I'm trying to learn a little more about the strange green creatures who rule the world, so now and then I'll post something here about a plant that got my attention.

Wood sorrel or Oxalis (family Oxalidaceae) is a diverse bunch of plants, but easy to recognize: straight succulent stems ending in clusters of three leaves that look like big clovers, but a little pointier, and the leaves fold up at night. They put out little pale flowers once in a while.

When you crush the leaves they have a pleasant tangy smell, and you can eat 'em, but don't overdo it; oxalic acid is yummy but can be bad for you.

This plant is almost impossible to kill. That's why I kept one in my house for years: it would shrivel up when I neglected it, but it bounced back with 20 new stems once it got a little water. Plus, the one I had was really pretty, the kind with deep purple leaves. The one that started growing in our front porch planter last year was plainer, green with little freckles, and not as sturdy; after a dry spell it looked 100% dead, leaves gone, stems all tiny and crinkly and brown.

The planter had been looking pretty sad for a long time anyway, so I threw away the dead plants, got rid of most of the crappy soil, bought some new dirt and put in a couple different things. Three or four days later, next to the violets, there were a half-dozen oxalis buds poking their necks out of the dirt (they come up like an upside-down U, with the leaves all curled up underneath)... and when I pulled them up, the roots went down at least six inches. This was in the new soil--if these guys were hiding out in the planter before, they must've come up from the very bottom. The next day: two more. The next day: two more. I've been taking one or two of them out of that planter every day for the last month. After being merciless for a while, I started to respect their determination, so I resettled some of them in a different planter; they're doing OK there but not spreading--they like the front porch a lot better.

So now I know why they call wood sorrel a weed. What I don't get is how it keeps growing new heads like that. Can someone help me out with the biology here? Don't plants need to photosynthesize to build stuff? How can a little critter that's never been above ground grow a half-inch clump of leaves and a seven-inch tail? Do they have a big secret bulb full of fuel down there somewhere? Are they eating the violets? Is it a perpetual motion machine?