So I gave up trying to ID a plant that had defeated me for several years, and sent the pictures off to an expert at the Wildflower Research Center...and this morning got the ID back. Texas Toothleaf (the narrow leaves have toothed edges). It's a weird looking plant in the spurge family, growing (on our place) only on the poorly consolidated gravel on the east side of the creek north of the creek woods.
Here's a whole plant of it; each stem (from an underground tuber) ends in a spike inflorescence.
And here's the inflorescence. That's it, that's all--just a yellow-green spike with little "things" sticking off it (they're pistils and stamens, really-truly.)