怎样用一个表单控制另一个表单!

try something like this:<xml id="xmldata">
<government>
  <province name="Beijing" value="1">
<city name="Beijing" value="2" />
  </province>
  <province name="Guang Dong" value="3">
<city name="Guang Zhou" value="4" />
<city name="Fo Shan" value="5" />
<city name="Shen Zhen" value="6" />
  </province>
  <province name="Zhe Jiang" value="7">
<city name="Hang Zhou" value="8" />
<city name="Weng Zhou" value="9" />
<city name="Ning Bo" value="10" />
  </province>
</government>
</xml><body onload="init()">
<form name="frmprovince">
Privince:<select name="provincelist" onchange="changeCity(this.selectedIndex);">
</select>
</form><form name="frmcity">
City:<select name="citylist">
</select>
</form>
</body>
<script language="javascript">
function fillList(oList, nodes)
{
  oList.options.length = 0;
  if (nodes)
  {
for (var i=0; i < nodes.length; i++)
{
var opt = new Option(nodes[i].getAttribute("name"),nodes[i].getAttribute("value"));
oList.options.add(opt);
}
  }
}function changeCity(n)
{
  if (n >=0)
  {
fillList(frmcity.citylist,xmldata.documentElement.childNodes[n].childNodes);
  }
}function init()
{
fillList(frmprovince.provincelist,xmldata.documentElement.childNodes);
changeCity(0);
}
</script>