Shadow of the Torturer chapters 17 to 20
Sorry this is late- there was lots going on at home, and now I'm relaxing in between moving items of furniture.
Summary: The rag shop is owned by a twin brother and sister. The brother takes a keen interest in the sword, Terminus Est, and seeks to buy it. In the shop, Severian is challenged by a masked hipparch to a duel- with a particular kind of poisonous flower, the avern.
Severian goes off with Agia (the sister) to the Botanic Gardens to pick his avern for the duel. On the way, they hire a fiacre, which as a result of a bet Agia makes crashes into an altar of the Pelerines, a religious order who worship the figure of the Conciliator. The Pelerine's treasure goes missing- they believe Severian when he says he knows nothing of it (perhaps having some reliable means to ascertain his honesty) but strip search Agia.
The two then visit the jungle garden in the Botanic Gardens. Severian recalls a tale Thecla told him of Father Inire and his Mirrors.