SELECT rpad(to_char(substr(the_path, 2)), 20, '.') || to_char(the_level + 1)
  FROM (
         SELECT sys_connect_by_path(col2, ',') || ',' || col1 the_path, level the_level, connect_by_root col2 root
           FROM seed_table s1
        CONNECT BY PRIOR col1 = col2
       ) branches
 WHERE NOT EXISTS --This ensures we're looking at a branch that goes all the way back to a root
       (
        SELECT 'foo'
          FROM seed_table s2
         WHERE s2.col1 = branches.root
       )
   AND NOT EXISTS --This ensures we're only report branches that go all the way to a leaf
       (
        SELECT *
          FROM (
                 SELECT sys_connect_by_path(col2, ',') || ',' || col1 the_path, level, connect_by_root col2 root
                   FROM seed_table s3
                CONNECT BY PRIOR col1 = col2
               ) branches2
         WHERE NOT EXISTS --This ensures we're looking at a branch that goes all the way back to a root
               (
                SELECT 'foo'
                  FROM seed_table s4
                 WHERE s4.col1 = branches2.root
               )
           AND substr(the_path, 1, length(the_path) - 2) = branches.the_path
       )
 ORDER BY 1;