A "stand" sounds much the same as a "clump" to me - different bamboos are more tightly clumped than others. And even running bamboos grow in clumps, just that they send out runners to form remote clumps. The runners are generally underground so not visible. The wikipedia article on Arundinaria gigantea describes it as growing in large "stands". A. gigantea is a running bamboo.

And Arundinaria gigantea does grow naturally in Texas according to a number of sources I have found. This one is quite explicit. http://www.discoverlife.org/mp/20o?s...naria+gigantea

But, no, bamboo does not have seed heads at the top. It has the seeds along the side branches. Here's an example on this page. http://jizogarden.wordpress.com/2009/04/
Another example here
http://www.realoasis.net/?p=450

So maybe you found some A. gigantea. Or maybe what you have seen is some other kind of grass. There are other large grasses that have segmented stalks like bamboo, but have seeds clumped at the top.