Question

Please help to explain why pine trees have needles instead of leaves and cacti have spines instead of leaves.

Answer 1

Developed a way to protect their foliage. Instead of flat leaves, pine trees have needles. These needles are the key to the Winter survival of the pine tree. Needles cut evaporation sotrees can save water!
Cacti have sines instead of leaves because they help protect them from getting eaten and they don't need leaves because they store water inside themselves. 
They both have adapteations to help them servive in very harsh conditions. Cacti use their adaptations to survive in scourching heat and pine trees can servive in freezing cold.