动态添加textbox，不能获取其值

动态添加textbox，不能获取其值，不知什么原因。

页面代码：

<body>
<form id="form1" runat="server">
<div>
<asp:Panel ID="Panel1" runat="server">
</asp:Panel>
<hr />
<asp:Button ID="Button1" runat="server" Text="添加" onclick="Button1_Click" />  
<asp:Button ID="Button2" runat="server" Text="获取" onclick="Button2_Click" />
</div>
</form>
</body>

后台代码：

protected void Button1_Click(object sender, EventArgs e)
{
for (int i = 0; i < 3; i++)
{
TextBox txt = new TextBox();
txt.ID ="txt"+i.ToString();
Panel1.Controls.Add(txt);
}
}
protected void Button2_Click(object sender, EventArgs e)
{
for (int i = 0; i < 3; i++)
{
TextBox txt = (TextBox)Panel1.FindControl("txt" + i.ToString());
if(txt!=null)
{
txt.Text = "ok";
}
}
}

页面生成后的源文件：
<div>
<div id="Panel1">

<input name="txt0" type="text" id="txt0" /><input name="txt1" type="text" id="txt1" /><input name="txt2" type="text" id="txt2" />
</div>
<hr />
<input type="submit" name="Button1" value="添加" id="Button1" />  
<input type="submit" name="Button2" value="获取" id="Button2" />
</div>

无法获取其值，总是为null，不知道什么原因。