extjs GridPanel添加到TabPanel，列标题总是不显示，急~~~

源码如下：
Ext.onReady(function(){
    var tabs=new Ext.TabPanel
     ({
            renderTo:"Container",
            width:800,
            autoHeight:true,
            deferredRender:false,
            region:'center',
            height:200
     });
var cm = new Ext.grid.ColumnModel
([  
{header:'Name', dataIndex: 'filename',sortable:true},
{header:'No', dataIndex: 'lineNo',sortable:true},
{header:'Date', dataIndex: 'importDate',sortable:true}
 ]);

var importRecord = Ext.data.Record.create([
     {name: 'filename', type: 'string'},
     {name: 'lineNo', type: 'string'},
     {name: 'importDate', type: 'string'}
]);
var data1 =[['A','B','C']];
var store1 = new Ext.data.Store
({ 
proxy: new Ext.data.MemoryProxy(data1),
reader: new Ext.data.ArrayReader({},importRecord)
}); store1.load();
var grid1 = new Ext.grid.GridPanel
({ 
store:store1, 
cm:cm, 
loadMask:true,
viewConfig:{forceFit:true},
autoHeight:true
});
tabs.add({
id:"grid1",
title:'ImportTool', 
layout:'fit',
items:grid1,
autoHeight:true
});  tabs.activate(0);
}); gridPanel里的数据（A,B,C）能显示出来，但是grid.ColumnModel定义的header（Name,No,Date）显示不出来。
急求高手解答，多谢！！！！！！