Theory question 101
Plant Responses and Hormones(a) In a tabular form, distinguish between tropic movements and nastic movements.
(b) Explain how light brings about tropic movement in the stem and root of a flowering plant.
(c) What would be observed when a growing seedling is placed horizontally?
(d) State five functions of the pituitary gland. (e) State two differences between growth in plants and growth in animals.
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(a) Tropic vs nastic: growth movement vs not a growth movement (turgor change); response to a one-sided (directional) stimulus vs response to a diffuse (non-directional) stimulus; direction depends on the stimulus (positive or negative) vs direction does not; slow vs fast; not reversible vs reversible; e.g. shoot bending to light vs closing of Mimosa leaves.
(b) Auxin (IAA) is made at the shoot and root tips.
One-sided light causes auxin to move to the shaded side.
In the stem, the high auxin makes cells on the shaded side grow faster, so the stem bends towards light (positive phototropism).
In the root, high auxin slows growth, so the shaded side grows less and the root bends away from light (negative phototropism).
(c) Auxin collects on the lower side of root and shoot due to gravity; the shoot tip curves upwards (negative geotropism) and the root tip curves downwards (positive geotropism).
(d) Any five: produces growth hormone; stimulates the thyroid (TSH); stimulates the adrenal cortex (ACTH); FSH and LH control the ovaries and testes; prolactin starts milk production; oxytocin causes contraction of the uterus during birth; ADH controls water reabsorption in the kidney; controls other endocrine glands (master gland).
(e) Any two: plants grow at meristems (tips) only, animals grow all over; plant growth is indefinite (throughout life), animal growth stops at maturity; plants can grow by secondary thickening, animals cannot; plant growth is towards stimuli (tropic) in parts.
Mention auxin redistribution in (b) and (c).