The Function of the Spines on a Cactus

Water Conservation

• Cactus spines are modified leaves and help the plant conserve water better than typical leaves might. Most cacti survive through drought by storing water in their stems, so preventing evaporation is vital. While typical leaves are prone to water loss, spines helps cacti conserve water. Meanwhile, the green stems of the cactus perform photosynthesis, providing the plant with energy.

Shading

• Cacti that grow in hot, dry desert regions tend to have a greater density of spines than do those from shady regions. The spines help shade the cactus from the harsh sun, creating a microclimate. This prevents the cactus from overheating, thereby protecting the chlorophyll and the plant's DNA. Though usually associated with the desert, a great number of cacti grow best in dark rainforests, semi-shade or cool, cloudy regions. These plants have shorter spines that are much less dense than their desert-dwelling counterparts.

Protection

• Spines help protect cacti from unwanted grazers, including humans who may pick the fruit or stems of the plant. With so much moisture stored in stems even during drought, cacti look delectable to thirsty creatures, many of which are slowed down by the painful spines. Regardless, some animals are able to consume the cactus, spines and all, without harm. The spines undoubtedly prevent humans from casually removing cactus flowers or otherwise damaging the plant.

Special Functions

• In a few species of cactus, the spines serve a special function as extrafloral nectaries. Their buds develop into glands that secrete a sugary solution to attract ants. These spines are shorter and broader but have the same spine formation as most cactus spines. The ants form a mutually beneficial relationship with the cactus, perhaps by defending it.

  In some tephrocacti, the spines appear thin and flat like blades of dry grass, helping them blend into the landscape.