如何通过script修改<td>的rowspan值，马上给分！！！！！！！！！！！！

<script id="clientEventHandlersJS" language="javascript">
<!--function window_onclick() {
TD1.rowSpan=2;
}//-->
</script>
</HEAD>
<body MS_POSITIONING="GridLayout" language="javascript" onclick="return window_onclick()">
<form id="WebForm4" method="post" runat="server">
<TABLE WIDTH="300" BORDER="1" CELLSPACING="1" CELLPADDING="1">
<TR>
<TD>1</TD>
<TD>2</TD>
<TD>3</TD>
</TR>
<TR>
<TD>1</TD>
<TD id="TD1" runat="server">2</TD>
<TD>3</TD>
</TR>
<TR>
<TD>1</TD>
<TD>2</TD>
<TD>3</TD>
</TR>
</TABLE>
</form>
</body>