Unit 8: Plant Physiology — Short Questions

Ans. Macronutrients are needed in relatively larger amounts in plants for growth and development. There are nine macronutrients in plants i.e., carbon, hydrogen, oxygen, nitrogen, potassium, calcium, phosphorus, magnesium and sulphur.

Micronutrients are needed in very smaller amounts in plants for growth and development. There are seven micronutrients i.e., iron, manganese, zinc, molybdenum, copper, chlorine and boron.

Ans. Water molecules possess kinetic energy which means that in liquid or gaseous form they move about rapidly and randomly from one place to another. So, greater the concentration of the water molecules in a system the greater is the total kinetic energy of water molecules. This is called water potential psi = Ψw.

Factors In plant cells, two factors determine water potential i.e., Solute potential (Ψs) and Pressure potential (ΨP). Pure water has maximum water potential which by definition is zero. Water moves from a region of higher Ψw to lower Ψw.

Ans. 1. The Apoplast Pathway
It is a continuous pathway that involves a system of adjacent cell walls in the plant roots. The apoplast pathway becomes discontinuous in the endodermis in the roots due to the presence of Casparian segments.

2. The Symplast Pathway
In symplast pathway, water and minerals move through interconnected protoplasts of root cells. The protoplasts of neighbouring cells are interconnected through plasmodesmata, which are cytoplasmic strands that extend through pores in adjacent cell walls. The symplast pathway is less important, except for minerals in the region of endodermis.

3. The Vacuolar Pathway
In vacuolar pathway, water and minerals move through cell membranes, cytoplasm and tonoplast (membranes of vacuoles) and vacuoles. They move from vacuole to vacuole and bypass the symplast and apoplast pathways. Movement in vacuolar pathway is negligible.

Ans. Hypotonic Solution A solution having reduced solute concentration relative to the intracellular environment of a cell. As a result, water enters the cell by osmosis resulting in the swelling.

Hypertonic Solution A solution having high solute concentration relative to the intracellular environment of a cell. As a result, water moves out of the cell which causes the cell to shrink due to loss of water, a condition called plasmolysis.

Ans. Halophytes are the plants that inhabit in saline soil with high concentrations of salts like NaCl, MgCl2, MgSO4, or saline water. On such a substratum, only such plants can grow which can tolerate a relatively high concentration of these salts.

Succulent leaves These plants have succulent leaves and sometimes the stem is also succulent. In certain cases, leaves are modified into spines.

Examples of halophytes are sea arrow grass and sea lavender.

Ans. Short-day plants The plants in which flowering induced by dark periods longer than a critical length.

Long-day plants The plants in which flowering induced by dark periods shorter than a critical length.

Ans. 1. Cell Division The cells of apical meristems undergo divisions and the number of cells is increased.

2. Cell Elongation: After the formation of new cells, their volume increases due to uptake of water.

3. Cell Differentiation: After the cells have got their final size and shape, elongation stops and cells are specialized to perform specific functions.

Ans. Vernalin Vernalin is a hormone which induces vernalization which is a process in biennial and perennial plants to stimulate flowering by exposure to low temperature (4 days to 3 months).

Florigen Florigen is a hormone produced in leaves in response to different concentration of P730; it travels through phloem to floral buds initiating flowering.

Ans. Thigmotropism It is the movement in response to stimulus of touch. When they come in contact with some solid object, the growth on the opposite side of contact increases and the tendril coils around the support. For example climbing vines.

Geotropism It is the movement of plant parts in response to gravity. For example, roots display positive geotropism and shoots show negative geotropism.

Ans. Apical Meristems Apical meristems are found at the tips of roots and shoots. They are primarily responsible for the extension of the plant body. These are perpetual growth zones found and are responsible for the increase in the number of cells at the tips of roots and stems. They play an important role in primary growth.

Intercalary Meristems The type of meristem situated at the base of internodes. They are actually the part of apical meristem, which are separated by permanent tissues. Play an important role in the production of leaves and flower, and are of temporary nature.

Ans. Nutrient A nutrient is a substance that provides the body with essential ingredients required for metabolism. Specific nutrients, such as carbohydrates, lipids, and proteins, serve as sources of energy.

Nutrition Nutrition refers to the collective processes involved in the intake and utilization of nutrients for growth, repair, and maintenance of activities in an organism. It is of two types i.e. autotrophic and heterotrophic nutrition.

Other nutrients, including water, electrolytes, minerals, and vitamins, are necessary for the metabolic process.

Ans. Role of Nitrogen
Nitrogen is necessary for plant growth as it plays an essential role in energy metabolism and the production of proteins.

Deficiency of nitrogen A deficiency of nitrogen results in leaf loss and stunted growth.

Role of Magnesium
Magnesium constitutes the core of the chlorophyll molecule and is therefore essential for photosynthesis. It promotes the absorption and transportation of phosphorus and also contributes to the storage of sugars within the plant.

Magnesium deficiencies result in weak stalks, loss of greenness in the oldest leaves, and the appearance of yellow and brown spots.

Ans. Role of Phosphorus
Phosphorus is a part of ATP. It also plays a role in promoting root growth and favours flowering in the aerial zone.

Deficiency of phosphorus A deficiency of phosphorus leads to delayed flowering, as well as the browning and wrinkling of the leaves.

Role of Potassium
Potassium is involved in water regulation and the transportation of the plant's reserve substances. It enhances the ability of plants to carry out photosynthesis, reinforces cellular tissue, and stimulates the uptake of nitrates.

Ans. Effect of Calcium Deficiency
Calcium provides stability to the cell wall and promotes the development of the cell wall. It also plays a role in cellular proliferation and maturation, and aiding in the development of seeds. Insufficient calcium leads to the development of yellow and brown patches on the leaves.

Effect off Sulfur Deficiency
Sulfur is a fundamental element in the metabolism of nitrogen. If there is a shortage of sulfur, the plant becomes lighter in colour.

Ans. Importance of Iron
Iron is essential for the synthesis of chlorophyll. It acts as a cofactor for several enzymes which are involved in energy transfer and nitrogen metabolism.

Deficiency Its deficiency results in interveinal chlorosis.

Importance of Manganese
Manganese is involved in the process of photosynthesis, nitrogen metabolism, carbohydrate metabolism and activation of enzymes.

Deficiency Its deficiency results in the premature falling of the leaf and delayed maturity.

Ans. Zinc
Zinc facilitates chlorophyll synthesis, root development and uptake of nutrients. Deficiency of zinc can lead to stunted growth.

Molybdenum
Molybdenum is critical for nitrogen fixation, nitrogen reduction, sulphur metabolism, phosphorus metabolism and iron utilization.

Deficiency Its deficiency can result in chlorosis of older leaves and stunted growth.

Ans. Zinc
Zinc facilitates chlorophyll synthesis, root development and uptake of nutrients. Deficiency of zinc can lead to stunted growth.

Chlorine
Chlorine is involved in stomatal regulation, osmotic adjustment and transport of nutrients. Deficiency: Its deficiency can affect the health and growth of plants.

Ans. Insectivorous plants supplement organic molecules into their food in addition to inorganic nutrients, acquired through the process of capturing and breaking down insects and tiny animals. All insectivorous plants are true autotrophs. However, their development accelerates when they capture prey. Apparently, nitrogenous compounds of animal body are of benefit to these plants. The captured insects are broken down by enzymes that are released by the leaves.

Pitcher plant, venus fly trap and sundew are some of the known insectivorous plants.

Ans. Pitcher plant has leaves modified into a sac or a pitcher, partly filled with water.

• The leaf's terminal portion is altered to create a hood, which partially covers the exposed opening of the pitcher.
• It has numerous stiff hairs that prevent little insects from crawling out once they fall inside it.

Ans. Venus-fly trap has a "trap" consisting of two lobes that are hinged at the end of each leaf. The inner surfaces of the lobes contain trichomes, which are hair-like projections that trigger the lobes to close rapidly upon contact with prey.

• The hinged traps are lined with fine bristles that interlock upon closure, preventing the prey from escaping.
• The trapped insect is then digested by the enzymes secreted from the glands on the leaf surface and the products are then absorbed.

Ans. Sundew catches its prey with shiny drops of "dew," where the plant's common name comes from. The leaves are covered with tiny hairs that look like tentacles.

• Each leaf has gland and has a single dew like drop at the tip.
• The insects, attracted by plant's odour are trapped by tentacles.
• The trapped insects are digested by enzymes and products are absorbed.

Ans. Stomata are tiny opening or pores present within the plant tissues which are necessary for gaseous exchange.

• These are typically found in leaves but can even be present in some stems.
• The stomata are surrounded by specialized cells or the guard cells that facilitate the opening and closing of the stomatal pores.
• Guard cells are bean-shaped and contain chloroplasts.
• Guard cells can open and close depending on environmental conditions. The opening and closing of stomata control the transpiration rate in plants.

Ans. - During daylight stomata open to allow CO2 to enter the plant for photosynthesis. The opening of stomata is primarily regulated by guard cells.

• At night when photosynthesis ceases due to lack of light, stomata typically close to conserve water. However, plants still respire, taking in O2 and releasing CO2. The closure of stomata at night helps to minimize water loss by transpiration.

Ans. Guard cells functions as multisensory hydraulic valves as they open and close in response of certain environmental factors like light, CO2 and humidity.

• They change shape due to water potential and release excess water through transpiration.

Ans. Starch Sugar Hypothesis
In 1856, German botanist H. Van Mohl proposed that guard cells in leaf epidermis are solely responsible for photosynthesis, producing sugars during the day. During day time as sugar concentration increases in guard cells, the water potential drops.

• Water moves into guard cells causing them to become turgid and open the stomata.
• At night, photosynthesis ceases, and sugars are converted to insoluble starch or used for respiration, leading to a decline in free sugars. Consequently, water moves out of guard cells and they lose turgor pressure. So, they become flaccid and close the stomata.

Ans. The opening of stomata in plants is facilitated by the active transport of potassium ions (K+) into guard cells, which reduces their osmotic potential. This influx of K+ leads to water entering the guard cells through osmosis, causing them to become turgid and open the stomata.

• Blue light enhances this process by acidifying the surrounding environment, promoting K+ uptake and subsequent water absorption.
• At night, K+ passively diffuses out of the guard cells, resulting in water loss and causing the guard cells to become flaccid, thereby closing the stomata.

Ans. Hormones are involved in stomatal movement in plants during stress conditions. At high temperature when leaf cells start willing, a hormone called abscisic acid, is released by mesophyll cells. This hormone stops the active transport of K+ into guard cells, overriding the effect of light and CO2 concentration. So, K+ pumping stops and stomata close.

Ans. Palisade tissue is primarily located just beneath the upper epidermis of the leaf. It consists of elongated, tightly packed cells that are rich in chlorophyll. The arrangement of these cells is organized to maximize light absorption and allowing plants to efficiently convert light energy into chemical energy.

Ans. [Diagram showing leaf cross-section with labeled parts: Cuticle, Upper epidermis, Palisade, Mesophyll, Spongy, Lower epidermis, Air space, Stoma, Guard cell]

Ans. The membrane that surrounds the vacuole is called the tonoplast. It has many active transport mechanisms that move ions into the vacuole, even when the concentration within the vacuole is higher than that of the surrounding fluid.

• Due to the elevated ionic concentration, water is drawn into the vacuole, resulting in turgidity and providing mechanical support to the plant's soft tissues.

Ans. The parenchyma tissue provides support to herbaceous plants and parts of larger plants.

• The parenchyma cells of the epidermis, cortex, and pith absorb water. This water creates an internal hydrostatic pressure known as turgor pressure that maintains the rigidity of cells.

Turgor pressure arises from the elevated osmotic pressure within the cell vacuole.

Ans. Collenchyma cells are specialized cells that are grouped in the form of strands or cylinders.

• They are found beneath the epidermis of young stems, leaf stalks and along the veins in leaves.
• Collenchyma cells lack secondary walls.
• Their primary walls are thickened at the corners, due to extra deposition of cellulose.
• They elongate when stem or leaf grows lengthwise. They provide support to the young parts of plant in which secondary growth has not taken place.

Ans. Sclerenchyma
This tissue also provides structural support to the plants. Typically, the cells of sclerenchyma tissue possess thick secondary cell walls.

• These walls are saturated with lignin, an organic compound that confers strength and rigidity to the walls.
• The majority of sclerenchyma cells are non-living.
• The main function of this tissue is to provide support to the various components of the plant.

Ans. There are three types of sclerenchyma cells which are fibres, sclerids and vessels.

1. Fibers (Tracheids) are elongated and cylindrical in shape. They can be found either as compact bundles inside the xylem or as bundle caps.

2. Sclerids are smaller in size as compared to fibers and are present in the seed coats and shells of nuts. Their function is to offer protection.

3. Vessels (Tracheae) are long tubular structures that are joined end to end to form a long water conducting pipe in xylem.

Ans. Solute Potential The solute potential or osmotic potential is a measure of the change in water potential (Ψw) of a system due to the presence of solute molecules. Ψs is always a negative value, so if more solute molecules are present, lower (more negative) is the Ψs.

Pressure Potential It is the part of water potential which is due to the pressure exerted by water it is denoted by ΨP. If pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. When water enters plant cells by osmosis, pressure may be built up inside the cell making the cell turgid and increasing the pressure potential.

Ans. Roots of plants provide large surface area for absorption by their extensive branching systems. Roots have tiny root hairs, which are actually extensions of epidermal cells of roots. Most of the uptake of water and minerals in roots takes place through root hairs.

From soil, water and minerals enter the root epidermal cells by active and passive transport. From root epidermis, they move to cortex, and then into the xylem tissue in the centre of root. Inside roots, water and minerals move in three different pathways to reach the xylem.

Ans. [Diagram showing transport pathways: Plasmode smata, Tonoplast, Vacuole, Cell wall, Cytoplasm, apoplast pathway, Symplast pathway, Vacuolar pathway]

Ans. Following are the structural components of xylem tissue:
1. Tracheids
2. Vessels
3. Xylem fibres
4. Xylem parenchyma

Ans. Tracheids Tracheids are elongated thin cells that have thick walls made of lignin. The ends of the cells are tapered and they are linked to each other by bordered pits, which enable the lateral movement of water between cells.

Vessels Vessels are shorter and broader compared to tracheids. They are arranged in a linear fashion, forming continuous channels. Perforation plates are present at the outer edges of these structures, enabling efficient movement of water.

Ans. Xylem fibres Xylem fibres are elongated cells with thickened lignified walls. At maturity, they are dead and enhance the structural integrity of the xylem. They offer additional structural support to the plant.

Xylem Parenchyma Xylem parenchyma are living cells with thin walls that have the ability to retain and hold nutrients and water. Xylem parenchyma cells participate in the lateral translocation of water and nutrients and can also contribute to the healing and regeneration of xylem tissue.

Ans. [Diagram showing components Fiber, Tracheid, Vessel, Xylem Parenchyma, with labels for Cytoplasm, Nucleus, Pit]

Ans. Transpiration is the process by which water evaporates from the surface of plant leaves, specifically through stomata. As water vapour exits the leaf, a negative pressure is generated within the leaf tissue.

• This negative pressure creates a pulling force, drawing water upward from the roots through the stem and toward the leaves.
• Transpiration, therefore, act as the primary driving force behind water transport in the xylem.

Ans. Adhesion Adhesion is the attraction between water molecules and the walls of the xylem vessels. Due to this attraction, water molecules stick to the walls of xylem vessels as they move upward. This property prevents any break in the water column within xylem. Adhesion thus play a crucial role in maintaining the continuity of the water column, especially in tall plants where gravity exerts a significant 'downward force on the water column.

Cohesion Cohesion refers to the attractive force between water molecules themselves, caused by hydrogen bonding. Water molecules within the xylem stick together, forming an unbroken column from the roots to the leaves. This cohesive property of water ensures that the "pull" initiated by transpiration at the leaf level extends down through the entire water column.

Ans. Tension is the negative pressure created by the pulling force of the transpiration at the leaf level. As water evaporates from the leaf surface, it creates a low-pressure area that extends through the xylem. This tension pulls the cohesive water column upwards. Tension is therefore vital for the continuous ascent of water within the xylem.

Ans. The TACT (Transpiration, Adhesion, Cohesion, Tension) also known as cohesion-tension theory, is a widely accepted model explaining how water moves against gravity through the xylem to reach all parts of the plant. This mechanism depends on both physical and chemical properties of water and the plant's interaction with its environment.

Ans. Phloem comprises of four major components

• Sieve elements
• Companion cells
• Phloem fibres
• Phloem parenchyma.

Ans. Phloem Tissue

• Phloem is a vascular tissue in plants responsible for the transport of organic nutrients, particularly the products of photosynthesis, from the leaves to other parts of the plant where they are needed or stored.
• The phloem is generally found on the outer side of both primary and secondary vascular tissue in plants with secondary growth and constitutes the inner bark.

Ans. The cells of phloem that transport sugars and other organic material throughout the plant are called sieve tube elements or cells. Sieve tube elements have 'sieve areas', which are the portions of the cell wall where pores interconnect the sieve tube elements. Sieve tube elements join to form long sieve tube, which is used to transport food from source to sink.

Ans. Phloem parenchyma stores substances, such as sugars, resins, latex, and mucilage, which are important for plant defence and moisture retention.

Ans. Passive theories of phloem transport include: Diffusion and pressure flow theory. The pressure-flow theory, also known as the mass-flow hypothesis, is the most widely accepted explanation for the transport of sugars in plants through the phloem. This process of translocation moves sugar from the source (where they are synthesized) to the sink (where they are consumed or stored). This theory was proposed by Ernst Munch in 1930. This theory relies on the principle of osmotic pressure differences between source and sink regions.

Ans. Growth in plants refers to a permanent increase in size, which can occur in various dimensions such as height, width, and mass.

• Throughout life, the plant adds organs such as branches, leaves, and roots. Its organs increase in size from the tips but the rate of growth is not uniform throughout the body.

Ans. In higher plants, growth is limited to certain regions known as growing points. These growing points consist of groups of cells, called meristems, that are capable of continuous cell division.

• There are three types of meristems in plants i.e., apical meristem, intercalary meristem and lateral meristem.

Ans. Apical Meristem The type of meristem that are found at the tips of roots and shoots. They are primarily responsible for the extension of the plant body. These are perpetual growth zones found and are responsible for the increase in the number of cells at the tips of roots and stems. They play an important role in primary growth.

Intercalary Meristem The type of meristem situated at the base of internodes. They are actually the part of apical meristem, which are separated by permanent tissues. Play an important role in the production of leaves and flower, and are of temporary nature.

Ans. Lateral meristems are cylinders of dividing cells present along the peripheral regions. They are responsible for growth in thickness of stems and roots. They are found in woody plants and are crucial for secondary growth.

• There are two main forms of lateral meristems; vascular cambium and cork cambium.

Vascular cambium is located between the xylem and phloem and is responsible for production of secondary xylem and secondary phloem.

Cork cambium is formed in the outer layer of stems and roots. This tissue produces cork cells which replace the epidermis and forms the outer protective bark.

Ans. Primary growth is responsible for an increase in the length of the plants. It is facilitated by the activity of apical meristems. Herbaceous plants generally display primary growth with little secondary growth as compared to woody plants.

The process of primary growth in plants occurs in three phases.
1. Cell Division
2. Cell Elongation
3. Cell Differentiation

Ans. - The cells of apical meristems undergo divisions and the number of cells is increased.

• It happens at the tips of apical meristems of root and shoot. The area of apical meristem where cell division occurs, is called zone of cell division.
• In this zone, cells are non-vacuolated and small. These cells have spherical nuclei in the centre of cytoplasm.

Ans. - During elongation phase volume of cell increases due to uptake of water.

• Plasticity of cell wall increases and wall pressure is reduced. It happens at a little distance from the tips of apical meristems.
• The area where cell elongation occurs, is called zone of cell elongation. In this zone, cells are vacuolated and large. They have nuclei in the peripheries of cytoplasm.
• During this phase, different cells elongate in different dimensions and the final size of cells is attained.
• For example, the cells which are determined to develop into pith, cortex etc. do not elongate much length-wise while the cells which are determined to develop into xylem tissue elongate more length-wise.

Ans. Cell Differentiation After the cells have got their final size and shape, elongation stops and cells are specialized to perform specific functions, thus is termed as phase of cell differentiation.

• Their cell walls become thicker and many new structural features develop.
• It happens in the area next to the zone of elongation. This area is called zone of cell differentiation.
• In this zone, cells are fully differentiated and each type of cell performs specific function.

Ans. Secondary growth refers to the increase in thickness or girth of stems and roots. It is due to the activity of lateral meristems, specifically the vascular cambium and cork cambium. It is more prominent in woody perennial plants, while herbaceous plants show only primary growth. The cells of vascular cambium divide and produce new cells on both of its outer and inner margins.

Ans. - Secondary tissues (particularly secondary xylem) cause increase in plant's thickness.

• The cells of vascular cambium divide and produce new cells on both of its outer and inner margins of vascular cambium make secondary phloem while the cells produced on its inner margins make secondary xylem.

Ans. Bark The region of mature stem outside of the vascular cambium, which contains secondary phloem, cork cambium and cork, is collectively called bark.

A concentric layer of wood formed each year in woody plants is called annual ring. Annual rings are formed due to the seasonal activity of the cambium layer in trees. These rings provide valuable information about the age of the tree and the environmental conditions experienced during each growing season.

Ans. The cambium is a meristematic tissue that generates new vascular tissues i.e., xylem and phloem.

• In spring: In spring, when conditions are favourable, the cambium is highly active, producing a large volume of xylem with wider vessels, resulting in lighter-coloured spring wood.
• In autumn: As the season changes to autumn, the cambium's activity decreases. It produces fewer xylem elements, which are narrower and denser, leading to the wood forms a complete annual ring.

Ans. Dendrochronology is the scientific method of dating and studying tree rings to analyze past climate conditions and events.

Ans. Plants regulate the rates of growth and the rate of metabolism in their cells. Special chemical messengers, called plant growth regulators or plant hormones regulate their growth. There are five major groups of plant growth regulators i.e., auxins, cytokinins, gibberellins, abscisic acid, and ethylene.

Ans. Auxin (IAA) is one of the most important plant growth regulator it.
1. Promotes cell division in cambium.
2. In roots, promote growth at very low concentrations. Inhibit growth at higher concentrations, e.g., geotropism. Promote growth of roots from cuttings and calluses.
3. Promotes bud initiation in shoots but sometimes antagonistic to cytokinins and is inhibitory.

Ans. Gibberellins are produced in apical portions of roots and shoots and transported to other parts.

• Promote cell enlargement in the presence of auxins. Also promote cell division in apical meristem and cambium.
• Promote 'bolting' of some rosette plants.
• Promote bud initiation in shoots of chrysanthemum callus.
• Sometimes may substitute for red light. Therefore, promote flowering in long-day plants, while inhibit in short-day plants.

Ans. Cytokinins are usually produced in roots, young fruits and in seeds.

Cytokinins promote cytokinesis during cell division. They increase the rate of DNA replication and the rate of RNA and protein synthesis. They perform the following functions:

• Promote stem growth by cell division in apical meristem and cambium.
• Inhibit primary root growth.
• Promote lateral root growth.

Ans. Abscisic acid (ABA) is synthesized mainly in mature green leaves, fruits and root caps. It performs the following functions:

• Inhibits stem and root growth notably during physiological stress e.g., drought and waterlogging.
• Promotes bud and seed dormancy.
• Promotes flowering in short day plants and inhibits in long day plants (antagonistic to gibberellins).

Ans. It is a natural product of the metabolism of plants. Inhibits stem growth, notably during physiological stress.

• Inhibits root growth
• Breaks root growth
• Promotes flowering in pineapple
• Promotes fruit ripening.

Ans. Osmoregulation refers to the process by which an organism maintains a stable internal equilibrium of water and dissolved substances, irrespective of the surrounding environmental conditions.

• Osmoconformers: Many marine organisms undergo osmosis without the requirement for regulatory systems since their cells have the same osmotic pressure as that of the sea are called osmoconformers.

• Osmoregulators: Organisms must actively acquire, retain, or eliminate water or salts in order to regulate their internal water and mineral balance are called osmoregulators.

Ans. Hydrophytes These are the plants adapted to aquatic environments (marine and freshwater) by specialized mechanisms of osmotic adjustments.

Freshwater hydrophytes grow in hypotonic environments, where water continuously enters their cells.

• These plants remove excess water through structures like hydathodes or vacuoles to avoid overhydration.
• They actively absorb essential ions, such as potassium and calcium, to maintain osmotic balance and compensate for the dilute surroundings.
• With thin or absent cuticles, these plants facilitate water exchange and often have reduced root systems, relying on direct nutrient and water absorption from their environment.

Examples of hydrophytes are water lilies, lotus, seaweeds and tape grass.

Ans. Mesophytes live in moderate environments that are neither too dry nor too wet.

• They prefer soil that is not waterlogged and has a moderate salt content and humidity.
• Mesophytes have well-developed roots and shoots with a fully formed vascular system.

Ans. Xerophytes are plants that are adapted to survive in dry conditions.

• They have special adaptations to minimize water loss and store water.
• Plants that store water are known as succulents.
• They possess fleshy stems that can store water and used when needed.
• Other adaptations in xerophytes include waxy coatings on leaves to reduce water loss, leaf dropping during dry periods, and leaf folding or repositioning to absorb sunlight efficiently.
• Examples of xerophytes include thorn trees, desert marigold, and blue agave.

Ans. Marine Hydrophytes Marine hydrophytes thrive in saline (hypertonic) conditions, where water tends to leave their cells. They excrete excess salts using specialized salt glands and synthesize organic solutes like proline, glycine, betaine, and sugars to retain water by increasing their internal osmotic potential. They have thick cuticles which further reduce water loss, and they exhibit halophytic traits to tolerate high salinity.

Halophytes Halophytes inhabit saline soil with high concentrations of salts like NaCl, MgCl2, MgSO4, or saline water. On such a substratum, only those plants can grow which can tolerate a relatively high concentration of these salts. These plants have succulent leaves and sometimes the stem is also succulent. In certain cases, leaves are modified into spines. Examples of halophytes are sea arrowgrass and sea lavender.

Ans. Halophytes growing in marshy places near seashore from a special vegetation known as the mangrove or tidal woodland. These are also called helophilous halophytes.

Ans. - High temperature denatures the enzymes and damages the metabolism.

• These proteins embrace enzymes and other proteins thus help to prevent their denaturation.

However, formation of ice crystals within protoplasm perforates membranes and organelles hence killing the cells.

Adaptations The plants native to cold region such as oaks, maples, roses and other plants have adapted to bring changes in solutes composition of the cells, which causes cytosol to super cool without ice formation, although ice crystals may form in the cell walls.

Ans. Low temperature

• Low temperature alters the fluidity of the cell membrane, because lipids of the membrane become locked into crystalline structures, which affects the transport of the solutes. The structure of the membrane proteins is also affected.
• Plants respond to cold stress by increasing proportion of unsaturated fatty acids, which help membrane to maintain structure at low temperature by preventing crystal formation. This adaptation requires time. Because of this reason, rapid chilling of plants is more stressful than gradual drop in air temperature.

Ans. Freezing temperature
Freezing temperature causes ice crystal formation. The confinement of ice formation around cell wall does not affect as badly and plants survive.

Ans. Tropic movements The growth movements in plants that are triggered by an external stimulus, are collectively called tropic movements or tropisms.

• Such movements occur as a curvature of whole organ towards or away from stimuli such as light, touch, chemical, water and gravity.
• Following are the major types of tropic movements in plants. 1. Phototropism 2. Geotropism 3. Thigmotropism 4. Chemotropism 5. Hydrotropism

Ans. Phototropism It is the movement of parts of plant, in response to stimulus of light and is caused by the differential growth of part of a plant like stem or root. The tips of shoots usually show positive phototropism while roots show negative phototropism.

Geotropism It is the movement of plant parts in response to gravity. Roots display positive geotropism and shoots negative geotropism.

Ans. Thigmotropism It is the type of movement in plants in response to stimulus of touch, for example climbing vines. When they come in contact with some solid object, the growth on the opposite side of contact increases and the tendril coils around the support.

Chemotropism It is the type of movement in plants in response to some chemicals is called chemotropism. For example, the sugary environment on the stigma of a flower induces pollen tube to grow inside the pistil/carpel to reach the ovule.

Ans. The response to changes in day length that enables plants to adapt to seasonal changes in their environment is termed as photoperiodism. Simply, it is response of plants to the length of day and night e.g., in strawberries flowering is induced by dark periods longer than a critical length.

Ans. Effect of photoperiodism was first studied in 1920 by Garner and Allard. They studied that tobacco plant flowers only after exposure to a series of short days.

• Tobacco plant naturally flowers under same conditions, in autumn, but flowering could be induced by artificially exposing to short days.
• With further studies they were able to classify flowering plants into long-day plants, which require long days for flowering, short-day plants, which require short days for flowering and day-neutral plants which flower without being influenced by photoperiod.

Ans. Phytochrome, a photoreceptor protein exists in two forms i.e., P660 and P730.

P660 is a quiescent form. It absorbs red light at a wavelength of 660 nm and is converted to active P730 which absorbs far red light at 730 nm and is converted to P660.

• In nature, the P660 to P730 conversion takes place in day light and P730 to P660 conversion occurs in the dark.
• The rate at which P730 is converted to P660 provides the plant with a "clock" for measuring the duration of darkness.

Ans. It has been found that red light inhibits flowering in short-day plants but promotes flowering in long-day plants, under conditions during which flowering normally takes place.

• This observation led to the hypothesis that the P730 to P660 interconversion might be the plant time-regulator for flowering.
• According to this hypothesis, P730, converted from P660 by the absorption of red light, would inhibit flowering in short day plants but promote flowering in long day plants.
• Because P730 accumulates in the day and diminishes at night, short day plants could flower only if the night were long enough, during which a great amount of P730 would not be completely inactivated, so that P730 would accumulate in the day and diminishes at night, short day plants could flower only if the night were long enough, during which a great amount of P730 would not be completely inactivated.

Ans. The process of induction of flowering in plants by exposing them to a certain low (chilling) temperature called vernalisation.

• For some plants, vernalisation is an absolute requirement while in some cases it simply assists in inducing flowering.

Significance

• Photoperiodism and vernalisation serve to synchronise the reproductive behaviour of plants with their environment, ensuring reproduction at favourable times of year.
• They also ensure that members of the same species flower at the same time, encouraging cross pollination for genetic variability.

Ans. See Q70 and Q71 of short questions.

Ans. See Q76 and Q77 of short questions.

Ans. See Q52 and Q54 of short questions.

Ans. See Q22, Q23, Q24, Q25, Q26 of short questions.

Ans. See Q82, Q83, Q84, Q85 of short questions.

Ans. The guard cells function as multisensory hydraulic valves. The two hypotheses which may explain the opening and closing of stomata are starch sugar hypothesis and influx of k+ ion.

Starch Sugar Hypothesis
In 1856, German botanist H. Van Mohl proposed that guard cells in leaf epidermis are solely responsible for photosynthesis, producing sugars during the day. As sugar concentration increases in guard cells, the water potential drops. Water moves into guard cells causing them to become turgid and open the stomata and vice versa.

Influx of K+ ion
The opening of stomata in plants is facilitated by the active transport of potassium ions (K+) into guard cells, which reduces their osmotic potential. This influx of K+ leads to water entering the guard cells through osmosis, causing them to become turgid and open the stomata.

Ans. Insectivorous plants supplement organic molecules into their food in addition to inorganic nutrients. These organic chemicals are acquired through the process of capturing and breaking down insects and tiny animals. All insectivorous plants are true autotrophs. However, their development accelerates when they capture prey. Apparently, nitrogenous compounds of animal body are of benefit to these plants. The captured insects are broken down by enzymes that are released by the leaves. Pitcher plant, venus fly trap and sundew are some of the known insectivorous plants.

Pitcher plant Pitcher plant has leaves modified into a sac or a pitcher, partly filled with water.

Venus-fly trap Venus-fly trap has a "trap" consisting of two lobes that are hinged at the end of each leaf.

Sundew catches its prey with shiny drops of "dew", where the plant's common name comes from.

Ans. Support in Plants
Supporting tissues play an important role in maintaining the structural integrity, support and flexibility of plants. These tissues consist of parenchyma, collenchyma, sclerenchyma, xylem and phloem.

1. Parenchyma
The parenchyma cells of the epidermis, cortex, and pith absorb water. This water creates an internal hydrostatic pressure known as turgor pressure that maintains the rigidity of cells.

2. Sclerenchyma
The cells of sclerenchyma tissue possess thick secondary cell walls. These walls are saturated with lignin, an organic compound that confers strength and rigidity to the walls.

Ans. Annual rings are formed due to the seasonal activity of the cambium layer in trees. This process is influenced by environmental factors and results in the production of two distinct types of wood each year: spring wood (early wood) and autumn wood (late wood).

These rings provide valuable information about the age of the tree and the environmental conditions experienced during each growing season.

Ans. Pressure flow theory The pressure-flow theory, also known as the mass-flow hypothesis, is the most widely accepted explanation for the transport of sugars in plants through the phloem. This process of translocation moves sugar from the source (where they are synthesized) to the sink (where they are consumed or stored). This theory relies on the principle of osmotic pressure differences between source and sink regions.

There is a higher water potential at the source end while a lower water potential at the sink end. The difference in water potential causes water to flow from source to sink. Since sucrose is dissolved in water, it is carried along from source to sink along with water.

Ans. Flowering can be induced by growing plants in controlled environment.

1. By variation in critical length and dark periods

• For inducing flowering in short day plants, keep them in light for short duration and increasing their dark period artificially.
• For inducing flowering in long day plants, keeping them in dark for shorter duration can induce flowering.

2. By temperature treatment
Biennial and perennial plants are stimulated to flowering by exposure to low temperature (chilling) from four days to three months, this process is also called vernalization.