Anatomy of Flowering Plants Class 11 Biology NCERT Solutions

Step-by-step solution

1. 1This is a drawing question, so the answer must tell the examiner which two outlines to draw and, more importantly, which structures to label on each, because the labels carry the marks. Draw each pair as a labelled transverse section, dicot on one side and monocot on the other, and label only the structures that differ.
2. 2(a) The root difference is built around the number of xylem bundles and the cambium, so these are the labels to make. On the dicot root label: the epidermis, the cortex which is the wide parenchymatous region, the endodermis, which is the innermost layer of the cortex and has the Casparian strips, the pericycle, which lies just inside the endodermis and is a single layer of parenchymatous cells, the radial vascular bundles, the pith which is small or entirely absent so the xylem reaches the centre, and the cambium, which is a strip of actively dividing cells between the xylem and the phloem. On the monocot root label: the epidermis, the cortex, the endodermis with the Casparian strips, the pericycle, the radial bundles, and above all the large and well-defined pith at the centre, which is the single most useful character for telling the two apart at a glance, and the polyarch condition, meaning the xylem bundles are more than six.
3. 3On the dicot root, also note that the xylem is usually two to four bundles arranged radially in a star, so it appears star-shaped in the transverse section, and it is exarch, that is the protoxylem is towards the periphery and the metaxylem towards the centre. On the monocot root the xylem bundles are more than six, or polyarch, so there are eight or more radiating from the centre, and the condition is also exarch. The cambium is present in the dicot root and absent in the monocot root, and this is the second decisive character.
4. 4(b) The stem difference is built around the arrangement of the vascular bundles and the cambium. On the dicot stem label: the epidermis, the hypodermis of collenchyma, which is the general cortex, the endodermis, which is the starch sheath, the pericycle, which may be sclerenchymatous, the vascular bundles, which are conjoint, open and arranged in a ring near the centre, the cambium as a ring between the xylem and the phloem, the pith, which is small or may be absent, and the medullary rays radiating outwards from the pith. On the monocot stem label: the epidermis, often with a siliceous cuticle, the hypodermis of sclerenchyma, which is the ground tissue, the vascular bundles, which are conjoint, closed, scattered and more numerous on the outside than inside, the bundle sheath of sclerenchyma surrounding each bundle, the large, well-defined and clear pith at the centre, and the absence of any cambium.
5. 5The three contrasts to mark on the drawing are therefore: the arrangement of the bundles, in a ring in the dicot and scattered in the monocot; the cambium, present in the dicot and absent in the monocot, so the bundle is open in one and closed in the other; and the pith, small or absent in the dicot and large and well defined in the monocot. Add the ground tissue difference, since the dicot cortex is collenchymatous and the monocot hypodermis is sclerenchymatous, and note the sclerenchymatous bundle sheath in the monocot, which Q3 turns on directly.

Step-by-step solution

1. 1This is a practical question and the answer should read as a procedure, with the observations and the inferences paired, so that each reason is attached to the character it justifies. NCERT's own specimen is the young stem of a sunflower, a dicot, and the answer can be written for that and then generalised.
2. 2The first step is the arrangement of the vascular bundles, and this is the character to look at first. If the vascular bundles are arranged in a ring near the centre of the stem, with the bundles of one ring at the same distance from the centre, the stem is that of a dicot. If the vascular bundles are scattered all through the ground tissue, with the bundles at the periphery smaller and those towards the centre larger, the stem is that of a monocot. So a ring means dicot, and scattering means monocot.
3. 3The second step is the cambium. If a strip of actively dividing, thin-walled cells is present between the xylem and the phloem of each bundle, so that the bundle is conjoint and open, the stem is a dicot. If no such cambium is present, so that the bundle is conjoint and closed, the stem is a monocot. This is the second decisive character, and it should be confirmed by looking for a cambium ring at the level of the pith as well.
4. 4The third step is the pith. If the pith at the centre is small, or is not clearly distinguishable from the ground tissue, or is absent altogether, the stem is a dicot. If the pith is large, well defined and clearly distinct from the rest of the ground tissue, and the ground tissue is pushed to the periphery, the stem is a monocot.
5. 5The fourth step is the ground tissue. If the cortex below the epidermis, the hypodermis, is collenchymatous, the stem is a dicot, and if it is sclerenchymatous and forms a hard band, the stem is a monocot.
6. 6The fifth and last step is the bundle sheath and the general condition of the epidermis. If each vascular bundle is surrounded by a sclerenchymatous bundle sheath, the stem is a monocot, and if the epidermis bears a siliceous cuticle, the stem is a monocot as well. The technique to mention is that the section should be taken from a young stem, stained with safranin and fast green, and observed under low power first to see the arrangement, since the arrangement is visible at low magnification whereas the cambium needs high power.
7. 7So the conclusion to write is: a ring arrangement of vascular bundles, the presence of a cambium, a small or absent pith and a collenchymatous hypodermis together identify a dicot stem, and scattered bundles, the absence of a cambium, a large well-defined pith and a sclerenchymatous hypodermis with bundle sheaths identify a monocot stem.

Step-by-step solution

1. 1Read the two features and identify each, then combine. Feature (a) gives three pieces of information at once, and each narrows the answer.
2. 2Conjoint means xylem and phloem lie on the same radius in the bundle, side by side, rather than on different radii as in a root, and conjoint bundles are a feature of the stem. So the material is a stem, not a root, and conjoint in any case excludes a root and excludes a leaf, whose xylem and phloem are separated by a bundle sheath.
3. 3Scattered means the bundles are not arranged in a ring, and this is the character of a monocot stem, since a dicot stem has its bundles in a ring. So scattered conjoint bundles identify a monocot stem.
4. 4The sclerenchymatous bundle sheath around each bundle confirms the monocot condition, since a sclerenchymatous bundle sheath is characteristic of the monocot stem, whereas in a dicot the region corresponding to it is the parenchymatous pericycle. So feature (a) alone identifies a monocot stem.
5. 5Feature (b) is a refinement. In a monocot stem the phloem parenchyma is absent, so the phloem consists only of sieve tube elements and companion cells, and the companion cells are associated with the sieve tubes. In a dicot stem the phloem parenchyma is present. So the absence of phloem parenchyma is a further confirmation of the monocot identification, and it is worth stating that the sieve tubes and the companion cells are the only components of the phloem in this material.
6. 6So the identification is a monocot stem, and every feature given is a character of it. The answer should also note the corroborating features to expect if the section were examined further, namely a large well-defined pith, the absence of a cambium, and a sclerenchymatous hypodermis.

Step-by-step solution

1. 1Define the term first, then build the diagram from the outside in, since the two guard cells with the pore between them is the whole idea, and the subsidiary cells and the epidermis are the surroundings. A T.S. of a leaf showing one stoma in surface view is the required figure.
2. 2Definition: a stoma is a minute pore or an opening present in the epidermis of leaves and of green stems, and the whole structure, that is the pore together with the two guard cells and the subsidiary cells, is called the stomatal apparatus or the stomatal complex. NCERT gives the definition as the pore along with the two guard cells.
3. 3First draw the surface view: draw a patch of the leaf epidermis showing the general epidermal cells, which are irregular in outline, and then within it one stoma. The stoma itself consists of the two kidney-shaped or dumb-bell-shaped guard cells lying side by side with their inner walls facing each other, and the narrow slit or pore between them. The guard cells are the only epidermal cells that contain chloroplasts, since all the other cells of the epidermis lack them, and it is the chloroplasts and the unique shape that make the guard cells distinct.
4. 4Now the subsidiary cells. The guard cells are surrounded by a few specialised epidermal cells called subsidiary cells, and these are not part of the stomatal apparatus proper, since the NCERT definition takes the apparatus as the pore and the two guard cells, but they should be drawn and labelled because they are what the question asks to be shown. The subsidiary cells surround the guard cells and in many plants they form the wall around the pore.
5. 5Now the T.S. or sectional view, which is the other half of the diagram and the part that shows the mechanism. In a transverse section the stoma is seen as a small chamber or pit in the epidermis, and the two guard cells project inwards, the pore opening to the outside and the cavity below it leading into the substomatal chamber, which is the intercellular space just below the stoma. The inner wall of each guard cell is thick and notched, and the wall next to the pore is the thick, and the outer wall and the wall away from the pore are thin, and this differential thickness is the whole basis of the movement.
6. 6Add the labels of the raised guard cells, the stomatal pore or slit, the substomatal chamber, the surrounding epidermal cells and the accessory or subsidiary cells, and mark the chloroplasts in the guard cells. Then give the mechanism in one sentence, which is what makes the diagram intelligible: because the inner walls are thick and the outer walls thin, when the guard cells lose water they become flaccid and the pore closes, and when they take up water they become turgid and the inner walls are pulled apart, so the pore opens. This is why the guard cells must have chloroplasts, since photosynthesis in them helps in maintaining the turgidity, and it is also why the guard cells are the only epidermal cells with chloroplasts.

Step-by-step solution

1. 1The three systems are the epidermal or dermal system, the ground tissue system and the vascular or conductive system, and the answer must name the tissues under each, so give the tissue and its component cells for each in turn.
2. 2The epidermal or dermal tissue system is the outermost covering of the plant body. Its main tissue is the epidermis, which is a single layer of cells, and it is made of the epidermal cells. In the older parts the epidermis is replaced by the periderm, which is made of cork, phellogen and phelloderm, and in the shoot tip the epidermis has the cuticle and the stomata. The tissue named under this system is the epidermis, or the periderm in the older stem.
3. 3The ground tissue system is the middle region, and it fills everything between the epidermis and the vascular bundles, so it is the largest of the three systems. Its three components are the cortex, which is the outer part, made of parenchyma; the pericycle, which lies between the endodermis and the vascular bundles, and consists of parenchymatous or sclerenchymatous cells; and the pith, which is the central part, made of parenchyma. So the three tissues are cortex, pericycle and pith, and the cells are mainly parenchymatous, though the pericycle of a dicot stem may be sclerenchymatous.
4. 4The vascular or conductive tissue system is the innermost system, and it is the transport system of the plant. It consists of two complex tissues, the xylem and the phloem, and the xylem is made of tracheids, vessels, xylem parenchyma and xylem fibres, and its principal elements are the tracheids and the vessels; the phloem is made of sieve tube elements, companion cells, phloem parenchyma and phloem fibres, and its principal elements are the sieve tube elements and the companion cells. Each of the xylem and the phloem may further be described as primary or secondary, but that subdivision is not required here.
5. 5The three systems and their tissues can then be summarised as: epidermal, with the epidermis; ground, with cortex, pericycle and pith; vascular, with xylem and phloem, and the memory hook is that the ground tissue is the middle and largest, the epidermal is the outer covering and the vascular is the inner transport network.

Step-by-step solution

1. 1Give the uses in the order in which NCERT lists them, since that order is also the most natural: the study of roots, the study of stems, the study of leaves, and then the study of flowers and seeds, and finally the general point about taxonomic and phylogenetic evidence. Each is a short paragraph with a reason in it.
2. 2Roots: the study of the root tells us how a plant absorbs water and minerals and how it is anchored, and it is practically useful in agriculture because knowing where a root grows and how deeply decides the depth of sowing and the spacing of crops. The root also shows the first appearance of the vascular tissue and of the lateral roots, so the study of the root is basic to the study of the whole plant.
3. 3Stems: the anatomy of the stem explains how water and food are conducted, and it is of direct use in making the wooden products of the plant, since the structure of wood, whether it is a dicot or a gymnosperm wood, decides its strength, its grain and its use in construction, furniture and paper. The presence of secondary growth explains the increase in girth, and the presence and position of the cambium explains the formation of wood and of rings, which is the basis of dendrochronology.
4. 4Leaves: the anatomy of the leaf explains photosynthesis and transpiration, and the study of the stomata and of the structure of the mesophyll shows how the leaf is adapted to its function of food manufacture and of water regulation, and the arrangement of the tissues in the leaf is the standard example used to illustrate the three tissue systems.
5. 5Flowers and seeds: the study of the floral anatomy is used in taxonomy and in plant breeding, because the characters of the calyx, the corolla, the androecium and the gynoecium are constant for a group and so serve as the basis of classification, and the study of the structure of the ovule and of the embryo sac and of the seed is the basis of all the work on hybridisation and on crop improvement.
6. 6The final and most important use is scientific: anatomical characters are among the most constant and least variable in a species, so they are used as the evidence on which taxonomy and phylogeny are based, and they also allow a plant that has lost its flowers or its leaves in the field to be identified from its internal structure, which is what makes anatomy essential in plant taxonomy.

Step-by-step solution

1. 1A dorsiventral, or dicot, leaf is the one with a dorsoventral internal structure, so the upper surface is different from the lower, and the whole answer is built around the contrast between the two surfaces. NCERT uses a mango leaf for this, and the diagram should be a vertical section of the leaf showing all the layers from the top surface to the bottom, plus a small surface view of the stomata to complete it.
2. 2Begin with the upper or adaxial epidermis. It is a single layer of compactly arranged, barrel-shaped, thin-walled and heavily cutinised cells, covered on the outer surface by a thick cuticle. The cuticle is present here because the upper surface is directly exposed, and the compact arrangement with the cuticle is what reduces loss of water from the surface that is most exposed to the sun. There are no stomata on the upper surface of a dorsiventral leaf, and in the surface view the epidermal cells appear polygonal, while the guard cells appear kidney-shaped, which is how the stomatal complex is recognised.
3. 3Next the palisade tissue, or palisade parenchyma. It lies below the upper epidermis and consists of several layers, three or four, of columnar, tightly packed, vertically elongated, chlorophyll-rich parenchyma cells, and the cells are packed so closely with very little intercellular space. The chloroplasts are numerous and the position is directly under the upper epidermis, so this is the chief food-manufacturing tissue of the leaf, and it is also called the chlorenchyma. The number of layers is greater on the sun side, and the cells of this tissue are the main site of photosynthesis.
4. 4Next the spongy parenchyma, or spongy mesophyll. It lies below the palisade and consists of several layers of rounded or oval, loosely arranged, thin-walled cells with large intercellular spaces, and the cells contain fewer chloroplasts than the palisade cells. The intercellular spaces are continuous with the stomatal pore, so this is the spongy tissue that allows the diffusion of gases, and it is the main site of respiration in the leaf, since the air spaces are the route for the exchange of CO2 and O2.
5. 5Then the vascular bundles. These are the veins of the leaf, and in a vertical section they appear in two sizes, the larger being the midrib and the smaller the lateral veins, and each bundle is a conjoint, open, collateral bundle, and it is surrounded by a bundle sheath, which in a dicot leaf is parenchymatous but which is sclerenchymatous in the midrib. Within the bundle the xylem lies towards the upper surface and the phloem towards the lower surface, so the leaf is dorsiventral in its vascular arrangement as well. The midrib also contains sclerenchymatous and parenchymatous tissue above and below the vascular bundle, forming the collenchymatous and sclerenchymatous patches.
6. 6Then the lower or abaxial epidermis. It is a single layer of loosely arranged, flat, thin-walled cells covered by a thin cuticle, less cutinised than the upper surface, and it bears most of the stomata, so the stomata are more numerous on the lower surface, and they open into the substomatal chambers. Because the lower surface is shaded and less exposed, it needs less cuticle and can afford to lose water through stomata, and this is the arrangement that reduces water loss in the dorsiventral leaf.
7. 7The contrast to close with is the whole point of the chapter: the upper surface is cutinised, has no stomata, has palisade tissue just beneath it and is the photosynthetic surface, and the lower surface has stomata and air spaces, thinner cuticle and the spongy tissue, and is the surface for transpiration, so the two surfaces are structurally different and the leaf is therefore called dorsiventral. The isobilateral condition of a monocot leaf, with the same structure on both surfaces and the bulliform cells on the upper epidermis, is the contrast to add if the examiner asks.