Problem 157 - Objective Biology for NEET

Question

Assertion: Leaves are modified into spines in Cacti. Reason: It protect plant from grazing animals and excessive transpiration

Step-by-Step Solution

Answer

Both the assertion and the reason are correct, and the reason is the correct explanation for the assertion. The leaves of cacti are indeed modified into spines to protect the plant from grazing animals and reduces water loss through transpiration - a necessary adaptation for survival in desert environments.

Step 1: Understanding the Assertion

The statement claims that in cacti, leaves have been modified into spines. This is true. Leaf modification into spines is one of the plant adaptations to survive in deserts. These spiny leaves perform several functions which are important for their survival, which leads to our next step: understanding the reason.

Step 2: Understanding the Reason

According to the provided reason, the modification happens to protect the plant from grazing animals and excessive transpiration. This is also correct. Thick, spiky leaves deter herbivores and provide protection for the plant. The lack of leaf surface area also helps to minimize water loss through transpiration.

Step 3: Combining Assertion and Reason

The assertion states a biological adaptation of cacti's leaves and the reason provides the rationale behind this adaptation, which is survival in harsh, arid conditions. Thus both the assertion and the reason are correct, and the reason is a correct explanation of the assertion.

Key Concepts

Leaf Modification

Plants have evolved various structural adaptations to thrive in challenging environments, such as deserts. These areas are characterized by extreme temperatures, limited water availability, and intense sunlight. One such adaptation is the modification of leaves. In many desert plants, typical large and flat leaves transform to serve other purposes, such as reducing water loss.

In cacti, leaves have evolved into spines or scales, significantly reducing their surface area. This adaptation plays a crucial role in minimizing water loss through transpiration. The smaller or nonexistent leaf surface directly translates to less exposure to the scorching desert sun and, hence, a decrease in water evaporated from the plant's surface. In addition to conserving water, these modified leaves also serve as deterrents to herbivores, protecting valuable water-storing tissues from being eaten.

Cacti Spines

The spines of cacti are one of nature's most fascinating transformations. Originally leaves, these spines have become sharp, hardened structures that serve multiple protective functions for the plant. They create a shade microhabitat over the surface of the cactus, crucial for reducing leaf temperature and water loss.

Spines also help to break the wind flow over the plant surface, which can otherwise increase transpiration. Moreover, cacti spines can trap a layer of moisture-rich air next to the plant's body during cooler nights, which is then used by the plant. Their intimidating appearance and texture make them an effective physical barrier against desert fauna that might see the cactus as a source of moisture.

Survival Strategies in Arid Conditions

Surviving the extreme arid conditions of deserts demands a wide array of strategies that plants have developed over millennia. Beyond leaf modification, desert plants often have deep or widespread roots that can tap into scarce water sources. They also have thick stems to store water, and their surface tissues often have a waxy coating to reduce evaporation.

Many desert plants exhibit a type of photosynthesis called CAM (Crassulacean Acid Metabolism), where they open their stomata at night to reduce water loss. These strategies together ensure the maximization of water intake, minimization of water loss, and protection from the extensive heat and light conditions that characterize desert environments.

Transpiration in Plants

Transpiration is the process by which moisture is carried through plants from the roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. In essence, it’s part of how plants breathe and is crucial for their survival. However, in desert environments, controlling transpiration is vital due to limited water availability.

Desert plants often have fewer stomata or have them sunk into the plant's flesh to decrease direct exposure to the sun's heat. Additionally, some plants only open their stomata during the cooler nighttime, significantly reducing water loss. This careful control of transpiration is key to survival in arid conditions, making it a cornerstone of desert plant adaptation strategies.