Protection

• Cactus spines form a protective barrier around the plant, keeping herbivores away from their soft and water-rich flesh. They also provide shade, protecting the plant from strong sunshine. Cacti that grow in hot, exposed deserts tend to have more spines than those growing in grasslands or forests, as they need more protection from the sun. In some species, such as Mammillaria plumosa, the spines are soft, white and feathery and serve no defensive purpose. Instead, their purpose is solely to shield the plant from the sun.
  
  

Water Retention

• The spines around a cactus trap a layer of warm, humid air around the plant. This reduces the amount of water lost by evaporation during photosynthesis. As many cacti grow in arid environments, their spines allow them to preserve as much moisture as possible.
  
  

Dispersal

• Some species of Mammilaria and Opuntia cactus have hooked spines and shallow root systems. They latch on to passing animals and then re-root when they drop off.
  
  

Other Functions

• Some desert cacti from South America have woolly spines that point downward. Water from sea mists condenses onto the spines and then drips down onto the soil at the base of the plant, where it is absorbed by the roots. Cactus spines also act as a thermal blanket, trapping a layer of warm air around the plant, protecting them from the cold.