Final answer:
Cacti are green due to chlorophyll necessary for photosynthesis. They have needles instead of broad leaves to minimize water loss and to protect against herbivores. The thick cuticle on the needles further reduces water evaporation under hot conditions.
Step-by-step explanation:
Cacti are green because they are filled with chlorophyll, which is necessary for photosynthesis, the process by which plants use sunlight to produce food. The green pigment in chlorophyll helps to absorb the sunlight efficiently. Cacti need needles because they are modified leaves that help to reduce water loss in their harsh desert environment.
These needles have a thick cuticle which minimizes water evaporation and they are also useful for the plant's defense against herbivores. Additionally, having needles instead of broader leaves reduces the surface area, diminishing water loss, yet these needles are rich in chlorophyll to maximize sunlight absorption even with their reduced size.