Problem 7 - AP Biology Premium Prep 2021

Question

Mimosa pudica is a plant often called the "sensitive plant" because when you touch the leaves, they immediately close up. One theory about the purpose of this type of movement is that herbivores avoid the plant due to this movement. The movement of Mimosa pudica is an example of (A) phototropism (B) gravitropism (C) aquatropism (D) thigmotropism

Step-by-Step Solution

Answer

The movement of Mimosa pudica when its leaves close up in response to touch is an example of thigmotropism. So, the correct answer is (D) thigmotropism.

Step 1: Understand the types of tropism

Tropism is the growth or movement of plants in response to specific external stimuli. Let's go through the given options: (A) Phototropism: This is the growth or movement of a plant in response to light. It is not related to touch. (B) Gravitropism: This is the growth or movement of a plant in response to gravity. It is also not related to touch. (C) Aquatropism: This is the growth or movement of a plant in response to water. Again, it's not related to touch. (D) Thigmotropism: This is the growth or movement of a plant in response to touch or contact with a solid object.

Step 2: Identify the correct type of tropism for Mimosa pudica

Based on the descriptions of each type of tropism in Step 1, we can conclude that the movement of Mimosa pudica when its leaves close up in response to touch is an example of thigmotropism.

Step 3: Answer

The correct answer is (D) thigmotropism.

Key Concepts

Thigmotropism

Thigmotropism refers to the way a plant's growth direction is influenced by touch. It is a form of movement that happens when a plant's body comes into physical contact with an external solid object. For example, the tendrils of climbing plants such as ivy will wrap around a support structure, clearly displaying this type of growth movement. Think of it like the plant is reaching out and grasping onto whatever it can to support itself, similar to how humans might hold onto a railing for balance.

The biological mechanism behind thigmotropism involves changes in the plant cells where contact is made. When touched, certain plant cells may elongate while others slow down their growth, causing the plant to bend or move towards the stimulus. This is particularly important for plants that do not have robust stems and need to attach themselves to stronger structures for support in their quest for sunlight and resources.

Mimosa pudica

Mimosa pudica, also known as the 'sensitive plant' or 'touch-me-not', is a fascinating example of plant behavior and thigmotropism. This plant has delicate compound leaves that fold inward and droop when touched or shaken, which can reduce the likelihood of damage by herbivores or environmental factors. This rapid folding is a defensive mechanism that makes the plant appear less appealing to animals.

Interestingly, the movement is not permanent; the leaves reopen within minutes. The mechanism behind this action involves the rapid movement of water out of the cells at the base of the leaflets, leading to a loss in cell pressure and leaf collapse. This fast response is quite unique in the plant kingdom, providing an excellent real-world illustration of thigmotropism.

Phototropism

Phototropism is the growth response of plants to light. Essentially, it's how a plant moves or grows towards a light source. Many indoor plants exhibit this behavior by bending towards windows or light fixtures, an adaptation to maximize their exposure to sunlight, which is critical for photosynthesis.

In the presence of light, plant hormones called auxins accumulate on the darker side of the plant, promoting cell elongation and allowing the plant to bend towards the light. Gardeners often rotate their potted plants to encourage even growth and prevent them from leaning too far in one direction, which is a direct response to the plant's phototropic tendencies.

Gravitropism

Gravitropism, sometimes called geotropism, is a plant's growth response to gravity. Roots typically show positive gravitropism as they grow downward into the earth, anchoring the plant and seeking out water and nutrients. On the flip side, shoots display negative gravitropism by growing upwards, away from the gravitational pull.

A plant’s perception of gravity involves specialized cells called statocytes which contain dense, starch-filled organelles called statoliths. These settle under the influence of gravity to the lower parts of the cell, providing positional information that directs the growth. Gravitropism ensures that plants can orient themselves correctly in their environment, with roots going deep into the soil for stability and nutrition and stems reaching upwards towards the light.