PL/SQL :: New Enhancement In 11G Sequence

I have Created sequence in oracle 10G and its working fine. its showing THE NUMBER IS 2

SQL> DECLARE NUM NUMBER;
  2  BEGIN
  3  select SERIAL_NO.NEXTVAL into num from dual;
  4  DBMS_OUTPUT.PUT_LINE('THE NUMBER IS ' ||NUM);
  5  END;
  6  /
THE NUMBER IS 2

PL/SQL procedure successfully completed.When i execute this code in oracle 11G then it does not show output.

SQL> DECLARE NUM NUMBER;
  2  BEGIN
  3  NUM :=SERIAL_NO.NEXTVAL;
  4  DBMS_OUTPUT.PUT_LINE('THE NUMBER IS ' ||NUM);
  5  END;
  6  /

PL/SQL procedure successfully completed. SQL> Why this is not showing output as THE NUMBER IS .