jquery.treeview problem : addClass("open");

jquery.treeview problem : addClass("open");

Hi!
 
When I put the class "open" directly on the tag <li> it works fine. The sub-menu open as supposed.
 
<li id="sousmenu1" class="open"><span>Title of the submenu</span>
      <ul>
            <li>Link 1</li>             

            <li>Link 2</li>             
            <li>Link 3</li>             
            <li>Link 4</li>
      </ul>
</li>

 
But when I try to do it using javascript it doesn't work!
 
$("#sousmenu1").addClass("open");
 
I know that it calls the right tag since I tried this code :
 
$("#sousmenu1"). hide ();  
 
And the sub-menu did hide. But in the same time if I write this :
 
$("#sousmenu1").addClass("open");
alert($("#sousmenu1").hasClass());
 
The alert returns me "false"!
 
I don't know where I do something wrong... :S