阅读下面这首古诗，回答后面小题。
                                                                                                    蒹葭
                                                                         蒹葭苍苍，白露为霜。所谓伊人，在水一方。
                                                                         溯洄从之，道阻且长。溯游从之，宛在水中央。
                                                                         蒹葭萋萋，白露未晞。所谓伊人，在水之湄。
                                                                         溯洄从之，道阻且跻。溯游从之，宛在水中坻。
                                                                         蒹葭采采，白露未已。所谓伊人，在水之涘。
                                                                         溯洄从之，道阻且右。溯游从之，宛在水中沚。