procedure TForm1.Button1Click(Sender: TObject);
  Function FindRoot(TreeNode : TTreeNode) : String;
  Var
    CurrentNode : TTreeNode;
  begin
    CurrentNode := TreeNode;
    while CurrentNode.Parent <> NIL DO
      begin
        CurrentNode := CurrentNode.Parent;
      end;
  Result:= CurrentNode.Text;
  end;
begin
    ShowMessage(FindRoot(TreeView1.Selected));
end;

procedure TForm1.TreeView1MouseUp(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
Var
  Node : TTreeNode;
begin
  Node :=TreeView1.GetNodeAt(X,Y);
  if Node<>nil then
  begin
    While Node.Level>0 do Node := Node.Parent;
    Showmessage(Node.Text);
  end;
end;