Extjs更换树状图节点图标问题。

Question

 var store = new Ext.data.TreeStore({
                    clearRemovedOnLoad : true,
                    autoLoad : false,
                    root : {
                        id: result.datas[0].key,
                        expanded: true,
                    },
                    fields : fields,
                    proxy : {
                        type : 'direct',
                        directFn : listOrgsAttributeFn,
                        reader : {
                            type: 'json',
                            root : 'datas',
                            messageProperty : 'message'
                        }
                    },
                    listeners:{
                        beforeexpand:{
                            fn:function(cmp, eOpts){
                            }
                        },
                        beforeload : function(store, operation) {  
                            var new_params = {//参数  
                                filterData : panel.params.queryData.f_tlj_code 
                            };  
                            Ext.apply(store.proxy.extraParams, new_params);
                        } 
                    },
                    sorters : [{
                        property : orderFieldName
                    }]
                });
                orgTree.bindStore(store);

    var orgTree = Ext.create('Ext.tree.Panel', {
        width: 150,
        //store: treeStore,
        rootVisible: false,
        frame: true,
        border : false,
            columns: [
            {leaf : false, dataIndex: 'orgshortname', xtype: 'treecolumn',width: 250}
            ],
            listeners :{

源码在上。节点如何更换为自定义的图标？