有谁知道吗？？javascript与asp.net如何获取表单动态元素！

<form name="Form1" method="post" action="ProdIndex.aspx?Typeid=2&amp;ProdTypeId=38" id="Form1">
  <table cellSpacing="0" cellPadding="0" width="98%" align="center" border="0">
    <tr> 
      <td vAlign="top" width="190">
        <script language="javascript">
//div的显示和隐藏
function showHideBlock(obj,block) {
if (block.length==null)
{
if (block.style.display == 'none')
{
block.style.display = '';
obj.src = '/images/_.gif';
}else
{
block.style.display = 'none';
obj.src = '/images/+.gif';
}
}
else
{
for(i=0;i<block.length;i++)
{
if (block[i].style.display == 'none')
{
alert('c');
block[i].style.display = '';
obj.src = '/images/_.gif';
}
else
{
alert('d');
block[i].style.display = 'none';
obj.src = '/images/+.gif';
}
}
}
}
</script> 
<TABLE id="Table8" cellSpacing="0" cellPadding="0" width="180" border="0">
          <TR> 
            <TD> <TABLE id="Table9" cellSpacing="0" cellPadding="0" width="180" align="left" border="0">
                <TR> 
                  <TD><IMG height="56" alt="" src="/images/left_pdulist_01.gif" width="180"></TD>
                </TR>
                <TR> 
                  <TD vAlign="top" width="180" background="/images/left_pdulist_02.gif"><table width="100%" border="0" cellspacing="0" cellpadding="0">
                      <tr> 
                        <td width="35" align="right">
<a href='###' onclick='showHideBlock(imguserBlock38,userBlock38)'><img src="/images/+.gif" border="0" id="imguserBlock38"></a></td>
                        <td height="24"><strong><font color=red>&nbsp;1 </font></strong><a class="title" href="/Product/ProdIndex.aspx?Typeid=2&ProdTypeId=38"><strong><font color="#653000">淋浴房</font></strong></a></td>
                      </tr>
                      <tr> 
                        <td></td>
                        <td><div id="div38"> 
                            <table id='userBlock38' style='DISPLAY:none'>
                              <tr> 
                                <td>&nbsp;</td>
                                <td valign="bottom">1.1 <a href="/Product/ProdIndex.aspx?Typeid=3&ProdTypeId=180"><font color="#CC6600">淋浴盆</font></a></td>
                              </tr>
                              <tr> 
                                <td>&nbsp;</td>
                                <td valign="bottom">1.2 <a href="/Product/ProdIndex.aspx?Typeid=3&ProdTypeId=181"><font color="#CC6600">淋浴屏</font></a></td>
                              </tr>
                              <tr> 
                                <td>&nbsp;</td>
                                <td valign="bottom">1.3 <a href="/Product/ProdIndex.aspx?Typeid=3&ProdTypeId=214"><font color="#CC6600">淋浴柱</font></a></td>
                              </tr>
                            </table>
                          </div></td>
                      </tr>
                    </table>
                    <script>showHideBlock(imguserBlock38,userBlock38);</script></TD>
                </TR>
                <TR> 
                  <TD><IMG height="14" alt="" src="/images/left_pdulist_03.gif" width="180"></TD>
                </TR>
              </TABLE></TD>
          </TR>
        </TABLE>
        <table id="Table4" cellSpacing="0" cellPadding="0" width="100%" border="0">
          <tr> 
            <td height="7"></td>
          </tr>
        </table>
      </td>
    </tr>
  </table>
</form>
哪位兄台帮我看看！咋回事呀，去掉form标签后不会报‘imguserBlock38’未定义的错误！一加就会，但是页面需要form提交！求助