaxial) epidermis, while in an isobilateral leaf they occur on both surfaces.

Extra depth. In the monocot (grass) leaf the guard cells are dumb-bell shaped; the two bulbous ends swell when turgid and pull the narrow middle parts apart. Stomata regulate transpiration, so they are the main route of water loss, whereas the waxy cuticle covering the epidermis prevents loss of water from the rest of the surface.

Conclusion: the stomatal apparatus consists of the stomatal pore, the two bean-shaped (dumb-bell shaped in grasses) chloroplast-containing guard cells with thin outer and thick inner walls, and the surrounding subsidiary cells; it regulates transpiration and gaseous exchange.

Concept. On the basis of their structure and location the tissues of the plant body are organised into three tissue systems: the epidermal tissue system, the ground (fundamental) tissue system and the vascular (conducting) tissue system.

Details of each system.

• Epidermal system: the epidermis is usually single-layered, made of elongated compactly arranged cells with a small amount of cytoplasm and a large vacuole; the cuticle (waxy) on the outside prevents water loss (absent in roots). Stomata regulate transpiration and gaseous exchange.
• Ground system: parenchyma cells are present in the cortex, pericycle, pith and medullary rays of primary stems and roots; collenchyma (e.g., in the dicot hypodermis) gives mechanical strength; sclerenchyma (e.g., pericycle patches, monocot hypodermis and bundle sheaths) gives rigidity; in leaves the chloroplast-containing thin-walled cells are the mesophyll.
• Vascular system: classified by the presence of cambium (open in dicot stems, closed in monocots) and by the arrangement of xylem and phloem (radial in roots; conjoint in stems and leaves).

Conclusion: the three tissue systems are the epidermal system (epidermal cells, stomata, trichomes and root hairs), the ground tissue system (parenchyma, collenchyma, sclerenchyma forming cortex, pericycle, pith, medullary rays and mesophyll) and the vascular system (xylem and phloem).

Concept. Anatomy is the study of the internal structure of plants — cells, tissues and their arrangement in organs. Plants have cells as the basic unit, cells form tissues and tissues form organs, and internal structure shows adaptations to diverse environments. The knowledge is useful in many ways:

1. Understanding structure–function relationships. Anatomy explains how the functions of the plant are carried out by its tissues: assimilation and storage of food (parenchyma, mesophyll), transport of water, minerals and photosynthates (xylem and phloem), mechanical support (collenchyma and sclerenchyma), protection and regulation of gas exchange (epidermis, cuticle, stomata). It is therefore the basis of plant physiology.
2. Identification and classification of plants. Anatomical features differentiate groups, for example monocots from dicots (ring vs scattered bundles, open vs closed bundles, number of xylem bundles in roots, dorsiventral vs isobilateral leaves). Anatomical characters also support taxonomy (the study of ultra-structure, anatomy and embryology is part of natural classification).
3. Understanding adaptations to the environment. Structures such as the cuticle, sunken or fewer stomata, bulliform cells (leaves curl in water stress), sclerenchyma and the thickness of the leaf show how plants adapt to habitats (dry, aquatic, etc.).
4. Understanding growth. Knowledge of the vascular cambium and secondary tissues explains the formation of wood and bark in dicots, and why monocots do not increase in girth.
5. Applied uses. (standard facts) Identification of timber and woods; detection of adulteration of crude drugs and spices; selection of crop varieties (e.g., lodging resistance depends on stem sclerenchyma); understanding the spread of pathogens through tissues (Katherine Esau showed that the curly top virus spreads through the phloem); and as the foundation for fields like plant breeding, horticulture and forestry.
6. Basis for later study. It provides the descriptive foundation that was later used in “reductionist” biology to frame research questions in physiology and evolutionary biology (Unit 2 introduction).

Conclusion: the study of plant anatomy helps us understand how tissues perform the plant’s functions, to distinguish monocots from dicots and other groups, to know the adaptations of plants to their environment, and to apply this knowledge in identifying timber and drugs, in agriculture and in understanding the spread of diseases.

Concept. A dorsiventral leaf is a leaf in which the upper (adaxial / dorsal) and lower (abaxial / ventral) surfaces differ in structure; it is characteristic of dicotyledons. A vertical section (V.S./T.S.) through the lamina shows three main parts — epidermis, mesophyll and vascular system (Fig. 6.5a).

1. Epidermis. It covers both the upper (adaxial) and lower (abaxial) surfaces and is covered with a conspicuous cuticle that reduces water loss. The abaxial epidermis generally bears more stomata than the adaxial epidermis; the latter may even lack stomata. Stomata (each with two guard cells) regulate gas exchange and transpiration.
2. Mesophyll. The tissue between the upper and lower epidermis. It possesses chloroplasts and carries out photosynthesis; it is made of parenchyma and has two types of cells:
   • Palisade parenchyma — placed adaxially (below the upper epidermis); made of elongated cells arranged vertically and parallel to each other; rich in chloroplasts, so most photosynthesis occurs here.
   • Spongy parenchyma — situated below the palisade cells and extending to the lower epidermis; oval or round and loosely arranged with numerous large spaces and air cavities between the cells that facilitate gaseous exchange.
3. Vascular system. Includes the vascular bundles seen in the veins and the midrib. The size of the bundles depends on the size of the veins, which vary in thickness in the reticulate venation of dicot leaves. The bundles are surrounded by a layer of thick-walled bundle sheath cells. In each bundle the xylem is towards the upper (adaxial) side and the phloem towards the lower (abaxial) side (conjoint, open).

For comparison — isobilateral (monocot) leaf (Fig. 6.5b). Similar, but stomata are present on both surfaces and the mesophyll is not differentiated into palisade and spongy parenchyma. In grasses, large, empty, colourless bulliform cells along the veins on the adaxial epidermis make the leaf curl inwards when flaccid (water stress) and expose the surface when turgid. Parallel venation is reflected in the nearly equal sizes of the vascular bundles (except the main veins).

Conclusion: a dorsiventral leaf shows an upper and a lower epidermis with cuticle (more stomata below), a chlorophyllous mesophyll differentiated into adaxial palisade and abaxial spongy parenchyma with air spaces, and vascular bundles in veins and midrib, enclosed by thick-walled bundle sheath cells, with xylem on the upper side and phloem on the lower side.