Why Do Cacti Have Thorns?

Reduce Transpiration

• Plants take up water through their roots and distribute it throughout their tissues using a system of vessels known as xylem. However, most plants only use about 1 percent of the water they take in. The rest escapes into the atmosphere as water vapor in a process known as transpiration. Transpiration occurs mainly through leaves. Stems also transpire, but to a lesser extent. Cacti minimize transpiration by making their leaves tiny and hard; the modified leaves are in fact spines.

Protection

• Spines serve several other functions for cacti. For example, spines discourage animals from eating the cactus. This is an important function since one of the other drought-survival adaptations seen in cacti is stem succulence, or the presence of specialized tissues in the cactus stem that store water. Without protective spines, thirsty desert animals would have a ready water supply in the form of cacti. The few cacti species that lack prominent spines typically produce toxic substances to repulse herbivores.

Reflect Sunlight

• Although lack of water is the main survival challenge for cacti, temperature is also an important concern. Minimizing transpiration allows the plant to store water but also minimizes the plant's main strategy for cooling. Spines deflect sunlight, especially in species with dense spination, such as cholla or golden barrels. Some cacti receive shade from what appears to be a thick covering of fur, but the hairs are actually long, threadlike spines. Examples include old man cactus (Cephalocereus senilis) and cacti in the genus Espostoa.

Other Uses

• Spines play additional roles in specific cactus species. For example, cacti native to the Atacama Desert of Chile receive water from fog that condenses on their spines and falls to the ground near their roots. Spines can also function the same way burrs do, to disperse the plant to new growing locations. Like burrs, cactus spines can catch in the fur or feathers of passing animals. This method of dispersal is common among jointed cacti, such as cholla, where individual segments detach and root.