阅读下面一诗，回答其后问题。
                                                                                                     蒹葭（节选）
                                                                            蒹葭苍苍，白露为霜。所谓伊人，在水一方。
                                                                            溯洄从之，道阻其长。溯游从之，宛在水中央。