Code
 Ext.onReady(function() {
            var tree = new Ext.tree.TreePanel({
                width: 250,
                height: 500,
                split: true,
                useArrows: true,
                lines: true,
                minSize: 200,
                maxSize: 300,
                autoScroll: true,
                bodyStyle: 'padding:10px',
                el: "divtree",
                root: new Ext.tree.AsyncTreeNode({
                    id: '0',
                    text: 'root'
                }),
                loader: new Ext.tree.TreeLoader({
                    dataUrl: 'AjaxData.ashx'
                })
            });
            tree.on('beforeload', function(node) {
            tree.loader.dataUrl = 'AjaxData.ashx?nodeid=' + node.id;
                
            });
            tree.render();
        });