Plant Physiology Archives

51. In which one of the following physiological processes, excess water es

In which one of the following physiological processes, excess water escapes in the form of droplets from a plant?

[amp_mcq option1=”Transpiration” option2=”Guttation” option3=”Secretion” option4=”Excretion” correct=”option2″]

This question was previously asked in

UPSC CDS-2 – 2018

Guttation is the process where plants lose water in the form of liquid droplets from specialized pores called hydathodes, usually located at the tips or margins of leaves. This phenomenon typically occurs when transpiration rates are low (due to high humidity) but water uptake by the roots is high (due to high soil moisture and root pressure). The positive root pressure forces water up the xylem and out through the hydathodes.
Transpiration is the loss of water vapor from the plant surface, primarily through stomata.
Secretion is the process where plants release substances produced by their metabolic activity (e.g., nectar, resins, oils).
Excretion is the removal of metabolic waste products, which plants often store internally or deposit in non-living tissues; they don’t have specialized excretory organs like animals.
The specific loss of water in the form of droplets is called guttation.

– Guttation is the loss of water as liquid droplets.
– It occurs through hydathodes.
– It happens when transpiration is low and root pressure is high.
– Transpiration is the loss of water as vapor.

Guttation is different from dew, which is water that condenses on the leaf surface from atmospheric moisture. Guttation water originates from inside the plant. Guttation droplets may contain dissolved mineral salts and sugars, unlike pure condensed water.

52. Which of the following is not a primary function of a green leaf?

Which of the following is not a primary function of a green leaf?

[amp_mcq option1=”Manufacture of food” option2=”Interchange of gases” option3=”Evaporation of water” option4=”Conduction of food and water” correct=”option4″]

This question was previously asked in

UPSC CDS-2 – 2018

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The primary functions of a green leaf are:
A) Manufacture of food (Photosynthesis): Green leaves contain chlorophyll and are the main sites for converting light energy, CO₂, and water into glucose (food). This is a primary function.
B) Interchange of gases: Leaves have stomata, pores that regulate the exchange of gases (CO₂ intake and O₂ release) with the atmosphere. This is a primary function essential for photosynthesis and respiration.
C) Evaporation of water (Transpiration): Water is lost from the plant mainly through stomata on the leaves in the form of water vapor. This process helps in the ascent of sap and cooling. This is considered a primary function related to water balance and transport.
D) Conduction of food and water: While vascular tissues (xylem for water, phloem for food/sugars) are present in the leaf veins and transport substances within the leaf and to/from the rest of the plant, the *conduction* itself is a transport process performed by these tissues, not a primary metabolic or exchange *function* of the leaf tissue as a whole. The primary functions are the synthesis (photosynthesis) and exchange processes.
Therefore, conduction of food and water is not considered a primary function of the leaf organ itself, unlike photosynthesis, gas exchange, and transpiration.

– Primary functions of leaves include photosynthesis, gas exchange (CO2, O2), and transpiration.
– Conduction is a transport process occurring within the vascular tissues (xylem and phloem) found within the leaf veins.
– The question asks for a primary *function* of the leaf, distinguishing between synthesis/exchange processes and transport.

The vascular bundles (veins) provide structural support and facilitate the transport of water to the photosynthetic cells and the transport of sugars away from the leaf. While essential for the leaf’s operation, this transport is a supportive function rather than a primary function like energy conversion or gas exchange.

53. Which of the following is/are the main absorbing organ/organs of

Which of the following is/are the main absorbing organ/organs of plants?

[amp_mcq option1=”Root only” option2=”Leaf only” option3=”Root and leaf only” option4=”Root, leaf and bark” correct=”option3″]

This question was previously asked in

UPSC CDS-2 – 2018

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Plants absorb substances from their environment through specialized organs.
Roots are the primary organs for absorbing water and mineral nutrients from the soil. They are well-adapted for this function with root hairs increasing surface area.
Leaves are the primary organs for absorbing carbon dioxide (CO₂) from the atmosphere, which is essential for photosynthesis. They also play a role in absorbing water vapor or even liquid water directly from the surface (e.g., dew), though this is usually not the main source of water compared to root absorption.
Bark, particularly older bark, is generally not a primary absorbing organ for water or nutrients from the substrate or air, although some gas exchange might occur through lenticels.
Considering the main substances absorbed by plants for their metabolism (water, minerals, CO₂), both roots (from soil) and leaves (from air) function as primary absorbing organs. Thus, “Root and leaf only” best describes the main absorbing organs among the given options.

– Roots absorb water and minerals from the soil.
– Leaves absorb carbon dioxide from the atmosphere.
– Both are essential absorptive processes for plant survival.
– Bark is generally not a primary absorbing organ.

While roots absorb water and minerals, leaves are the main sites of CO₂ absorption, which is crucial for photosynthesis. Some specialized structures like epiphytic roots or insectivorous leaves might have unique absorption functions, but in general terms for typical vascular plants, roots and leaves are the main absorbing interfaces with the environment (soil and atmosphere respectively).

54. Xylem is a type of complex tissue in plants for upward conduction of w

Xylem is a type of complex tissue in plants for upward conduction of water. Which one of the following xylem tissues consists of living cells?

[amp_mcq option1=”Tracheid” option2=”Vessel” option3=”Xylem parenchyma” option4=”Xylem fibre” correct=”option3″]

This question was previously asked in

UPSC CDS-1 – 2023

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Xylem is composed of four main types of elements: tracheids, vessels, xylem parenchyma, and xylem fibres. Among these, only xylem parenchyma cells are living cells.

Tracheids and vessels are the main water-conducting elements and are dead, hollow tubes at maturity. Xylem fibres are typically dead and provide mechanical support. Xylem parenchyma cells are involved in storage of food materials (starch, fats) and lateral conduction of water and minerals.

Phloem, another complex plant tissue, is responsible for the translocation of sugars and also contains living cells, such as sieve elements and companion cells.

55. In grasses, intercalary meristem is usually located at

In grasses, intercalary meristem is usually located at

[amp_mcq option1=”root tip” option2=”lateral sides of stem” option3=”base of leaves” option4=”shoot tip” correct=”option3″]

This question was previously asked in

UPSC CDS-1 – 2023

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Intercalary meristems are located at the base of internodes or leaves in monocotyledonous plants like grasses. They are derived from apical meristems and are situated between mature tissues.

The presence of intercalary meristems allows grasses to regrow quickly after being grazed or cut, as they contribute to the elongation of stems and leaves from points other than the tips.

Apical meristems (at root and shoot tips) are responsible for primary growth (increase in length). Lateral meristems (like vascular cambium and cork cambium) are responsible for secondary growth (increase in thickness).

56. Plants contain a variety of sterols like stigmasterol, ergosterol, sit

Plants contain a variety of sterols like stigmasterol, ergosterol, sitosterol etc., which very closely resemble cholesterol. These plant sterols are referred as :

[amp_mcq option1=”Phytosterols” option2=”Calciferols” option3=”Ergocalciferols” option4=”Lumisterols” correct=”option1″]

This question was previously asked in

UPSC CDS-1 – 2016

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Plant sterols that resemble cholesterol, such as stigmasterol, ergosterol, and sitosterol, are referred to as Phytosterols.

Phytosterols are a group of naturally occurring sterols and stanols found in plants. They have structures similar to cholesterol but differ in the side chain. Common examples include beta-sitosterol, campesterol, and stigmasterol. They are primarily found in vegetable oils, nuts, seeds, and legumes.

Calciferols are forms of Vitamin D (like D2 and D3). Ergocalciferol is Vitamin D2, derived from ergosterol, a phytosterol found in fungi and yeast. Lumisterols are isomers formed during the photolysis of previtamin D3. While related to sterol chemistry, ‘Phytosterols’ is the specific term for the group of plant sterols resembling cholesterol.

57. Which one of the following tissues is responsible for increase of girt

Which one of the following tissues is responsible for increase of girth in the stem of a plant?

[amp_mcq option1=”Tracheid” option2=”Pericycle” option3=”Intercalary meristem” option4=”Lateral meristem” correct=”option4″]

This question was previously asked in

UPSC CDS-1 – 2019

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Lateral meristems are responsible for secondary growth, which is the increase in the diameter or girth of stems and roots. The two main types of lateral meristems are the vascular cambium (producing secondary xylem and phloem) and the cork cambium (producing the periderm, including cork).

Lateral meristems cause growth in girth (secondary growth) in plants.

Tracheids are components of xylem tissue involved in water transport and support. Pericycle is a layer of cells in the root from which lateral roots arise and sometimes contributes to secondary growth. Intercalary meristems are located at nodes in monocots and are responsible for increases in stem length between nodes (primary growth).