Fondling cacti with your bare hands is often ill-advised. These spiny plants are icons of plant defense. Cactus spines are actually modified leaves/bud scales. They develop from a bundle of cells called "primordia" that are nearly indistinguishable from leaf primordia. Unlike leaves, cactus spines are not living tissue. The genes for leaf development are shut off in these cells and instead, genes for wood fibers are ramped up, creating the stiff structures many of us have had to pry out of our skin.
It is easy to assume that spines are simply there for defense. For a lot species they certainly do the trick. However, for many species, spines serve another important purpose - they provide shade. This is exemplified by the fact that cacti growing in rainforests and cloudy highlands often have reduced or no spines at all.
For cacti living in the sun-baked regions of the world, sunburn is a serious issue to contend with. Sensitive photosynthetic machinery can become damaged under full sunlight and damaging UV rays wreak havoc on the genome. As such, any adaptation that can shelter sensitive tissues is advantageous.
Spines also buffer the cactus from huge temperature swings. They create a boundary between air immediately surrounding the cactus and the cold nighttime air of these arid climates. This insulation can come in handy as desert temperatures can drop quite low when the sun goes down.
Another benefit spines have is to catch and direct water to the base of the plant. Rain is often scarce in these habitats and spines and when it does occur, a cactus needs to be ready. Water collects on the spines and then runs down to the base. Spines also act as dew catchers. As the air condenses, the spines act as a sort of filter for water, causing it to accumulate around the plant. It is a way of taking advantage of every last drop available.
Though they can offer protection, many of these shade spines are too thin and flexible to deter a hungry herbivore. Still, most cacti are not plants we want to run into. Nonetheless, the evolution of this family is fascinating.
Photo Credit: Frank Vincentz (Wikimedia Commons)