Plant hormones

Roots normally grow downward into the soil, along the direction in which gravity acts. This directional growth is called positive geotropism or positive gravitropism. Shoots generally show the opposite response and grow away from gravity, while light and air do not define the normal direction of root growth. Hence, option A is correct.

Auxin is a plant growth regulator that promotes cell elongation and helps coordinate several growth responses. Its unequal distribution can also cause bending, such as the curvature of a shoot during phototropism. Digestion, blood circulation, and respiration are not functions assigned to this plant hormone. Therefore, option A correctly identifies growth.

Abscisic acid, commonly abbreviated as ABA, is generally described at this level as a growth-inhibiting plant hormone. It promotes dormancy and helps plants respond to stress, such as water shortage, partly by causing stomatal closure. Auxin, gibberellin, and cytokinin are chiefly associated with growth promotion and development, although hormones can have context-dependent effects. Therefore, option A is correct.

Gibberellins are plant hormones that promote cell elongation and division, so they are especially associated with increased stem length and internode growth. They also influence seed germination and other developmental processes. Blood sugar, heartbeat, and nerve impulses are animal physiological functions, not the principal role of gibberellin in plants. Hence, option A is the best answer.

Cytokinin is a plant growth hormone whose major function is to stimulate cell division, especially in actively growing tissues. When cytokinin promotes the formation of new cells, the plant can develop shoots and other structures. Blood clotting, heartbeat, and digestion are animal processes and are not functions of this plant hormone. Therefore, option A is correct.

During wilting or water stress, a plant must reduce the loss of water through transpiration. Abscisic acid, commonly called ABA, signals the guard cells around stomata to close them. This conserves water in the plant. Insulin, adrenaline, and thyroxine are animal hormones and do not regulate stomatal closure. Hence, option A is correct.

Plants lack a nervous system, brain, and bones, so they cannot coordinate responses in the animal way. They respond mainly through chemical messengers such as plant hormones and by changing their growth. For example, auxin can cause a shoot to bend toward light. Therefore, option A is correct; the other choices describe structures or processes that plants do not use for nervous coordination.

The governing concept is chemical coordination in plants. Plants lack a nervous system, so specialised chemical messengers regulate growth, development, and directional responses such as bending toward light or gravity. These substances are called plant hormones or phytohormones, making option A correct. Digestive juice belongs to animal digestion, blood transports materials in animals, and a nerve is a nervous-system structure rather than a plant growth regulator.

Auxin is a plant hormone that chiefly promotes cell elongation and supports growth responses. Its unequal distribution in a stem is especially important in phototropism, because cells on the shaded side elongate more and the stem bends toward light. Therefore option A is correct. Leaf fall is commonly associated with ethylene or abscisic effects, seed dormancy with abscisic acid, and flower colour mainly with pigments.

The governing concept is a plant tropic response to water. Directional growth of a plant organ toward a moisture source is called positive hydrotropism. Roots commonly show this response, helping them grow toward regions where water is available, although gravity and other stimuli may also influence their path. Thus option A is correct. Phototropism responds to light, chemotropism to chemicals, and thigmotropism to touch or contact.

The governing concept is plant hormonal coordination. Auxin is a plant hormone that promotes cell elongation, especially in young shoots, and it also helps produce directional growth such as phototropism. Therefore, option A, Auxin, is correct. Insulin regulates blood glucose in animals, adrenaline supports emergency responses, and thyroxine regulates metabolism, so these distractors are not the major plant-growth hormone.

Gibberellin is a plant growth hormone. Its important school-level effect is promoting stem elongation, which increases the length and overall growth of shoots; it can also support processes such as seed germination. Therefore, option A is correct. Decreasing blood sugar is associated with insulin, increasing heartbeat is related to animal responses such as adrenaline, and sound detection involves sensory receptors and the nervous system.

Abscisic acid, commonly abbreviated as ABA, is generally classified as a plant growth-inhibiting hormone. It can promote seed dormancy, help close stomata during water stress, and oppose some growth-promoting effects. Thus option A is correct. Option B describes growth promoters such as auxin or gibberellin, option C refers to insulin, and option D refers to animal emergency hormones such as adrenaline.

Plants do not possess a brain, spinal cord, or a nervous system like animals. Their control and coordination mainly depend on chemical messengers called plant hormones, together with local growth responses and changes in cells. Therefore option A is correct. Nerves, the brain, and the spinal cord are parts of animal nervous coordination, so options B, C, and D are unsuitable for plants.

Plants lack a brain, nerves, and a blood circulatory system like animals, but they can still sense and respond to stimuli. Plant hormones and changes in cell growth or water balance produce directional growth, such as shoots bending toward light or roots growing with gravity. Some parts also show movement. Therefore option A is correct; B, C, and D describe animal mechanisms that plants do not use.

The governing concept is plant hormonal coordination. Auxin is a plant hormone that promotes cell elongation in suitable tissues and helps regulate growth responses such as phototropism, in which a shoot bends toward light. Hence option A is correct. Lowering blood sugar and increasing heartbeat are animal-regulation functions, while stopping respiration is not the normal role of auxin.

Geotropism, or gravitropism, describes a plant growth response to gravity. A positive response occurs in the direction of the stimulus. Therefore, a root showing positive geotropism grows downward, along the direction of gravitational pull. Growth against gravity is negative geotropism and is typical of the shoot. Light response and touch-related shrinking describe different plant responses, so they are not correct here.

The governing concept is a tendril’s touch-guided growth response. A tendril is a slender, specialised structure that bends and coils when it contacts a support, helping the plant climb and obtain better light. A main root anchors and absorbs water, a petal attracts pollinators, and a cotyledon stores or supplies nourishment to the young seedling. Thus option A is correct.

The governing concept is the specific function of plant hormones. Cytokinins stimulate cell division and support growth in actively dividing tissues, so option A gives a scientifically correct hormone-function pair. Abscisic acid generally inhibits growth and helps plants respond to stress, while gibberellin affects plant growth rather than blood sugar. Auxin regulates plant growth and tropic responses, not heartbeat; therefore B, C, and D are incorrect.

Growth of a plant part in the direction of light is called positive phototropism. Stems usually bend towards light, making this a common example. Geotropism is a growth response to gravity, not to light. Exam tip: associate growth movement caused by light with phototropism.