Who is the written by when mark worthington is the director?
CREATE TABLE "table1_22570439_1" (
  "series_num" real,
  "season_num" real,
  "episode_title" text,
  "written_by" text,
  "directed_by" text,
  "u_s_viewers_millions" text,
  "original_air_date" text
);
SELECT "written_by" FROM "table1_22570439_1" WHERE "directed_by"='Mark Worthington';