Plant Physiology

Step 1: Understanding biogas production.
Biogas production occurs through anaerobic digestion of organic matter in a biogas plant. This process involves multiple groups of microorganisms acting sequentially.
Step 2: Role of methanogens.
Organic acids such as acetic acid are finally converted into methane-rich biogas by methanogenic archaea. The most important methanogens involved are Methanococcus and Methanobacillus.
Step 3: Analysis of options.
(A) Correct — these are methanogenic microorganisms responsible for methane formation.
(B) Incorrect — these are fermentative bacteria, not methanogens.
(C) Incorrect — nitrogen-fixing bacteria, unrelated to biogas formation.
(D) Incorrect — involved in decomposition, not methane production.
Step 4: Conclusion.
Acetic acid is converted into biogas by enzymes of Methanococcus and Methanobacillus.

Step 1: Identifying functions of structures.
Different plant structures are specialized for specific physiological processes such as transpiration, absorption, guttation, and translocation.
Step 2: Matching each structure correctly.
Stomata: Involved in transpiration, so (i)-b.
Hydathode: Responsible for guttation, so (ii)-c.
Root hair: Main site of absorption of water and minerals, so (iii)-a.
Tracheary elements: Involved in conduction and translocation of water, so (iv)-d.
Step 3: Conclusion.
The correct matching is (i)-b, (ii)-c, (iii)-a, (iv)-d, which corresponds to option (C).

Step 1: Understanding apomixis.
Apomixis is a form of asexual reproduction in plants where seeds are produced without fertilization. This results in progeny that are genetically identical to the parent, eliminating segregation of traits.
Step 2: Analyzing the options.
(A) apomictic: Correct — Apomictic seeds result in progeny without genetic segregation.
(B) dicotyledonous hybrid: Incorrect — Dicotyledonous hybrids undergo segregation of traits in the progeny.
(C) monocotyledonous hybrid: Incorrect — Like dicotyledonous hybrids, monocotyledonous hybrids also experience genetic segregation in the progeny.
(D) polyembryonic: Incorrect — Polyembryonic seeds result in multiple embryos, but they still undergo genetic segregation.
Step 3: Conclusion.
The correct answer is (A) apomictic, as apomictic seeds do not undergo genetic segregation.

Step 1: Understanding population density.
The density of a population is affected by factors such as natality (birth rate), mortality (death rate), immigration, and emigration. An increase in population density typically results from high natality and low mortality.
Step 2: Analyzing the options.
(A) high mortality: Incorrect — High mortality would decrease the population density, not increase it.
(B) high natality: Correct — High natality, or birth rate, leads to an increase in population density.
(C) low emigration: While low emigration can contribute to population growth, it is not as significant as high natality.
(D) high emigration: Incorrect — High emigration would reduce the population density.
Step 3: Conclusion.
The correct answer is (B) high natality, as the increase in population density is driven by high birth rates.

Step 1: Guttation is the process by which plants excrete excess water in liquid form through specialized pores called hydathodes, primarily found on the edges of leaves. This occurs when soil moisture is high and transpiration is low, typically at night. The exuded water contains dissolved substances like sugars, amino acids, and minerals.

Step 2: The process of transpiration, on the other hand, is the evaporation of water from plant surfaces, mainly through stomata. This is the main method of water loss in plants. Guttation is different in that it involves the release of liquid water, whereas transpiration involves water vapor.

Step 3: The other options are not correct:
- (B) Transpiration: Involves water vapor, not liquid form.
- (C) Evaporation: Refers to the change of water from liquid to vapor, which is not the case here.
- (D) Condensation: Refers to the process where water vapor turns into liquid, which is not relevant to this question.

Conclusion: Guttation is the correct process for the excretion of liquid water in plants.

The leaves are the site of photoperiod perception in plants. The process involves:
Photoreceptors such as phytochromes in leaves detect the duration of light and dark periods.
Upon perceiving the correct photoperiod, the leaves produce a flowering signal, commonly known as florigen.
Florigen is transported to the shoot apical meristem, where it induces flowering by activating flowering genes.
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Final Answer:

Step 1: Exudation of xylem sap at the cut ends of the stem, commonly observed in plants, occurs due to root pressure. Root pressure is generated when water is absorbed by the roots and pushed upwards through the xylem, causing the sap to be exuded through cut surfaces.

Step 2:
- (B) Leaf is involved in transpiration, not in the exudation of xylem sap at cut ends.
- (C) Stem is where xylem sap is transported, but it is the root pressure that causes sap exudation at the cut ends.
- (D) Gland does not play a role in sap exudation from the xylem.

Conclusion: Root pressure is responsible for exudation of xylem sap, making the correct answer.