Spines are leaves. A cactus did not drop leaves, a cactus reshaped leaves, and the cactus spines purpose comes down to two jobs. Spines shade the stem, and spines break up the drying winds that pull water off the plant surface. You are looking at modified leaves in the plainest sense, and that is a lot of work to ask of something you would file under thorn.
Look at a saguaro up close and you can see how the layout pays off. The stem carries 19 to 25 deep lengthwise ridges, and every ridge is dotted with clusters of bristle-like spines. So the surface doing the plant's food-making is the same surface being shaded and guarded. That one green skin has to feed the plant and hold water while anything hungry tries for a bite. Broad leaves would only have added more surface for the sun and wind to dry out.
The swap starts in the stem. Cactus pads and trunks are modified stems with a waxy coating, so the plant makes food in the stem rather than in any leaf. Once the stem took that over, the leaves had no work left, and they were free to become something else. What you get now is a spine growing out of an areole, the small cushioned bud on the stem surface that sits where a leaf node would sit on any other plant.
Spine shape follows the job. A saguaro carries fine bristles in tight clusters strung along the ridges, spread thin and even across a very tall plant. The Arizona barrel cactus goes the other way with hooked spines tough enough for fishhooks, and people have used them that way. The cactus spines purpose stays the same in both, shade first and defense right behind.
Prickly pear adds a second layer. Its areoles carry glochids, tiny barbed hairs that come loose at the lightest touch and lodge in your skin. That is why brushing a pad bothers you more than a jab from one big spine. A large spine you can see, and you can pull it out. Glochids come off by the dozen, they are fine enough for you to miss, and they irritate the skin they land in.
NPS Organ Pipe reports that prickly pear areoles bear glochids, tiny barbed hairs that irritate skin. Those hairs cause most of the trouble, not the large spines you can see coming. Keep bare skin away from the pads, and stand spiny plants back from walkways and out of reach of children and pets.
The stem pays for this arrangement. It holds the water and runs the food-making under a seal of wax, all day, with the desert sun on top of it. Much of that food-making happens at night, when cooler air means less water lost to dry air. And spines still do not put a cactus off the menu, since rodents gnaw the pads and both bears and people eat prickly pear fruit. Armor slows a browser down without stopping one.
Placement does more for you than any handling trick. Give a prickly pear a spot where you never have to squeeze past it, and keep a barrel cactus off the edge of a path where your leg finds it first. Indoors, set your pots where a curious hand cannot reach across them. The plant will never move, so you plan around it, and a spine cannot tell your hand from a hungry rodent.
Read the full article: Cacti Are Not Hard to Kill, Just Hard to Read
