The seeds of this herb contain up to 50 percent mucilage, so they swell in the gut, thus relieving diarrhea. Just don't use more than two teaspoons at a time, as more may cause abdominal distress. Commission E recommends using one to two teaspoons of dried oak bark to make an astringent tea. Pomegranate, an herb mentioned in the Bible, is often used to treat diarrhea. I'm not surprised, because the seeds are astringent.
Psyllium is useful for relieving constipation.
Its high mucilage content also makes it useful for treating diarrhea. By absorbing a great deal of water, psyllium adds bulk to stool. You should watch how you react to this herb if you have allergies, however. If allergic symptoms develop after you take it once, don't use it again. One of the most astringent plants around is conventional tea in conventional tea bags.