dynamic select menu doesn't work for 'form_tag'

Hi everyone,

      I have tried with the dynamic menu using
"http://railscasts.com/episodes/88-dynamic-select-menus " site.
i implemented and it was working fine for 'form_for' tag.
But i need to do the same function with 'form_tag' as like as,
<% javascript 'dynamic_states' %>
<%form_tag do%>
<p>
  <label for="person_country_id">Country:</label>
  <%= collection_select :id, Country.find(:all), :id, :name, :prompt =>
"Select a Country" %>
</p>
<%end%>

child records doesn't changed to its corresponding parent when i use
'form_tag'....
i want to do with 'form_tag'
Please help me....