Sometimes you’ve got to give your enemies credit, and that’s definitely the case with a little weed that infests my back yard. It’s called “little western bittercress,” or Cardamine oligosperma, and it is an amazing example of evolution in action. First of all, it is well camoflauged against the ground—its dark color and thin lines make it hard to pick out against red dirt and oak leaves—but most astonishing are its seed pods.
They’re about an inch long, and very slender, essentially tubes, that look like antennae beside the flower in this photo (stolen from this site). They’re called siliques. Inside them, the seeds sit on top of one another like bullets in the magazine of an automatic pistol. As the summer heat dries the pods out, their sides lose elasticity, and this gradually builds up a great deal of tension. When the time is just right, the slightest bump against the side of the tiny pod—a raindrop is enough, let alone a hapless homeowner walking across the ground—will cause these sides to spring open and roll themselves up to the top, very like a pull-down window shade that’s been let go too fast. This happens with explosive force, often throwing the sand-grain-sized seeds two or three feet high, and perhaps three or four feet horizontally. During the hottest months of (our exceedingly hot El Dorado County) summer, walking through the garden can look like a little fireworks show with these seeds blasting into the air. I’ve even had them poke me in the eye sometimes. I wish there were a video of this posted online somewhere, because it’s a remarkable thing to watch. Nasty as the things are, it’s still a pleasant reflection for a belated Darwin Day.
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