get control from a PlaceHolder

 hi.
im using a place holder to add a table on mymain page ... 
 however after add table on the placeHolder I dont know how get control of table again?
what i want to do is read datas from insade the table, but main page doesnt recognize my table....
 
code page.aspx ::
<asp:PlaceHolder id="PlaceHolder1" runat="server"/>
code page.aspx.cs::
 protected void showTable()
    {
        Table tbl = new Table();
        ...
        PlaceHolder1.Controls.Add(tbl);
    }
 but i cant call placeholder from a differente side of my code...
protected void btn_Click(object sender, EventArgs e)
    {
         TextBox txt = new TextBox();
   & ...