MOVEMENT REVIEW QUESTIONS (With proposed answers)

Questions: 

(a) Two potted plants A and B were placed in the school laboratory for observations. Plant A was placed in a room with unilateral source of light and another plant B grown on the normal land soil but when touched it closes its leaves. 

(i) What kind of response was shown by each plant A and B? 

(ii) Differentiate the response shown by plant A from that shown by plant B. 

(b) In three points state the significance of the response shown by plant A in plant growth in (a) above.

Answers:

Let's analyze the responses of plants A and B:

(a)

(i) Kind of response shown by each plant:

• Plant A: Plant A, placed with a unilateral source of light, will exhibit phototropism. Specifically, it will show positive phototropism by bending its stem and leaves towards the light source.
• Plant B: Plant B, which closes its leaves when touched, exhibits thigmotropism or a seismonastic movement. In this case, it's a rapid, touch-induced movement, often referred to as a nastic movement (specifically, a seismonastic or thigmonastic movement) because the direction of the response is not determined by the direction of the stimulus (touch).

(ii) Difference between the response shown by plant A and plant B:

Feature	Response in Plant A (Phototropism)	Response in Plant B (Thigmonastic/Seismonastic Movement)
Stimulus	Unilateral light	Touch
Directionality	Directional; the plant grows/bends towards the light	Non-directional; the leaves close regardless of the direction of touch
Growth Involvement	Involves differential growth of plant parts	Primarily involves rapid changes in turgor pressure in specialized cells
Speed	Relatively slow, a growth response occurring over time	Relatively fast, a rapid movement occurring within seconds

(b) Significance of phototropism (response shown by plant A) in plant growth:

1. Maximizing Light Absorption for Photosynthesis: Positive phototropism ensures that the leaves of the plant are oriented towards the primary light source. This maximizes the absorption of light energy, which is essential for the process of photosynthesis. Photosynthesis is the fundamental process by which plants produce their food (glucose) and oxygen, directly fueling their growth and development.

2. Optimizing Energy Production for Overall Growth: By efficiently capturing sunlight, the plant can produce more energy through photosynthesis. This increased energy production supports various metabolic processes necessary for overall plant growth, including the synthesis of new tissues, development of roots, stems, and leaves, and eventually reproduction.

3. Facilitating Survival and Competition: The ability to grow towards light allows the plant to compete effectively with neighboring plants for this vital resource. In environments where sunlight may be limited or partially blocked by other vegetation, phototropism enables the plant to reach areas with more light, increasing its chances of survival and successful growth.