What Are the Spines on the Cactus Used For?

Origin and Anatomy

• The spines on a cactus are modifications of leaves or bud scales. Cactus plants still have leaves located at the base of the spine clusters, but these are often microscopic. The spines are simply a fibrous core covered in a hard epidermis. The majority of the spine is made up of dead cells, with living cells located only at the bottom to allow the spine to grow. The density, thickness and length of the spines can vary greatly from variety to variety.

Protection

• The most-obvious purpose for the spines on a cactus is protection. In a harsh, dry climate, there are many animals that would like nothing more than to eat the soft, moist flesh of a cactus, and the hard, sharp spines serve as a deterrent to keep these animals away. Cactus spines can break off and become lodged in an animal's flesh, providing a lasting reminder to leave such plants alone. Contrary to urban legend, however, there are no species of cactus that can shoot their spines at intruders.

Shade

• Many species of cacti have gradually spread from cool, shady climates to environments with extreme sunlight. A common defense is to produce hundreds of spine clusters, densely covering the entire plant, which not only protect against animals but provide shade from the scorching sun. The effect is like holding a piece of gauze over your face: some light will get through, but a lot of it is blocked. This partial shade keeps the cactus from drying out.

Attracting Insects

• Some cactus plants have hollow spines, through which they can pump a sugary nectar to attract ants and other insects. The effect is the same as in flowers: as the insects feed on the nectar, they also cross-pollinate the cactus plants. This nectar isn't actively pumped by the cactus, but builds up over time and oozes out of holes in the spines' epidermis. These specialized spines are often shorter and broader than those used for defense or shade.