Ext4.0 tree为啥不会向后台发送请求，求解

    var store = Ext.create('Ext.data.TreeStore', {
        proxy: {
            type: 'ajax',
            url: 'getTreeMenuAction'
        }
    });

    var tree = Ext.create('Ext.tree.Panel', {
        title: 'Simple Tree',
        width: 250,
        height: 600,
        store: store
        //rootVisible: true
    });