Recently, I've acquired a Cephalocereus senilis:
The often disheveled-looking tufts of white hair inspired this plant's common name: the old man cactus. The primary purpose of all this hair isn't to make the cactus look like an unkempt retiree yelling at younger cacti to get off his lawn, but it is a hilarious side benefit. It is definitely the reason why I now have one. But, if it's not aiming to delight easily-amused botanists, what could the old man cactus possibly be doing with its Einstein hairdo?
As it turns out, the hair is an offbeat play on a survival strategy employed by many plants. Old man cacti have taken to the extreme a tendency common among plants in a diverse variety of environments: the development of a hairy layer called a “tomentum.” The old man cactus's tomentum helps the plant stay cool as a cucumber in the hot desert sun when the traditional route of cooling used by plants, transpiration, is unavailable.
What is transpiration? Think about it this way: when your body gets hot, you get sweaty. The purpose of sweat is to provide evaporative cooling – as the water evaporates, it takes heat away from your body. Plants also have this ability, by allowing water to evaporate through their mouth-like stomata: a process known as transpiration. In dry environments, however, sweating to cool down is a waste of precious water, and so desert plants keep their stomata closed during the day and open them at night in order to take in carbon dioxide for photosynthesis while minimizing water loss. In environments where low humidity is a challenge, a tomentum can provide extra protection against drying out by trapping a layer of moist air close to the surface of the leaf or stalk.
The drawback to this method is that now this layer of air is very humid, and so it can't take any more water from transpiration. And, as anyone who's had long hair in the summer knows, this layer is also very effective at trapping heat along with the humidity. This is definitely not something that appeals to our old man cactus. So the plant has to turn to another strategy to get out of the corner it's painted itself into. The solution is more than a little counterintuitive. It turns out that if having hair heats a plant up, the solution to cooling down is to grow more.
All those hairs are white for a reason: to reflect light before it has a chance to get absorbed and turned into heat. Just as wearing a white t-shirt instead of a black one can help keep you cool in the sun, so does turning the hairs of the tomentum white and reflective keep the cactus from heating up when it doesn't have a more traditional cooling route available. Growing more hair means the old man cactus can reflect more light. The result is that the old man cactus gets to have its cake and eat it, too: it turns the structure that helps it hang onto water into a nice reflective sunscreen, and the way to make it more effective is to make it look even cooler.
Old man cacti, like most cacti, are relatively easy to care for and are popular ornamentals and houseplants. However, do note that they are listed as endangered. Because they are slow-growing and slow to flower, they are vulnerable to over-harvest by unscrupulous collectors who don't want to take the time to develop more valuable stock. If you decide to adopt your own old man cactus, make sure that it comes from a reputable source who propagated rather than collected their inventory.