初学，ext tree没显示出来。

下午刚刚练习下ext  tree.有2个问题不明白。请教下大家1.页面<div id='tree_test'></div>然后js  var tree = new Ext.tree.TreePanel({
renderTo: 'tree_test',（这里换成el:'tree_test'行不行。他们区别是什么，分别怎么用）                  ....
         }）
2.我在网上找个例子没有显示出来。求大家帮忙看看
页面<div id='tree_test'></div>
js:   
Ext.onReady(function(){
     
       var tree= new Ext.tree.TreePanel({
            el:'tree_test',
            loader:new Ext.tree.TreeLoader()
        });
        var root = new Ext.tree.AsyncTreeNode({
            text:'我是根',
            children:[{
                text:'01',qtip:'我是鼠标提示',//如果有qtip属性,则必须要有Ext.QuickTips.init()这句话
                children:[{
                    text:'01-01',
                    leaf:true   //这里为true代表他下面已经没有子节点，不需要加载出来
                },{
                    text:'01-02',
                    children:[{
                        text:'01-02-01',
                        leaf:true
                    },{
                        text:'01-02-02',
                        leaf:true
                    }]
                },{
                    text:'01-03',
                    leaf:true,
                    href:"http://www.baidu.com",//通过点击节点，可以链接到一个地址
                  //  hrefTarget:'_blank'   
                }]
            },
            {
                text:'02',
                leaf:true,
                //icon:'user_female.png'  //自定义节点的图标
            }]
        });
        tree.setRootNode(root);
        tree.render();
root.expand();
        Ext.QuickTips.init(); //初始化接点提示
        tree.expandAll();//展开所有节点
  })