Explanation:

Cacti are plants that thrive in arid and semi-arid regions. They have adapted to these harsh environments by developing several unique features, including spines. Here's why spines in cacti are modified leaves:

Structure of Spines:

Spines are usually sharp, pointed structures that protrude from the cactus stem. They are made up of the same tissues as the rest of the plant, but they are modified in structure to serve a different function. Spines are typically small, thin, and needle-like, and they can be either straight or curved.

Function of Spines:

The primary function of spines in cacti is to protect the plant from predators and environmental stressors. Spines deter animals from eating the plant, and they also reduce water loss from the plant's surface by providing shade and reducing air movement around the stem.

Similarity with Leaves:

Spines in cacti are modified leaves because they originate from the same tissue as leaves, the shoot apical meristem. During development, the meristem produces a small bump on the surface of the stem, which eventually develops into a spine. The spine is composed of the same tissues as a leaf, including the epidermis, mesophyll, and vascular tissues.

Difference with Leaves:

However, spines are structurally different from leaves. They lack the broad surface area and chloroplasts of leaves, and they do not photosynthesize. Instead, spines are specialized for their protective function, and they often have a hard outer layer and a hollow interior.

Conclusion:

In conclusion, spines in cacti are modified leaves that have adapted to serve a protective function in arid and semi-arid environments. Although they are similar in structure to leaves, they are structurally and functionally different, and they play a crucial role in the survival of cacti in harsh environments.