Problem 154 - Objective Biology for NEET

Question

Assertion: Phylloclade (flattened stem of plant like opuntia) helps the plant to grow in dry habitats. Reason: Very low transpiration occurs from stems.

Step-by-Step Solution

Answer

Both the assertion and the reason are correct and are related, as Phylloclades, the flattened stems, do contribute to the survival of plants such as Opuntia in dry habitats, due to their role in reducing transpiration and promoting water conservation. This enables the plant to thrive in conditions where other plants might struggle due to the lack of water.

Step 1: Understanding phylloclade

Phylloclade is a modified stem that has taken over the function of leaves in certain plant species like cacti (opuntia). This modification is typically seen in plants growing in dry habitats for minimizing water loss.

Step 2: Understanding transpiration

Transpiration is the process in which water is carried through plants from roots to small pores on the underside of leaves, from where it changes to vapor and is released to the atmosphere. The majority of transpiration typically happens via the leaves.

Step 3: Association of phylloclade and transpiration

Since the phylloclade is a modified stem taking the function of the leaf, it reduces the surface area where water loss can happen, leading to very low transpiration. The low transpiration rate helps the plants to retain more water in their system, aiding their survival in dry habitats.

Step 4: Verification of Assertion and Reason

The assertion that phylloclades help the plant grow in dry habitats is correct as it promotes lower transpiration, ensuring the plant retains more water. The reasoning is also accurate as it correctly explains why transpiration is lower in plants with this type of modification. As a result, the assertion and reasoning are indeed linked.

Key Concepts

Transpiration

Transpiration is an essential process in plants that involves the movement of water from the roots through the plant and out of small pores on the leaves called stomata. This process is crucial for several reasons:

• It helps in nutrient uptake by creating a vacuum that pulls up water and nutrients from the roots.
• It cools the plant surface, similar to how sweating helps cool animals.
• It maintains the flow of water in plants, keeping them hydrated.

In typical conditions, most water loss happens through the leaves via transpiration. However, in dry habitats, where conserving water is vital, plants have evolved mechanisms to minimize transpiration. This helps them to retain more water and survive in harsh environments.

Modified Stem

In some plants, especially those found in arid regions, stems undergo special modifications to fulfill roles typically associated with leaves. These modified stems, known as phylloclades, take on a flattened appearance, resembling leaves. Phylloclades are important for several reasons:

• They contain chlorophyll, enabling them to carry out photosynthesis.
• Their flattened structure reduces surface area, lowering water loss through transpiration.
• They are often thick and fleshy, allowing them to store water efficiently.

These adaptations make modified stems highly efficient in minimizing water loss and optimizing water use, contributing significantly to a plant's survival in dry conditions.

Dry Habitats

Plants in dry habitats, such as deserts, face unique challenges due to the limited availability of water. These regions have specific climatic conditions:

• Low precipitation, sometimes going years with minimal rainfall.
• High temperatures that can cause rapid evaporation of any available moisture.
• Intense sunlight, increasing the need for water conservation.
• Often, soil in these areas is low in moisture content.

To thrive in such environments, plants have developed several adaptations like reduced leaf size, thick cuticles, and extensive root systems. These adaptations help them minimize water loss and maximize water uptake, ensuring their survival and growth.

Cacti

Cacti are iconic inhabitants of dry habitats known for their unique adaptations, which allow them to survive in extreme conditions. Key features of cacti include:

• Phylloclades: Most cacti have these modified stems that carry out photosynthesis and minimize water loss.
• Spines: Instead of leaves, cacti have spines which reduce water loss and provide shade.
• Thick, waxy cuticle: This covering helps retain moisture within the plant.
• Specialized root system: Cacti have extensive root networks that quickly absorb moisture from the soil.

These features make cacti specialists in conserving water, thriving where few other plants can. Their ability to store and carefully manage water resources is integral to their survival strategy in arid landscapes.