Plant evolution - tree to cactus

I've talked with the kids before about how cacti used to be trees that evolved and changed into their current state. I think it came up first because Sayer named the cactus we pass on our drive home and always says 'hi!' to it when we drive by, and eventually asked how cacti came to be. Well, a week in Tucson with cacti everywhere and we revisited the topic ... several times.

One or the other of the kiddos asked, not *how* the trees turned into cacti, but *why.* So I explained that in the desert it was important to hold on to water, because it was scarce, and that plants lose most of their water through their leaves. The leaves breathe and some of the water in their leaves is breathed out and evaporated away by the hot desert air. So some trees found it advantageous to make their leaves smaller, so as to lose less water. But, when they made their leaves smaller they made less food, because they did not have as much green area full of chlorophyll to collect energy from the sun. So some of the trees found it advantageous to make their bark green, too. And with more green area to make food, smaller and smaller leaf sizes were needed, so they kept shrinking, and the branches that had been used to reach all those leaves out so they could gather the most sun also shrank back until just a green trunk remained. A green trunk with the remnants of branches or attachment points for now-gone leaves. A green trunk that could hold on to water in the desert, which made it attractive to birds and rodents who could poke into it to get the moisture out. At which point the plants found it advantageous for those old branch/leaf points to be come sharp and pointy, to deter animals from harming it. And so, from leafy tree to cactus.

I think, in the 11 days (? I lost count) we were in Tucson, we had this discussion about 3 times. And then Duncan played teacher and explained this lesson to Jen as we were driving home from an outing to visit a friend in Elgin. :D

I think we still have to work on understanding that this process takes time. Lots and lots and lots of it. And that it's not directed - the plant isn't consciously making decisions about what will work best in it's environment, just that those that luck into an adaptation that gives them an advantage survive and pass along those traits when others die ...