Morphological Defence: Acacia and Cactus

Plants possess various structural adaptations, known as morphological defences, to protect themselves from herbivores and harsh environmental conditions.

Role of Thorns

• Thorns are sharp, pointed structures derived from modified plant parts. They serve as a primary defence mechanism by deterring animals from feeding on the plant.
• In Acacia species, thorns are typically modified stems or stipules, effectively discouraging grazing.
• In Cactus species, the spines are actually highly modified leaves, providing defence against herbivores and also helping to reduce water loss and regulate temperature.

Comparison with Other Options

• Leaf modifications (like spines in cacti) contribute to defence, but 'thorns' is a more encompassing term for the sharp structures providing primary defence in both groups. Leaves themselves are often eaten if not modified.
• Stem modifications can form thorns (as in Acacia), but 'thorns' specifically describes the defensive structure. The stem itself is a larger organ.
• Pseudostems are characteristic of plants like bananas, formed by rolled leaf sheaths, and are not the primary defence mechanism in Acacia or cactus.

Therefore, thorns represent the most common and significant means of morphological defence for both Acacia and cactus plants.