How to keep the XML text in the UGUI.Text that has been load？

I use UGUI to show text of my xml but The new text appears, the old text is gone I want all my text on the screen what should I do

here`s code

using UnityEngine; using System.Collections; using System.IO; using System.Xml; using UnityEngine.UI; public class new_XmlLoda : MonoBehaviour { public int index = 0; public string dialogue;
public float dia_interval = 1.0f;
// Use this for initialization void Start () {

 }
 
 // Update is called once per frame
 void Update () {

     XmlDocument xml = new XmlDocument();

     xml.Load("E:/unity5project/project_AVG/project_avg/Assets/Resources/duihua_1.xml");

     XmlNode xnl = xml.SelectSingleNode("dialogue");

     XmlNodeList xl = xnl.ChildNodes;

     dia_interval -= Time.deltaTime;
     if (dia_interval < 0)
     {
         dia_interval = 1.0f;
         index += 1;
     }
     if (index < xl.Count)
     {
        //how to keep this on the screen?
         dialogue = xl.Item(index).InnerText;

     }
 }

}