A HOTSPOT OF DIVERSITY and ENDEMISM IN SOUTH TEXAS
If you’re ever driving Highway 281 South of San Antonio, be sure to stop a lot. And if any of your passengers complain, threaten to leave them on the side of the road. This area, known colloquially as the “Texas Sandsheet” is a hotspot of quite a few plants that grow nowhere else in the world. As we’ve seen many times, a change-up in the geology or substrate of a given region offers a selection pressure for plants to speciate and for new species to evolve. We see this with serpentine, with gypsum, with bare limestone, and with….sand. These “Eolian” sands were deposited over millions of years from the Southeast and the nearby Gulf. Pictured above are some of the rarities that grow on this substrate. If you drive this area between March and August, when temps are the hottest (and most dangersous lol), you will be treated to some rarities and very beautiful, very cool plants that you won’t see if you’re speeding past them at 80 mph. This is the home of the (not yet blooming) Silverleaf Sunflower, Helianthus argophyllus, an annual sunflower with wooly leaves that resembles a Q-tip and which can grow to heights of 15’ in a single season. It has been hybridized with crop sunflowers in various regions to produce a more heat and drought tolerant sunflower. It’s an amazing plant and a beast and does well in cultivation. It’s seeds are also important as hell for migrating birds.
