Select option html tag in twig Symfony

For my project I used Symfony framework. I need to use the select option to generate a list for my form.

Here is the code:

Form:

<form method="post" {{ form_enctype(form)}} action="{{ path('my_path')}}">
    {{form_errors(form)}}
    <div name="nature">
        {{form_label(form.nature,"(*) Nature sample")}}
        {{form_errors(form.nature)}}
        <select name="nature" id="nature">
            <option value ="Other">Other</option>
            <option value ="Adn">ADN</option>  
        </select>
    </div>
    {{ form_widget(form) }}
    <input type="submit" value="Next" class="btn btn-primary" />
</form>

FormType:

    public function buildForm(FormBuilderInterface $builder, array $options){
    $builder
            ->add('nature')
            ->add('origin');
     }

Controller:

public function madeDemandAction($id, Request $request)
{
     $em = $this-> getDoctrine() -> getManager();
     $sample = new Sample();
     $repository = $this ->getDoctrine()
                 ->getManager()
                 ->getRepository('BsBundle:Demand')
                 ->find($id);

     $demand = $repository;
     $form=$this ->createForm(new SampleType, $sample);

     if($request ->getMethod() == 'POST')
     {
       $form->handleRequest($request);
       if($form->isSubmitted() && $form->isValid())
       {
         dump($request);
         $inforequest=$form->getData();
         dump($inforequest);
         $em = $this->getDoctrine()->getManager();
         $em->persist($inforequest);
         $em->flush();
         return $this->redirect($this->generateUrl('homepage'));
       }
     }
     return $this ->render('bs_connected/human_demand.html.twig'
     , array('form'=>$form ->createView()
          , 'inforequest'=>$inforequest
          ));
   }

The problem is when I select an option on my form, the field is not load on my database.