Question

explain why the various plants appear the way they do in each of the following demonstrations: phototropism: gravitropism: etiolation: holly leaf abscission: leaf movement in the sensitive plant ( mimosa pudica):

Answer 1

Phototropism- It is the phenomenon of growth of a plant in response to light. Plants grow towards light to allow maimum absorption of light for photosynthesis. This process occurs due to different concentration of a growth hormone called Auxin. The incoming makes the auxin hormones to concentrate in the cells of the shaded regions of the plants and thus promoting their elongation. This results in bending of the plant towards light. Plant stem show positive phototropism ( bent towards light ) and plant root show negative phototropism( bent away from light).

Gravitropism: It is the phenomenon in which an organisms growth occurs due to the effect of gravity. Plant roots show positive gravitropism (Grow in the direction of graviatation) and plant stem show negative gravitropism (Grow opposite to gravitation.). Plant does this growth pattern so that roots can avail nutrients and water from soil. Gravity sensing organelles such as Statoliths and growth hormone Auxin together brings about this growth pattern.

Etiolation: It is a process of growth pattern in plants grown in complete or partial absence of light. In this process plants usually have long internodes making longer stems, less number of leaves, weak cell wall and yellowish colour due to less chlorophyll. This kind of elongated growth pattern occurs to ensure maximum availability of light. Less leaves and chlorophyll ensures maimum storage of energy by reducing photosynthesis, transpiration and respiration etc. This kind of growth pattern also occurs due to the different distribution of Auxin hormone.

Holly leaf Abscission: Leaf abscission( shedding of leaves) in Holly plant occurs when the leaf of the plant undergoes senescence and finally dies. When a plant leaf grows its photosynthesis capability gradually increases and reaches its maximum when the leaf attains its maximum size. After that it starts undergoing decay. This mostly happens because of competion of pull of nutrients from other parts of the plant. This abscission process may also hasten due to deficiency of nutrients in plants, lack of water etc.

Leaf movement in sensitive plant ( Mimosa pudica):It is a creeping herb with a very slender stem. Its leaflets close inwards when stimulated by touching or warming or shaking to protect it from outside harm. The leaflets when stimulated produces a action potential which travels throughout the plant leaf and to the petiole( where the leaflets are attached to the stem). This causes water to flow out from the cells of the petiole and it loses its turgidity. As a result the leaflets collapse and bend inwards.