Monday, June 4, 2012

Salt Lovers' Week: meet the halophytes

Water, water everywhere, nor any drop to drink... Coleridge knew it, sailors have always known it, and anyone who's been to the beach certainly knows it: salt water is no drop to drink. Water with as much salt as is found in seawater can, in fact, be lethal. It literally dehydrates the drinker,1 making you thirstier even as you guzzle it down. Humans can't survive on salt water.

Neither can plants, and for much the same reasons. Salt water dehydrates them just as it dehydrates us, sucking the moisture out of their tissues and leaving them to wilt and die. There's a reason that ancient armies would salt an enemy's farmland if they wanted to destroy it utterly: salted ground can't support crops. Not even barnyard weeds will grow in land seeded with salt.

Like virtually everything in biology, though, this prohibition isn't absolute. Salty ground is harsh and unforgiving, but many species of plants -- halophytes -- have grown to live in it anyway. Dunegrasses take up the salt, then spit it right back out. Glasswort takes it up, puts it aside, and lives with it anyway. Tamarisks can tolerate salt well enough to take over the salt-crusted desert waterholes of the American West. Some plants can even live fully immersed in extremely salty water -- they must have managed it, if they were to survive the primordial oceans.

This week, look for a post each day, each dealing with one of these extraordinary plants as it thumbs its nose at the killing salts.

1. By osmosis. If you have two aqueous solutions (things-dissolved-in-water) side-by-side, the water will tend to move from the less-concentrated to the more-concentrated one. The liquid in your tissues has less stuff dissolved in it than seawater does, so the salt water literally sucks the water out of your body. Nasty.

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