CSS selector not working?

Hey guys, thanks in advance for any help or input. I am having trouble understanding CSS selectors and i have read the docs..

I am trying to refer to the UL with the id dropdown in my stylesheet. I was under the assumption that i was able to refer to any elements like:

#dropdown ul  
{
}

This method however does not seem to work :s.. Am i misunderstanding CSS selectors? The elements in my actual code are nested deeper than this structure but i presume the principle is the same?

<div id="wrapper">

    <ul id="dropdown">
      <li class="sub"><a href="#">Dropdown</a>

      <!-- Sub Menu -->
      <ul>
        <li><a href="#">Item 1</a></li>
        <li><a href="#">Item 2</a></li>
      </ul>
      <!-- End Submenu -->

    </li>
  </ul>

</div>


/* Dropdown Menu */
#dropdown ul 
{
  font-family: Arial, Verdana;
  font-size: 14px;
  margin: 0;
  padding: 0;
  list-style: none;
}

#dropdown ul li 
{
  display: block;
  position: relative;
  float: left;
}

#dropdown li ul 
{
   display: none; 
}

#dropdown ul li a 
{
  display: block;
  text-decoration: none;
  color: #ffffff;
  border-top: 1px solid #ffffff;
  padding: 5px 15px 5px 15px;
  background: #2C5463;
  margin-left: 1px;
  white-space: nowrap;
}

#dropdown ul li a:hover 
{ 
   background: #617F8A; 
}

#dropdown li:hover ul 
{
  display: block;
  position: absolute;
}

#dropdown li:hover li 
{
  float: none;
  font-size: 11px;
}

#dropdown li:hover a 
{ 
   background: #617F8A; 
}

#dropdown li:hover li a:hover 
{ 
   background: #95A9B1; 
}