extjs tree 的问题，求帮助

html 代码
<html>
<head>
    <title>Extjs 布局</title>
    <!--引用Extjs4.0类库开始-->
    <script type="text/javascript" charset="gb-2312" src="../extjs/bootstrap.js"></script>
    <link href="../extjs/resources/css/ext-all.css" rel="Stylesheet" />
    <!--引用Extjs4.0类库结束-->
</head>
<script type="text/javascript">
    Ext.require([
    'Ext.tree.*',
    'Ext.data.*',
    'Ext.tip.*'
    ]);     Ext.QuickTips.init();       Ext.onReady(function () {
            var store = Ext.create('Ext.data.TreeStore', {
                proxy: {
                    type: 'ajax',
                    url: 'json_tree_dept.aspx',
                    root:'items'
                },
                root: {
                    text: '部门',
                    id: 'root',
                    expanded: true
                },
                folderSort: true,
                sorters: [{
                    property: 'text',
                    direction: 'ASC'
                }]
            });            var tree = Ext.create('Ext.tree.Panel', {
                id: 'tree',
                store: store,
                width: 250,
                height: 300,
                viewConfig: {
                    plugins: {
                        ptype: 'treeviewdragdrop',
                        appendOnly: true
                    }
                },
                renderTo: document.body
            });
        });
</script>
<body>
</body>
</html>
服务器传回来的json是：
{success:true,items:[
{children:[
{text:'轩辕春秋集团',id:'9',href:'www.baidu.com',hrefTarget:'main',leaf:false,children:[
{text:'江西轩辕春秋',id:'2',href:'www.baidu.com',hrefTarget:'main',leaf:false,children:[
{text:'总经办',id:'8',href:'www.baidu.com',hrefTarget:'main',leaf:true}
]},
{text:'江西春秋实业',id:'3',href:'www.baidu.com',hrefTarget:'main',leaf:true},
{text:'湖南春秋',id:'11',href:'www.baidu.com',hrefTarget:'main',leaf:true},
{text:'湖南轩辕春秋',id:'13',href:'www.baidu.com',hrefTarget:'main',leaf:true}
]}
]}
]}没有效果。会的帮帮我啊