accessing content of dynamically created TextBoxes

<%@ Page Language="C#" MasterPageFile="MasterPage.master" AutoEventWireup="true" CodeFile="test1.aspx.cs" Inherits="member_area_databases_defra_canine_equine_test1" Title="Untitled Page" %>
 
<%@ Register Assembly="RadTreeView.Net2" Namespace="Telerik.WebControls" TagPrefix="radT" %>
 
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
 
    <script language="Javascript">
    
        var tree = "<%= RadTreeView1.ClientID %>"; 
        var selected_node;
        
        function UpdateClick()
        {
            //__doPostBack('__Page', selected_node.value);
        }
 
        function AfterClientClick(node)
        {
            //selected_node = node
        }
    
    </script>
    
    <div class="frame"> 		                        
        <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
 
        <asp:UpdatePanel runat="server" ID="up1" UpdateMode="Always" >
            <ContentTemplate>
            
                <div class="frame"> 
                    <table>
                        <tbody>
                            <tr>
                                <td>
                                    <div class=frame>
                                        <b>Sample UID</b>
                                    </div>
                                </td>
                                <td>
                                    <div class=frame>
                                        <asp:TextBox runat=server ID=sample_uid></asp:TextBox>
                                    </div>
                                </td>
                            </tr>
                            <tr>
                                <td>
                                    <div class=frame>
                                        <b>Species</b>
                                    </div>
                                </td>
                                <td>
                                    <div class=frame>
                                        <asp:DropDownList runat=server ID=species>
                                            <asp:ListItem Selected=true Text=? Value=-1></asp:ListItem>
                                            <asp:ListItem Text=dog Value=dog></asp:ListItem>
                                            <asp:ListItem Text=horse Value=horse></asp:ListItem>
                                        </asp:DropDownList>
                                    </div>
                                </td>
                            </tr>
                        </tbody>
                    </table>
                </div>
        
                <asp:Button ID=update_attributes runat=server OnClick=update_button_Click CssClass=btn Text="Update Attributes" Visible=false/>
        
                <div class="frame"> 
                    <asp:Panel ID="Panel1" CssClass="ControlPanel" runat="server">
                        
                    </asp:Panel>
                    
                </div>
                
                        <radT:RadTreeView
                            ID="RadTreeView1"
                            runat="server"
                            Height="1000px"
                            Skin="Web20" 
                            CheckBoxes =true 
                            AutoPostBackOnCheck=true AutoPostBack=true OnNodeClick=NodeClicked OnNodeCheck="HandleCheck" AfterClientClick="AfterClientClick"
                            > 
                        </radT:RadTreeView>  
                    
            </ContentTemplate>
        </asp:UpdatePanel>
 
    </div>
    
</asp:Content>